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American Mineralogist: Journal of Earth and Planetary Science:

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American Mineralogist Volume 106

September 2021

Stable and transient isotopic trends in the crustal evolution of Zealandia Cordillera

Schwartz et al. use zircon δ18O and Lu-Hf isotope analyses to explore the interplay between spatial and temporal magmatic signals in the Mesozoic Zealandia Cordillera. They find that Mesozoic pluton chemistry was controlled by both crustal/upper mantle architecture and a transient process involving a mantle-controlled flare-up event that involved mantle melts contaminated with recycled sediments. Their data demonstrate that spatial and temporal signals are intimately linked, and when evaluated together they provide important insights into the role of stable and transient arc magmatic trends in Cordilleran batholiths.

An evolutionary system of mineralogy, Part V: Aqueous and thermal alteration of planetesimals (~4565 to 4550 Ma)

The evolutionary system of mineralogy classifies minerals based on their roles in planetary evolution. The first five parts of the system summarize all known minerals that emerged prior to the formation of planet Earth. Most of those minerals are preserved in the meteorites that still fall to Earth today. In Part V, by Hazen and Morrison, the focus is on the varied secondary minerals that formed in planetesimals more than 4.5 billion years ago through the often linked processes of aqueous alteration and thermal metamorphism. In this contribution Hazen and Morrison catalog 166 secondary meteorite minerals, including the earliest known appearances of species with the chemical elements Co, Ge, As, Nb, Ag, Sn, Te, Au, Hg, Pb, and Bi. Among the varied secondary meteorite minerals are the earliest known examples of halides, arsenides, tellurides, sulfates, carbonates, hydroxides, and a wide range of micas and clay minerals.

Cr2O3 in Corundum: Ultra-high contents under reducing conditions

Griffin et al. describe a suite of corundum xenocrysts in Cretaceous, mafic, pyroclastic rocks on Mt Carmel, Israel, which includes rubies with the highest recorded contents of Cr2O3. Many coexist with K- and Na-beta alumina phases and native chromium, requiring extremely reducing conditions, indicating oxygen fugacities circa 5 log units below the Iron-Wustite buffer. Zoning patterns indicate a progressive decrease in oxygen fugacity during crystallization (Fig. 1), which is interpreted as tracking the interaction of oxide melts with mantle-derived methane and hydrogen near the crust-mantle boundary.

Plagioclase population dynamics and zoning in response to changes in temperature and pressure

In this paper by Andrews, the numerical model SNGPlag tracks the numbers, sizes, morphologies, and compositional zoning of plagioclase crystals through time in response to changes in magmatic conditions. Low growth rates and dissolution result in significant fractions of time (>>50%) missing from the final crystal record. Growth of complexly zoned phenocrysts requires the addition of new magma. Crystal records are biased towards more recent intervals and periods of decreasing temperature. Crystallization (or dissolution) acts to return magmas to near-equilibrium crystal fractions within hundreds of days.

Limited channelized fluid infiltration in the Torres del Paine contact aureole

This paper by Siron et al. describes the metamorphic evolution of the Torres del Paine contact aureole that experienced a limited amount of fluid infiltration from the crystallizing granite. This evolution was tracked using hydrogen and oxygen isotopes and Cl concentrations in biotite. The potential of these different tracers to track fluid-rock interactions is evaluated and highlights the sensitivity of hydrogen vs. oxygen isotopes for such use. Cl content in biotite is also a promising tool for such work.

Quantitative determination of the shock stage of L6 ordinary chondrites using X-ray diffraction

The mean lattice strain and grain size factor for olivine and orthopyroxene of 14 L6 chondrites were determined by Imae and Kimura from analyses based on Williamson-Hall plots via X-ray diffraction. The combination of mean lattice strain with grain size factor enables the construction of a shock barometer.

A new method to rapidly and accurately assess the mechanical properties of geologically relevant materials

This new method by Deng et al. will enable geological scientists and engineers to rapidly determine the elastic behavior of any rock or mineral in a simple manner with a robust statistical response. It will also facilitate the determination of elastic properties of rocks and minerals as a function of composition, structure, hydration, or other physical variables. This will undoubtedly impact the development of geologic-based deformation models through knowledge of the constituent material properties for validation and prediction accuracy.

Two-stage magmatism and tungsten mineralization in the Nanling Range, South China: Evidence from the Jurassic Helukou deposit

In this study, Cao et al. report LA-ICP-MS zircon U-Pb and molybdenite Re-Os ages of the Helukou W deposit, which is a representative W deposit located in the Guposhan district, SW of Nanling Range, South China. This geochronological data aims to reveal persistent (Early to Jurassic) granitic magmatism and related W-Sn mineralization events in the Nanling region. In addition, the in-situ trace element and Sr isotopes compositions of scheelite from the skarn- and altered granite-type ores of the Helukou W deposit are reported, with the purpose of outlining the nature of the ore-forming fluids in the long-lived magmatic-hydrothermal system. The data reveals two stages of magmatism and tungsten mineralization in the Guposhan ore field during Early Jurassic (~180 Ma) and Middle Jurassic (~165 Ma). This 20 m.y. long-lived successive magmatism and mineralization in the Nanling Range will provide new insights into the magmatism, mineralization, and metallogenesis in this region.

Constraints on scheelite genesis at the Dabaoshan stratabound polymetallic deposit, South China

Stratabound polymetallic deposits serve as one of the most important reserves of base metals in South China, but the genetic relationship between stratabound base metal mineralization and porphyry mineralization remains in dispute. Scheelite occurs as a characteristic mineral in many stratabound polymetallic deposits and in the adjacent porphyry system. In this paper, Su et al. use scheelite geochemistry, zircon, and hydrothermal apatite U-Pb dating to constrain the genesis of scheelite in stratabound orebodies at Dabaoshan. By integrating scheelite CL textures, trace element characteristics, and the new dating results, they demonstrate that ore-forming fluids responsible for scheelite formation in stratabound orebodies were derived from the Dabaoshan porphyry system. Furthermore, Jurassic porphyry mineralization at least partially contributed to Cu mineralization in the stratabound orebodies. This work suggests that scheelite geochemistry, combined with geochronology studies, can contribute to our understanding of stratabound orebody genesis.

Crystal chemistry of schreibersite, (Fe,Ni)3P

Schreibersite is the most common phosphide mineral and the main carrier of reduced phosphorus in the celestial bodies. The present paper by Britvin et al. reports on previously obscured trends in Fe and Ni distribution across schreibersite structures, which are directly related to crystallization histories of metal-rich planetary interiors.

Elastic geobarometry: how to work with residual inclusion strains and pressures

Gilio et al. discuss the difference between various approaches to retrieve the residual pressures and residual strains from Raman measurements of inclusions. They provide general guidelines on how to deal with inclusion strain, measurements, inclusion pressure, and their uncertainties.

Controls on tetrahedral Fe(III) abundance in 2:1 phyllosilicates - - Discussion

Controls on tetrahedral Fe(III) abundance in 2:1 phyllosilicates - Reply

New Mineral Names

Book Review: Geochronology and Thermochronology

August 2021

Highlights and Breakthroughs: Crustal melting: deep, hot, and salty

Lamadrid and Steele-MacInnis provide perspective on the study by Ferrero et al.: High pressure, halogen-bearing melt preserved in ultra-high temperature felsic granulites of the Central Maine Terrane, Connecticut (U.S.A.).

Petrogenetic and tectonic interpretation of strongly peraluminous granitic rocks and their significance in the Archean rock record

Strongly peraluminous granitic rocks first become abundant in the rock record in the Neoarchean. In this study by Frost and Da Prat (MSA Presidential Address by Carol Frost), six suites of Neoarchean strongly peraluminous granitic rocks are described from the Wyoming province. These are shown to form in both subduction and collisional environments from a variety of sources, including hornblende-plagioclase rocks, biotite-bearing gneisses, and two-mica metapelitic rocks. The appearance of strongly peraluminous granitic rocks in the rock record signals the development of strong, thick felsic continental crust and the formation of the first supercontinents.

Partial melting and P-T evolution of eclogite-facies metapelitic migmatites from the Egere Terrane (Central Hoggar, South Algeria)

This contribution by Arab et al. improves our understanding of high-pressure metapelites from Central Hoggar, provides very advanced P-T modeling, and shows the first description of evidence of partial melting phenomena from Hoggar. This study resolves numerous problems regarding the metamorphic evolution of the Egere area and their geodynamic implication within the Tuareg belt.

High pressure, halogen-bearing melt preserved in ultra-high temperature felsic granulites of the Central Maine Terrane, Connecticut (US)

Ferrero et al. performed micro-petrology experiments on preserved droplets of melt in garnet from the ultrahigh temperature felsic granulites of the Central Maine Terrain (Connecticut, U.S.A.). The results reveal a history of metamorphism and melting at a depth consistent with the orogenic roots, ~70 km, twice the previous estimates for these rocks. The high-pressure melt here preserved is peculiar: it has an uncommonly high mafic component, coupled with significant amounts of carbon dioxides and halogens (chlorine and fluorine). Whereas the first phenomenon results from the extreme melting temperatures, the halogens abundance likely relates to the presence of brines (highly saline liquids) at depth during melting. The latter is a direct evidence that brines may play a role in promoting melt production when the crust attains temperature in excess of 1000 °C, where melt production is actually expected to be hindered by the extremely dehydrated character of the rocks.

Targeting Jarosite -- Clay-mineral mixtures for Mars exploration

Hinman et al. compared, with multiple techniques, the detectability and abundance of organic matter associated with clay minerals and the sulfate salts, alunite, and jarosite, from a hydrothermal area in Yellowstone National Park, Wyoming, U.S.A. Spectroscopy and diffractometry produced comparable mineral identifications in sample matrices. The matrix material (clay minerals or sulfate salts) had no effect on the detection of organic matter by Raman spectroscopy. However, mineral matrix composition had a significant effect on the detectability of organic matter by laser-desorption, Fourier transform mass spectrometry (LD FTMS). Indeed, LD FTMS detected diverse organic compounds in samples containing jarosite, more so than in samples dominated by aluminosilicate minerals, despite higher organic content in the later matrix. Further, organic compounds were detected more frequently in jarosite samples than in aluminosilicate samples. Consequently, jarosite-bearing sites on Mars could be potential targets for life detection, especially if associated with clay minerals.

Zirconolite from Larvik Plutonic Complex, Norway, its relationship to stefanweissite and nöggerathite, and contribution to the improvement of zirconolite end-member systematics

Haifler et al. performed crystal-chemical characterization of a complex zirconolite from Larvik Plutonic Complex, Norway. A concept called "edgemembers" was introduced to quantitatively express compositional variation of a complex solid solution. Moreover, a modified scheme of zirconolite end-member set and redefinition of a composition space was suggested.

Nanomineralogy of hydrothermal magnetite from Acropolis, South Australia: Genetic implications for iron-oxide copper gold mineralization

The Acropolis prospect is a magnetite dominant iron-oxide copper gold (IOCG) prospect in the Olympic Dam district (South Australia). Verdugo-Ihl et al. used complementary microbeam techniques (HAADF STEM, XMg-in-magnetite thermometry, and nanothermobarometry using ilmenite-magnetite pairs) to characterize titanomagnetite from veins hosted in volcanic rocks and Ti-poor magnetite from a granite body stratigraphically higher in the sequence. Hydrothermal titanomagnetite from Acropolis is comparable with magmatic magnetite in granites across the district and should typify early alkali-calcic alteration. Open-fracture circulation, inhibiting additional supply of Si, Ca, K, etc., during magnetite precipitation, prohibits formation of silician magnetite hosting calc-silicate NPs, as known from IOCG systems characterized by rock-buffered alteration of host lithologies. Obliteration of trellis textures during subsequent overprinting could explain the scarcity of this type of hydrothermal magnetite in other IOCG systems.

Effect of magnesium on monohydrocalcite formation and unit cell parameters

Vereshchagin et al. report the phase diagrams for monohydrocalcite (MHC), calcite, aragonite, and dypingite precipitation, as a function of pH and Mg/Ca ratio in solution. They demonstrate that elevated magnesium content and low temperature are favorable for MHC formation. On the basis of changes in the unit cell parameters, the possibility of the significant incorporation of magnesium into the crystal structure of MHC has been unequivocally proven. The increase in Mg in MHC is accompanied by an increase in water content, which leads to multidirectional changes in a and c cell parameters.

Formation pathway of norsethite dominated by solution chemistry under ambient conditions

"Dolomite problem" is a long-standing puzzle in geology. Zhang et al. investigated low-temperature formation of norsethite &91;BaMg(CO3)2&93; as a dolomite-analog under different solution chemistry conditions. The results reveal that the initial Mg/Ba ratio in solution is a crucial factor controlling the formation pathway of norsethite. At low Mg/Ba ratios, a multistep pathway occurs, i.e., a precursor witherite first forms, followed by norsethite precipitation and transformation from witherite to norsethite. In contrast, at Mg/Ba ratio > 20, norsethite can be directly precipitated from aqueous solution. This is the first report on direct precipitation of dolomite analogs under ambient conditions, and the findings provide new insights into the formation pathways of dolomite analogs and thus low-temperature dolomite.

A model for the kinetics of high-temperature reactions between polydisperse volcanic ash and SO2 gas

Scientists have collected volcanic ash from eruptions and noticed that tiny salt crystals exist on their surfaces. Wadsworth et al. use experiments and mathematics to work out how quickly those salt crystals grow. Because the salts contain sulfur, they propose that some of the sulfur that would have ended up in the atmosphere actually ends up on ash surfaces. They further propose that these equations could tell us just how much sulfur is removed from the atmosphere in this process.

Redox control and measurement in low-temperature hydrothermal experiments

Fang and Chou propose a new oxygen buffer technique for hydrothermal experiments carried out at temperatures below 450 °C and demonstrate its success in redox control and monitoring. Their work extends the low-temperature limit of previously well-developed redox control techniques to, at least, 200 °C, and it has promising prospects of application in low-temperature hydrothermal experiments.

Heat capacity and thermodynamic functions of partially dehydrated sodium and zinc zeolite A (LTA)

Dickson et al. offer a thermodynamic perspective into cation exchange, water sorption, and sorbate-induced structural transformations in zeolites. Heat capacities and thermodynamic functions are reported for industrially important zeolite A and for the ion-exchanged version zinc zeolite A. In addition, zeolitic water in sodium zeolite A is compared with that in other zeolites. Inflections common to all the zeolitic water heat capacities suggest that the hydration-influenced framework flexibility that has been found in zeolite A may be present in other zeolites as well.

P-V-T measurements of Fe3C to 117 GPa and 2100 K

McGuire et al. measured the unit-cell volume of an iron carbide, Fe3C, by X-ray diffraction at simultaneous extreme pressure and temperature conditions relevant to the deep interior of the Earth. These measurements are used to construct a thermodynamic model of Fe3C for the Earth's core. They find that existing measurements support two possible scenarios for the Earth's core in the Fe-C system: (1) Fe3C is the stable end-member at inner core conditions but cannot explain the inner core density; and (2) Fe7C3 is stable and may explain the inner core density at relatively low temperatures.

New Mineral Names


Book Review

July 2021


Tourmaline composition and boron isotope signature as a tracer of magmatic-hydrothermal processes

Qiu et al. in their paper make three key points. They found that tourmalines crystallized from magmatic-hydrothermal fluids with different oxygen fugacities and salinities; that degassing during fluid evolution leads to significant B isotope fractionation and increases oxygen fugacity; and that tourmaline composition and the boron isotope signature can be tracers of magmatic and hydrothermal processes, as well as used to distinguish mineralized from barren host-granites.


Deformation and strength of mantle relevant garnets: Implications for the subduction of basaltic-rich crust

Garnet is an abundant mineral in the upper mantle, and there have been no previous measurements of both its room-temperature strength and deformation mechanism. Vennari et al. deformed three compositions of garnet between two diamonds and probed its deformation with X-rays perpendicular to the compression axis. These measurements allowed them to determine garnet's resistance to plastic deformation and show that its strength is similar to other mantle phases. Vennari et al. also provide a modeling analysis to explore the deformation mechanism on an atomistic scale when axial stress is applied.

Ultra-reduced phases in ophiolites cannot come from Earth's mantle

It is claimed that ophiolites carry highly reduced minerals and that the source regions of ophiolites in the Earth's mantle are located in the transition zone or lower mantle. By showing experimentally that highly reduced phases such as moissanite (SiC) are unstable in a FeO-bearing mantle, Ballhaus et al. refute this hypothesis. They caution that accessory diamond and ultra-reduced phases may not provide sufficient evidence to question existing models of ophiolite genesis.


Olivine from aillikites in the Tarim large igneous province as a window into mantle metasomatism and multi-stage magma evolution

Wang et al. report the first aillikite in China. Phlogopite- and carbonate-rich metasomatic veins are involved in the source of the aillikite magma. Phenocrystic olivines in the aillikite are cognate cumulates formed by multi-stage crystallization at temperatures of 906-1136 degrees C.

Precise determination of the effect of temperature on the density of solid and liquid iron, nickel, and tin

Kamiya et al. precisely measured the density of solid Sn, Ni, Fe, and liquid Fe using a high-temperature furnace. The density of solid Sn shows a drastic decrease near melting temperature (Tm). By contrast, the densities of solid Ni and Fe decrease linearly with increasing temperature up to Tm without any drastic decrease near Tm. The thermal expansion of liquid Fe was also determined to depend on temperature. The results are important for understanding the elastic behavior of solid and liquid metals at high temperatures.


Timescales of crystal mush mobilization in a volcanic system based on olivine diffusion chronometry

Caracciolo et al. applied diffusion chronometry to olivine crystals erupted at different ages from a single volcanic system. They carried out a chemical characterization of olivine crystals coupled with a detailed Fe-Mg, Mn, and Ni diffusion study from temporally diverse (sub-glacial to historical time) eruptive units to retrieve timescales of crystal mush disaggregation in the volcanic system in Iceland. One important implication of their study is that magma replenishment and mush disaggregation events, possibly triggered by rifting-assisted processes, do not result in imminent eruption. In addition, only a few data exist on timescales of magmatic processes in mid-Ocean Ridge settings, and this is the first detailed study of the temporal variation of crystal-mush-to-eruption timescales in any volcanic system in any magmatic setting.


Chemical reactions in the Fe2SiO4-D2 system with a variable deuterium content under a pressure of 7.5 GPa

The reactions examined in the paper by Efimchenko et al. are expected to occur under natural conditions in the interiors of the icy satellites and lead to the removal of fayalite from rocks and minerals due to its dissolution in hydrogen fluid or replacement by iron, ferrosilite, or silica depending on the molar ratio H2/Fa.


High-pressure syntheses and crystal structure analyses of a new low-density CaFe2O4-related and CaTi2O4-type MgAl2O4

The high-pressure polymorphs of MgAl2O4 are considered important and abundant components that are stable under lower-mantle conditions in alumina-rich rocks, such as basalt, upper continental crust, and sediment. In this study, Ishii et al. synthesized single crystals of CaTi2O4 (CT)-type, CaFe2O4 CF-type, and a novel MgAl2O4 phases at 27 GPa and 2500 °C by conventional multi-anvil technique. They also synthesized CT-type MgAl2O4 at 45 GPa and 1727 °C using an advanced multi-anvil technique that they developed. Their single-crystal X-ray diffraction measurements showed that the novel phase has a completely new structure with a smaller density than the ambient pressure phase of spinel, despite its high-pressure synthesis, indicating that this phase is a back-transformed phase from a high-pressure phase during the recovery. This finding solves the crystal structure of a previously unknown phase. CT-type MgAl2O4 has been considered to be stable at more than 40 GPa. Ishii et al.’s finding that CT-type phase is synthesized even at 27 GPa indicates that the CT phase in natural-rock compositions may be stable in wide lower mantle conditions, especially upwelling hot plume conditions. They also suggest that the novel phase can be found in shocked meteorites and can be used as a shock indicator.

Phase diagram and thermal expansion of orthopyroxene-, clinopyroxene-, and ilmenite-structured MgGeO3

Despite the ubiquity of studies of low-pressure analogs for Earth-forming phases, these systems are often incompletely characterized. Frequently, parts of the system remain unmeasured, but an analogue is only as good as its characterization. Here, Hunt et al., in their open access paper, complete the characterization of the lowest pressure phases of MgGeO3 and reconcile the disagreement between previous measurements. The reconciliation points to important differences between MgSiO3 and its low-pressure analog MgGeO3.

Mass transfer associated with chloritization in the hydrothermal alteration process of granitic pluton

This study by Yuguchi et al. focuses on the petrography and mineral chemistry of hornblende chloritization, K-feldspar chloritization, and fracture-filling chlorite, in order to better understand mass transfer among the reactants, products, and hydrothermal fluids during chloritization. A combination of this study and Yuguchi et al. (2015) dealing with biotite chloritization reveals the characteristics of mass transfer in the overall chloritization process.


Non-linear effects of hydration on high-pressure sound velocities of rhyolitic glasses

Gu et al. measured sound velocities in hydrous rhyolitic glasses containing up to ~6 wt% H2Ot at pressures to 3 GPa. The results show that hydration has a non-linear effect on sound velocities at high pressure, in contrast to the linear effects of hydration previously observed at ambient pressure. These results imply that the different water species, OH- and H2Om, play unique roles in altering sound velocities in rhyolitic glasses. The non-linear effects of hydration found here provide some insight into the behavior of hydrous silicate melts in felsic magma chambers at depth.


Crystal chemistry and high-temperature vibrational spectra of humite and norbergite: fluorine and titanium in humite-group minerals

The humite-group minerals on the brucite-olivine join may be important carriers of H2O into the Earth’s deep interior, as well as water reservoirs in subduction zones and the upper mantle in extremely hydrous compositions. In this study, Liu et al. carried out single-crystal X-ray diffraction and used in-situ high-temperature Raman and Fourier transform infrared (FTIR) measurements on natural humite and norbergite samples to study the effects of F and Ti incorporation on the hydrogen behavior in these hydrous minerals. The OH-stretching modes above 3450 cm-1 are affected by the neighboring H-H repulsion, while the ones below 3450 cm-1 could be explained by F/Ti substitutions, both of which could relieve the H-H repulsion effect. The Raman-active OH bands below 3450 cm-1 are determined by Ti-substitution, while the IR-active ones are contributed by both F and Ti substitutions.  Both the structure refinements and high-temperature vibrational spectra on natural humite samples could provide microscopic insight for understanding F and Ti effects on the stability field of the hydrous minerals (such as humite-group minerals) in subducting slabs.


Exomorphism of Jacobsite Precipitates in Bixbyite Single Crystals from the Thomas Range in Utah

Peter et al. investigated single crystals of bixbyite via transmission electron microscopy (TEM), revealing a lamellar 3D network consisting of the tetragonal manganese-silicate braunite. Additionally, nanometer-sized precipitates were found and identified as jacobsite, a cubic iron-manganese-spinel, not previously known to occur in the Thomas Range. The presence of spinel implies higher formation temperatures than described in the literature, possibly acting as a novel geo-thermometer. Interestingly, TEM observations indicate that the shape of the crystallites is exomorphously determined by the braunite lamellae, leading to their unusual cuboid morphology, which is, to the best of our knowledge, the first report on exomorphism of a naturally formed spinel.


Ferropyrosmalite-bearing fluid inclusions in the North Patagonian Andes metasedimentary basement, Argentina: A record of regional metasomatism

Sosa et al. present new cathodoluminescence, Raman spectroscopy, and microthermometric data of fluid inclusions in metamorphic quartz segregations from metasedimentary rocks of northwestern Patagonia (Argentina). This information is combined with heating experiments of fluid inclusions, which allowed studying equilibrium conditions of ferropyrosmalite with clino-amphibole, providing novel insights into their genetic conditions and mineral equilibria. Implications for fluid evolution during regional metasomatic processes are included in this work.


Memorial of Alden Bliss Carpenter (1936-2019)

New Mineral Names


June 2021

Cation partitioning among crystallographic sites based on bond-length constraints in tourmaline-supergroup minerals

Bacik and Fridrichova determined the positions of major, minor, and trace elements in the tourmaline structure based on the calculated bond lengths. Major elements can be divided into the structural sites based on the chemical and structural data, but it is far more complicated for minor and trace elements due to their low contents. Predictions can be made based on bond-length constraints for each site. Theoretical bond-length calculation from ideal bond valences for each ion and coordination can predict ion site preference in the structure.

Magnesio-lucchesiite, CaMg3Al6(Si6O18)(BO3)3(OH)3O, a new species of the tourmaline supergroup

Scribner et al. describe magnesio-lucchesiite, a new species of the tourmaline supergroup. The mineral originates from the replacement of mafic minerals by boron-rich metasomatic fluids; it may also form in metacarbonate rocks during crystallization of common uvitic tourmaline. High miscibility with other tourmaline end-members indicates the large petrogenetic potential of magnesio-lucchesiite in Mg,Al-rich calcsilicate rocks, as well as contact-metamorphic, and metasomatic rocks.

Raman spectroscopic quantification of tetrahedral boron in synthetic aluminum-rich tourmaline

Kutzchbach et al. describe the correlation of the tetrahedral B content in synthetic Al-rich tourmaline determined by EMPA analyses with the summed relative intensity of all OH-stretching bands between3300–3430 cm–1. Applying the regression equation to natural 4B-bearing tourmaline from the Koralpe (Austria) reproduces the EMPA-derived value. This demonstrates that Raman spectroscopy provides a fast and easy-to-use tool for the quantification of tetrahedral boron in tourmaline. The knowledge of the amount of tetrahedral boron in tourmaline has important implications for the better understanding and modeling of B-isotope fractionation between tourmaline and fluid/melt, widely used as a tracer of mass transfer processes.

Thermal expansion of minerals in the pyroxene system and examination of various thermal expansion models

Based on the new data for thirteen chemically distinct pyroxenes, Hovis et al. provide a review of thermal expansion data and models for clino- and orthopyroxenes and model changes in thermal expansion with temperature according to the Fei empirical and Kroll physical models of thermal expansion. Conclusions are drawn about the effects of Fe2+- Mg substitution, Ca-Na concentration, and other chemical substitutions on the magnitudes of volume and axial thermal expansion. In part, differences in thermal expansion can be related to the concentration of M1 polyhedra along the b axis and relative charges of the substituted cations in that site. These results have implications for the modeling of geophysical and thermodynamic behavior of high-temperature pyroxene-bearing assemblages.

Incorporation of tetrahedral ferric iron in hydrous ringwoodite

Thomson et al. synthesized hydrous Fo91 ringwoodite crystals at 20 GPa and high-temperature conditions using a multi-anvil press. Recovered crystals were analyzed using electron microprobe analysis, Raman spectroscopy, infrared spectroscopy, synchrotron Mossbauer spectroscopy, single-crystal X-ray diffraction, and single-crystal Laue neutron diffraction to characterize the chemistry and crystallography of the samples. Analysis of the combined datasets provides evidence for the presence of tetrahedrally coordinated ferric iron and multiple hydrogen incorporation mechanisms within ringwoodite. Tetrahedral ferric iron is coupled with cation disorder of silicon onto the octahedrally coordinated site. Cation disorder in mantle ringwoodite minerals may be promoted in the presence of water, which could have implications for current models of seismic velocities within the transition zone.

The evolution of saponite: An experimental study based on crystal chemistry and crystal growth

Zhang et al.’s synthesis experiments on saponite show that element distributions within clay minerals are related to the synthesis temperature gap between starting materials and resultant products. Larger Mg- or Ni- clusters can form when the respective synthesis temperature of the starting materials and of the resultant products are close (i.e., low T). Thus, the clustering is not only related to given temperature conditions but also to the characteristics of the starting materials in terms of crystallinity degree and particle size. Crystal-chemistry and crystal growth could be used to predict the environmental transition and element migration. Similar experiments with more complexities could offer potentials to predict the mineral geochemistry and better understand and parametrize natural processes.

Hydroxylpyromorphite, modern description and characterization of a mineral important to lead-remediation

Olds et al. describe the structure and properties of a new mineral, hydroxylpyromorphite. Hydroxylpyromorphite is a key phase used for remediation of Pb-polluted aquifers and its ability to remove Pb from drinking water has been heavily studied. Their description provides a deeper understanding to the anion column arrangements in pyromorphites and other Pb-bearing apatites, which may help guide and optimize Pb sequestration, or reveal how anion and cation substitution affects important properties such as solubility, reactivity, and mobility of Pb in natural systems.

Experiments on two techniques for the removal of barite from detrital zircon

Martin et al. performed experiments on two techniques for the removal of barite from detrital zircon: conventional milling method and a new chemical technique. Milling in a ball mill fractures and removes zircon along with barite. Milling thus can introduce bias into a detrital zircon sample. Boiling in aqueous Na2CO3 solution converts barite to barium carbonate, which can be separated from zircon by hand or by hot nitric acid bath. Boiling in concentrated HNO3 did not affect the U-Pb or Lu-Hf isotopic systems in zircon and boiling in Na2CO3 solution followed by HNO3 did not disturb the Lu-Hf system. Boiling in concentrated Na2CO3 solution followed by HNO3 did affect the U-Pb isotopic system in zircon. These results highlight the importance of proper technique during zircon isolation to minimize the introduction of bias into the sample.

Discovery of terrestrial allabogdanite (Fe,Ni)2P, and the effect of Ni and Mo substitution on the barringerite-allabogdanite high-pressure transition

Britvin et al. report the discovery of a high-pressure phosphide, allabogdanite, (Fe,Ni)2P, in phosphide assemblages confined to the surficial metamorphic rocks at the Dead Sea basin in Israel. High-pressure high-temperature experiments show that the low- to high-pressure phase transition for natural (Fe,Ni)2P from the Dead Sea occurs at a pressure of 25 GPa (~250 kbar) and a temperature of ~1400 °C.

Pseudomorphic 9-line silician ferrihydrite and Fe-rich serpentine-group minerals in FeTi-oxide rich ferroan peridotite, Laramie anorthosite complex, Wyoming, U.S.A.

Evans et al. describe a unique occurrence of the mineral ferrihydrite as a pseudomorph after a sulfide mineral in a Larimie Complex, Wyoming, peridotie. Ordinarily, ferrihydrite is a supergene mineral occurring as mine waste. In this new occurrence, ferrihydrite displays elemental covariations never before described in this mineral. Co-variations of Si, Mg, and total Fe represent a crystal chemical response to subsurface altering solutions and its superior (9-line) crystallinity.

Synthesis and characterization of Fe(III)-Fe(II)-Mg-Al smectite solid solutions and implications for planetary science

Fox et al.’s detailed characterization of the full compositional range of smectite minerals using techniques analogous to those employed in planetary exploration will help improve these capabilities for identification and compositional quantification, and in turn better the ability to interpret alteration environments throughout the solar system. They synthesize and report on the spectroscopic properties of a suite of smectite samples that span the intermediate compositional range between Fe, Mg, and Al end-member species, including both Fe(III)- and Fe(II)-dominated samples.

Evidence for a two-stage particle attachment mechanism for phyllosilicate crystallization in geological processes

He et al. describe a nonclassical crystallization process for phyllosilicates forming at elevated temperatures in magmatic and metamorphic environments whereby oriented attachment of building blocks occurs along the (001) plane and/or the 1 direction simultaneously. Nanoparticles are basic building blocks for coarsening of phyllosilicate crystals. The results suggest that the microstructure of magmatic and high-grade metamorphic minerals may have the potential to reveal the evolution history of melts and high-temperature metamorphic processes.

Multiple generations of tourmaline from Yushishanxi leucogranite in South Qilian of western China record a complex formation history from B-rich melt to hydrothermal fluid

Liu and Jiang studied elemental and boron isotopic compositions of tourmaline in leucogranite dike in the Yushishanxi Nb-Ta mining area in the Yushishan district of the South Qilian orogenic belt in western China. They identified five distinct types of tourmaline that show significant chemical variations and core-rim zoning trends. Detailed petrological, geochemical, and isotopic studies reveal four major generations of tourmaline growth from magmatic to hydrothermal, all of which belong to the alkali group and schorl-dravite solid solution series. The authors propose that the isotopically light fluids may have derived from B release during mica breakdown within the surrounding metamorphic rocks.

Zhanghuifenite, Na3Mn2+4Mg2Al(PO4)6, a new mineral isostructural with bobfergusonite, from the Santa Ana mine, San Luis province, Argentina

Yang et al. describe the physical and chemical properties of a new mineral, zhanghuifenite, and its crystal structure determined from single-crystal X-ray diffraction data, illustrating its structural relationships to bobfergusonite in particular and alluaudite-type minerals in general. Furthermore, they predict the existence of three more new minerals in the bobfergusonite group based on the data.

Re-examined heterotype solid solution between calcite and strontianite and Ca-Sr fluid-carbonate distribution: an experimental study in the system CaCO3-SrCO3-H2O at 0.5-5 kbar and 600 degrees Celcius

Schiperski et al. present a revised phase diagram in the system CaCO3-SrCO3 at 600 °C and 0.5-5.0 kbar based on new experimental data and thermodynamic calculations. In addition, for the first time Kd-values of Ca/Sr carbonate-fluid are presented at this condition. Results show that Sr has a high tendency for being accumulated into carbonates rather than in the fluid, which is a substantial difference from silicate-fluid systems.

On the formation of arrays of micro-tunnels in pyrope and almandine garnets

Rabier et al. show that the intricate tunnels imaged in pyrope and almandine garnets found in soils and river sediments can be fully explained by abiogenic etching of dislocation microstructures contained within the minerals. There are striking geometric similarities between these tunnels and dislocation networks that have been documented in numerous natural and synthetic garnets.

Book review

May 2021

How to apply elastic geobarometry in geology

This paper by by Fabrizio Nestola highlights the new "EntraPT" web application, published by Mazzucchelli et al. (2021) in American Mineralogist, which provides all recent advances of elastic geobarometry within a single platform. This application allows the user to interpret the residual strain of anisotropic inclusions in an intuitive and consistent manner. At the same time, the platform provides the tools needednto perform calculations of the residual pressure and of the entrapment pressure and temperature of isotropic and anisotropic systems using a self-consistent set of thermoelastic properties.

Early Archean alteration minerals in mafic-ultramafic rocks of the Barberton greenstone belt as petrological analogs for clay mineralogy on Mars

The upcoming Mars2020 rover analyses of Jezero crater will investigate phyllosilicate and carbonate-bearing outcrops in delta sediments, as well as basaltic bedrock containing olivine, chlorite, and serpentine (e.g., Goudge et al. 2017, Bramble et al. 2017; Amador et al., Brown et al. 2020). If chlorite- and serpentine-bearing rocks are encountered by Mars2020, the rock textures and spectral analyses in this study by Grosch et al. will provide ground truth to understand potential formation conditions for those rocks.

Characterization of the metasomatizing agent in the upper mantle beneath the northern Pannonian Basin based on Raman imaging, FIB-SEM, and LA-ICP-MS analysis of silicate melt inclusions in spinel peridotite

This study by Liptai et al. presents a combination of analytical methods applicable to silicate melt inclusions as an alternative to microthermometry. The combined analyses allow the acquisition of both geochemical properties and 3D structure of the daughter phases. With the use of these methods, composition, origin and crystallization history of a trapped metasomatizing melt was revealed in the upper mantle of the northern part of the Pannonian Basin (Central Europe).

The potential for aqueous fluid-rock and silicate melt-rock interactions to re-equilibrate hydrogen in peridotite nominally anhydrous minerals

Hydrogen diffusion chronometry provides first-order constraints on how quickly mantle mineral water contents can be overprinted by hydrothermal alteration and melt-rock interactions. Peridotite xenoliths likely completely lose their mantle-inherited water contents prior to eruption due to fast H re-equilibration timescales. Re-equilibration occurs in as little as 10 minutes at 1000 °C to several of years at 600 °C, suggesting that both melt- and/or aqueous fluid-rock interactions can explain why many peridotites have water contents decoupled from trace elements and that are too high to reflect residues of melting. Lynn and Warren also posit in this contribution that hydrogen has the unique potential to be used at low-T (e.g., 300 °C) to investigate the timescales of serpentinization.

Oxygen isotope ratios in zircon and garnet: A record assimilation and fractional crystallization in the Dinkey Dome peraluminous granite, Sierra Nevada, California

Quintero et al. demonstrate that δ18O variations in zircon and garnet from the Dinkey Dome peraluminous granite in the Sierra Nevada, California, show how a peraluminous pluton is constructed from multiple batches of variably contaminated melts, suggesting that early-stage arc magmas sample crustal heterogeneities in small melt batches. Further progressive invigoration of the arc more effectively blends with mantle melts in source regions. Peraluminous magmas provide key details of the nascent arc and pre-batholithic crustal structure.

An evolutionary system of mineralogy, Part IV: Planetesimal differentiation and impact mineralization (4566 to 4560 Ma)

The evolutionary system of mineralogy attempts to classify minerals based on their historical context, considering when and how the minerals formed, in addition to the traditional approach of documenting major element chemistry and atomic structure. In Part IV of their series, Morrison and Hazen consider minerals formed in the partially molten interiors of asteroids that began to separate metal-rich cores from silicate-rich mantles, as well as minerals transformed by high-pressure impacts. The 130 new natural kinds of minerals considered here all formed within the solar system's first 10 million years -- a time when high pressure first became an important factor in the formation of minerals.

Constraints on deep, CO2-rich degassing at arc volcanoes from solubility experiments on hydrous basaltic andesite of Pavlof Volcano, Alaskan Peninsula, at 300 to 1200 MPa

Mafic magmas that enter Earth's crust from the mantle are commonly estimated to have appreciable CO2 concentrations, but direct experimental determinations are few of CO2 solubility at pressures appropriate for the continental lower crust and underlying uppermost mantle. This study by Mangan et al. determines CO2 solubility in that pressure range for basaltic andesite, a widespread subduction zone magma type, across a range of dissolved H2O concentrations suitable for arc magmas. Results show that basaltic andesite can retain substantial dissolved CO2 in the deep arc crust, consistent with geochemical estimates, but that published models of magmatic CO2 solubility are inaccurate for mid-crustal to uppermost mantle pressures, some overestimating and others underestimating the measured concentrations. Systematic studies varying individual melt compositional components may be required to improve understanding of controls on solubility and to develop robust models of H2O-CO2 degassing in the deep arc crust.

Electrical conductivity of diaspore, δ-AlOOH and ε-FeOOH

This study by Wang and Yoshino addresses the measurement of the electrical conductivity of three hydrous minerals: diaspore, δ-AlOOH, and ε-FeOOH. Investigations on such phases became hot topics because of their wide pressure-temperature stability field (Sano et al. 2008; Nishi et al. 2017; Duan et al. 2018; Hu et al. 2016, 2017). Electrical conductivity measurement of these minerals is also useful to understand deep water cycle. This study could provide important insights on the electrical behaviors of hydrous minerals and water cycle in the Earth's interior. Three significant outcomes were obtained from this study. (1) Diaspore shows a negative pressure dependence on the conductivity because of the ineffectual change of the length of O1H bond with increasing pressure. (2) δ-AlOOH and ε-FeOOH have shorter H...O2 bonds and longer O1H bonds which can account for the relatively higher conductivity than diaspore. (3) ε-FeOOH indicates higher conductivity than δ-AlOOH, which might be caused by the contribution of a small polaron conduction. The result of this study could be applied to the trends of the conductivity-depth relation for various lithologies including hydrous minerals in subduction zones such as altered oceanic crust, hydrous peridotites, and subducted sediments.

A reassessment of the amphibole-plagioclase NaSi-CaAl exchange thermometer with applications to igneous and high-grade metamorphic rocks

The existing calibrations of amphibole-plagioclase and amphibole-only thermometers yield inaccurate temperature estimates. Three new expressions of the amphibole-plagioclase NaSi-CaAl exchange thermometer are calibrated in this work by Molina et al. that yield an overall similar precision (plus or minus 50°C), but are significantly more accurate. The new expressions can be used in a wide range of igneous and high-grade metamorphic rocks that bear subcalcic to calcic amphibole and oligoclase or more calcic plagioclase

Structural variations across the nepheline (NaAlSiO4)-kalsilite (KAlSiO4) series

Antao and Hovis obtained the crystal structure of 19 samples within the (Na,K)AlSiO4 series on a fine compositional scale. Well-defined structural variations occur across the Ne-Ks series. Natural samples are not yet available to give a complete understanding of Ne-Ks solid series, which is overcome by using "synthetic" ion-exchanged, powder samples instead of single crystals. Natural samples with limited solid solutions plot close to their "synthetic" samples, so their results apply to natural systems. However, it would be interesting to see how the results for other natural samples with different chemistries, if discovered, compare to the present results. Phase transitions in minerals and their reversibility continue to intrigue researchers. Synchrotron high-resolution powder X-ray diffraction uses a highly intense beam that is sensitive to fine-scale intergrowths or overlapping multi-phases with subtle change in symmetry that cannot be resolved with conventional powder X-ray diffraction. The P31c to P63 transition in kalsilite may be monitored using the 111 reflection that is clearly observable with HRPXRD. K-rich nepheline and kalsilite occur in volcanic rocks as high temperatures expand the aluminosilicate framework to accommodate large K atoms. Particularly, P63 Ks occurs in volcanic rocks, and low temperature P31c Ks occurs in metamorphic rocks. The high to low transition is not completely reversible.

The (chemical) potential for understanding overstepped garnet nucleation and growth

This paper by Nagurney et al. utilizes phase equilibria modeling to understand the energetic consequences of metamorphic minerals, specifically garnet, not forming at thermodynamic equilibrium. They compare the chemical potentials of garnet-forming oxide components (MnO, CaO, FeO, MgO, Al2O3) between two calculations: one in which Gibbs free energy is minimized and one in which the minimization proceeds under identical conditions but in the absence of garnet. This allows them to study the energetic differences between a garnet-bearing and a garnet-absent calculation. Their results highlight that immediately up temperature of garnet-in, differences in the chemical potentials between the two calculations are commonly minimal for some components. In all twelve examples used in this study the chemical potential of Al2O3 (mu-Al2O3) diverges between garnet-bearing and garnet-absent calculations at greater P-T conditions than that of MnO, CaO, FeO and MgO. This P-T point at which mu-Al2O3 diverges is a function of bulk-rock MnO content. Further, results emphasize that immediately up temperature of garnet-in the total volume of garnet in a rock is commonly calculated to be minimal. These results highlight the necessity for petrologists to interrogate phase equilibria modeling data when interpreting the amount of overstepping of the garnet-in reaction.

EntraPT: an online platform for elastic geothermobarometry

During geodynamic processes minerals in rocks can recrystallize at depth in the Earth at high pressures and temperatures and new minerals form. During crystallization, growing minerals can trap other minerals within them as inclusions. After millions of years, some of these rocks are exposed on the Earth's surface and become available to researchers for direct observation. The inclusions deform during exhumation because of the removal of external pressure and temperature. By measuring the residual deformation in the inclusion and applying specific calculations it is possible to estimate the conditions (pressure and temperatures) of the recrystallization; the time when the inclusion was entrapped in its host. This in turn gives us a wealth of information to understand large scale geological processes such as the subduction and the collision of tectonic plates. In this Open Access paper, Mazzucchelli et al. present EntraPT, a new freely accessible online application that eases the interpretation of the residual deformation of inclusions and performs the calculations to estimate the entrapment conditions. EntraPT gives a standardized procedure to make it easier for researchers to check, store, and share their data.

Sodium nanoparticles in alkali halide minerals: Why is villiaumite red and halite blue?

Radiation damage in natural alkali halides causes alkali ions to form metallic nanoparticles that produce vivid bright colors. In this paper by Calas et al., the Lorentzian lineshape of the main absorption band characterizes a Surface Plasmon Resonance effect due to Na colloids, about 2.5-3 nm large. As the resonance wavelength depends on the refractive index of the mineral, the position of this band shifts between the two minerals. The resulting change of the transmission window causes a dramatic change of color from red villiaumite to blue halite. This approach may also help understand the purple color of irradiated fluorite, which also arises from the presence of metal colloids.

April 2021

Gamma-enhancement of reflected light images: A rapid, effective tool for assessment of compositional heterogeneity in pyrite

Zhu et al. developed a gamma correction method to display impurity-induced reflectance variation of pyrite at grain scale. This method enhances optical digital signal differences obtained under reflected light using an optical microscope. Incorporation of As in pyrite reduces the total number of electrons that could be excited to produce reflected light, resulting in decrease of reflectance. Gamma correction is a rapid, effective tool for the assessment of compositional heterogeneity in pyrite and other ore minerals with negligible bireflectance. Gamma-enhanced images may help constrain links between textures and compositions of minerals prior to subsequent quantitative analyses.

Thermal metamorphic history of Antarctic CV3 and CO3 chondrites inferred from the first- and second-order Raman peaks of polyaromatic organic carbon.

Yesiltas et al. determined the first- and second-order carbon Raman peak parameters in an effort to infer asteroidal thermal metamorphic history. The second-order carbon peaks are quantitatively studied for the first time, which contain information that the first-order peaks don't. Peak metamorphic temperatures of the investigated meteorites have been estimated based on the width of the D band as well as the calculated Raman spectral curvature.

A quantitative description of fission-track etching in apatite

Aslanian et al. propose a quantitative model of fission-track etching in apatite and use it for calculating the shapes and dimensions of etched tracks in different faces. Their model replaces an earlier model and invalidates certain restrictive dating practices based on it. This can have a profound effect on fission-track dating. Their measurements show that the length of confined tracks increases with etch time at a decreasing average rate that differs from the track-etch rate as well as from that of undamaged apatite. Step-etch experiments are shown to be useful for reducing or eliminating procedure-related effects from the track-length data and so for accessing more fundamental track properties resulting from formation and annealing.

Spectroscopic analysis of allophane and imogolite samples with variable Fe abundance for characterizing the poorly crystalline components on Mars

Poorly crystalline nanophase minerals that occur as weathering products on Mars contain critical information about the evolution of the early martian climate. Jeute et al. present compositional, structural, and spectroscopic data on a set of synthetic Mars analog nanophase aluminosilicates, including imogolite and allophanes with varying Al-Si ratios. They show that changes in the Al-Si ratio can be remotely detected, and that this measurement will help evaluate current models for the climatic evolution of Mars.

The relationship between 207Pb NMR chemical shift and the morphology and crystal structure for the apatites Pb5(AO4)3Cl, vanadinite (A = V), pyromorphite (A = P), and mimetite (A = As)

NMR spectroscopy of single crystals is the method of choice to precisely determine the full tensors of NMR interaction parameters via orientation-dependent measurements. Zeman et al. conducted a series of single-crystal NMR experiments on natural minerals, in particular on lead-bearing compounds, with the aim to extract the chemical shift tensor of 207Pb in high precision. Going beyond mere tensor determination, they noted that within the mineral family of vanadinite, pyromorphite, and mimetite, the NMR chemical shift may be related to some structural parameters such as unit-cell volume. From evaluating the number of the NMR resonances and their respective line widths, information about the mosaicity of these minerals could be derived.

Effect of cationic substitution on the pressure-induced phase transitions in calcium carbonate

Martirosyan et al. studied a member of the (Ca,Sr)CO3 solid solution series using in situ Raman spectroscopy at pressures up to 55 GPa and temperatures up to 1273 K. They observed crystallization of the CaCO3-II-type structure at 1273 K and 2 GPa. A new high-pressure modification, Sr-calcite-IIIc, was detected at 7-14 GPa. Thus, substitution of Ca2+ with Sr2+ promotes the formation of structures with larger cation coordination numbers such as aragonite, CaCO3-VII, and post-aragonite at lower P-T conditions compared to pure CaCO3.

Immiscible-melt inclusions in corundum megacrysts: Microanalyses and geological implications

Xu et al. identified two types of inclusions in the corundum megacrysts from Changle, China. Type I inclusion consists of a dark part (DP) and a bright part (BP), which were formed due to liquid immiscibility at ~1200 C; the former is composed of quartz, corundum, and amorphous substance-1, and the latter is composed of baddeleyite and amorphous substance-2. Type II inclusion is composed of zircon, quartz, and amorphous substance-3. The novel inclusions, together with other previously found mineral inclusions in Changle corundum, demonstrate that both the alkaline felsic melt and carbonatitic melt existed, and they derived from metasomatized mantle. The ages of zircon inclusions show that the corundum megacrysts crystallized from syenitic-type differentiated of earlier underplated basalts at the crust-mantle boundary and were brought up by later episodic basalt eruptions.

Water quantification in olivine and wadsleyite by Raman spectroscopy and study of errors and uncertainties

Martinek and Balfan-Casanova demonstrate that Raman spectroscopy allows the study of water quantification in different phases of fine polymineralic samples of complex composition, with a wide range of measurable water contents. The water contents of olivine and wadsleyite can be measured using this method with a simple sample preparation. Despite being around 25%, the uncertainties on water concentration are sufficiently low to infer the presence of dehydration that was induced by melting or other geologic processes.

High-pressure and high-temperature vibrational properties and anharmonicity of carbonate minerals up to 6 GPa and 500 C by Raman spectroscopy

Carbonate minerals play a dominant role in the deep carbon cycle. Farsang et al. measured the high-pressure and high-temperature vibrational properties of all aragonite-group and calcite-group carbonate minerals up to 6 GPa and 500 C by Raman spectroscopy in order to understand their anharmonicity under crustal and upper mantle conditions.

Vanadium-induced coloration in grossite (CaAl4O7) and hibonite (CaAl12O19)

Ardit et al. studied the unusual coloration in hibonite (purple) and grossite (light violet) crystals, caused by high concentrations of vanadium. Characterization of these specimens by means of single-crystal X-ray diffraction and absorption spectroscopy (aided by EMPA chemical analyses) provides information of both long- and short-range characteristics of their crystal structures.

Incorporation mechanism of tungsten in W-Fe-Cr-V-bearing rutile

Majzlan et al. examined the position of tungsten in the structures of two common minerals, rutile and hematite. It is of interest because these and related minerals serve as vehicles that enrich the sediments in the element tungsten. These sediments can be later melted and transformed into fertile magmas that form ore deposits. Tungsten was found to enter the structure of rutile, but in hematite, there exist nanoinclusions of iron-tungsten oxide intergrown with the host mineral. Rutile is a better vehicle for tungsten; hematite can serve as such under specific moderately reducing conditions.

Titanium diffusion profiles and melt inclusion chemistry and morphology in quartz from the Tshirege Member of the Bandelier Tuff

Boro et al. describe that melt Inclusions in quartz from the chemically zoned Tshirege Member of the Bandelier Tuff record the compositional evolution of a caldera-forming magma chamber. Variable melt inclusion faceting suggests timescales of millennia for crystal mush generation and crystal storage. Titanium concentration zoning in quartz suggests shorter timescales of decades to centuries for recharge and mobilization of said mush to produce an eruptible, zoned magma body.

Vasilseverginite, Cu9O4(AsO4)2(SO4)2, a new fumarolic mineral with a hybrid structure containing novel anion-centered tetrahedral structural units

Pekov et al. describe the unique structure of a new mineral, vasilseverginite Cu9O4(AsO4)2(SO4)2, which can be considered as a hybrid of the structures of popovite Cu5O2(AsO4)2 and dolerophanite Cu2O(SO4). The concept of hybridization of mineral species developed in this study may give new ideas for the preparation of novel structural architectures on the border of stability fields of chemically and structurally simpler compounds. The discovery of vasilseverginite indicates the existence of polynuclear oxocentered copper clusters in a gaseous phase, which may be a form of transport of Cu by volcanic gases.

Priscillagrewite-(Y), (Ca2Y)Zr2Al3O12: A new garnet of the bitikleite group from the Daba-Siwaqa area, the Hatrurim Complex, Jordan

Galuskina et al. report the discovery of a new garnet, priscillagreowte-(Y), belonging to the bitikleite group of the garnet supergroup. The mineral was found in apatite-bearing varicolored spurrite marble in the Daba-Siwaqa area, central Jordan. Priscillagrewite-(Y) is interpreted to be a relic of the high-temperature association formed in the progressive stage of the peak pyrometamorphism conditions, when the temperature could have reached close to 1000 C. The authors suggest that there is a reasonable chance of finding priscillagrewite-(Y) in ultrarefractory calcium-aluminum-rich inclusions from chondrites, then it can be the third garnet species originating in the solar nebula.

Stoefflerite, (Ca,Na)(Si,Al)4O8 in the hollandite structure: A new high-pressure polymorph of anorthite from martian meteorite NWA 856

Stoefflerite is the high-pressure polymorph of anorthite. Tschauner et al. show that in Earth's transition zone, it is a component of the important carrier of K, liebermannite. In shock-metamorphic environments, it marks high-pressures at temperatures above the Hugoniot but below formation of stable phases.

Recycled volatiles determine fertility of porphyry deposits in collisional settings

There have long been debates about whether copper and gold come from the mantle or crust. Previous hypotheses dominantly relied on the metal concentration, which is lower in both Earth's mantle and crust. The volatile elements Cl and S are the most important factors controlling the transport of ore metals in magma; therefore, their source and evolution can be a key to understanding the genesis of Cu-Au deposits. Xu et al. show that geochemical and Rb-Sr isotopic data on apatite from 12 porphyry systems across Iran, Tibet, and western China, can distinguish fertile magmas from barren magmatic suites and indicate the importance of volatiles, recycled from previous oceanic subduction, in collisional settings. This study is of importance to the Earth's volatile evolution and economic geologists.

Nitrogen diffusion in silicate melts under reducing conditions

Nitrogen and argon are the two most abundant elements in the current atmosphere, but the elemental composition of Earth's interior and surficial reservoirs may have evolved over time. In this study, Boulliung et al. provide the first constraints on nitrogen diffusion in natural-like silicate melt with implications on N/Ar fractionation during reducing magmatic events, such as Earth's magma ocean stage. The result implies a N/Ar fractionation during reducing magmatic events, such as Earth's magma ocean stage.

March 2021

An evolutionary system of mineralogy, Part III: Primary chondrule mineralogy (4566 to 4561 Ma)

The evolutionary system of mineralogy relies on varied physical and chemical attributes, including trace elements, isotopes, solid and fluid inclusions, and other information-rich characteristics. To understand processes of mineral formation and to place natural condensed phases in the deep-time context of planetary evolution, Hazen et al. add Part III of their evolutionary system that considers the formation of 43 different primary mineral phases in chondrules, which are igneous droplets that formed early in the history of the solar system, more than 4.56 billion years ago.

Raman Spectroscopy Study of Manganese Oxides - Layer Structures

Micro-Raman is a powerful tool for identification and characterization of biotic and abiotic Mn oxide phases from diverse natural settings (including on other planets) and thereby can provide new insights into the roles of these phases in our environment. Post et al. provide results from what they believe is the most comprehensive analysis of the Raman spectra for layer-structure Mn oxide phases to date, collected from a large number and variety of natural and synthetic samples, drawing from the Smithsonian Institution's extensive collection of Mn oxide specimens, and elsewhere. In many cases, the specimens have been characterized in detail using supplementary techniques. Additionally, Post et al. present representative spectra from different specimens, localities, and crystal orientations. A major goal of this study is to provide a comprehensive base of information (a spectral database is provided as supplementary data) that can be used for identifying the various Mn oxide mineral phases, with an emphasis on natural samples. Finally, Post et al. explore spectral trends for some specific phases that provide insights about composition, crystal structure, symmetry, and in some circumstances, Mn oxidation states.

Raman signatures of the distortion and stability of MgCO3 to 75 GPa

Zhao et al. report the Raman modes of natural magnesite, MgCO3 , up to 75 GPa at room temperature. They detected abnormal behavior in MgCO3, including the splitting of Raman peaks of T and v4 modes at approximately 30 and 50 GPa, respectively. The phenomena are assigned as MgCO3-Ib and MgCO3-Ic produced by the rotation of MgO6 octahedra. The distorted environment of the chemical bond would greatly improve the stability of magnesite over a large pressure and temperature range in relation to its melting or decomposition. Both experimental and theoretical evidence indicates that the diversity of distorted structural environments, including corner-sharing CO4 tetrahedra forming C3O9 three-membered rings. Compared to the low-pressure threefold coordinated carbonates (CO3)2-triangles in the structure, the tetrahedrally coordinated carbonates are expected to exhibit substantially different reactivity and different chemical properties in the liquid state. These crystallographic characteristics in carbonates may play an important role in deep carbon reservoirs and fluxes in the deep Earth. Furthermore, the bonding strength in MgCO3 changes through lattice distortion and structural transition, likely impacting the distribution of carbon and magnesium isotopes in the deep mantle.

Competitive adsorption geometries for the arsenate As(V) and phosphate P(V) oxyanions on magnetite surfaces: Experiments and theory

Adsorption of arsenate and phosphate on magnetite was studied by Liang et al. using in situ ATR and 2D-COS. Monodentate mononuclear and bidentate binuclear complexes dominate in phosphate adsorption. Arsenate forms bidentate binuclear complexes with fewer outer-sphere species. Arsenate displays a higher competitive ability than phosphate. The competitive ability is related to adsorption geometry and the heterogeneity of surface active sites.

Probing transformation path from aluminum (oxy)hydroxides (boehmite, bayerite, and gibbsite) to metastable alumina: A view from high-resolution 27Al MAS NMR

Kim and Lee investigated the dehydration paths to metastable alumina from various aluminum (oxy)hydroxide precursors (i.e., boehmite, bayerite, and gibbsite) in the low-temperature range (~300 °C) using high-resolution 27Al NMR. The results confirm that the phase transformation paths depend on the type of precursor minerals. The precursor-dependent structural evolution in the low-temperature range helps to understand the geological processes involving metastable phases and their dehydration in the Earth's surface environments.

Crystal structure of K-cymrite and kokchetavite from single-crystal X-ray diffraction

Romanenko et al. report their investigation of K-cymrite (KAlSi3O8+H2O) and kokchetavite (KAlSi3O8, IMA-2004-011), which were earlier identified as mineral inclusions in ultra-high pressure metamorphic crustal rocks. However, their crystal structures previously were only guessed on the basis of powder X-ray diffraction patterns. Romanenko et al. present the crystal structures of K-cymrite and kokchetavite by single-crystal X-ray diffraction. For kokchetavite a new space group and unit cell were identified. In addition, the spectroscopic and Thermogravitational data provide important information for the identification and interpretation of these phases in mineral inclusions.

Fluid source and metal precipitation mechanism of sediment-hosted Chang'an orogenic gold deposit, SW China: constraints from sulfide texture, trace element, S, Pb and He-Ar isotopes, and calcite C-O isotopes

Yang et al. highlight that ore metals in sediment-hosted disseminated orogenic gold deposits can be sourced from both deep fluids and local wallrock and that fluid-rock interaction behaved as a key control on ore precipitation.

Iron isotope fractionation in reduced hydrothermal gold deposits: A case study from the Wulong gold deposit, Liaodong Peninsula, East China

Pyrite and pyrrhotite are the major Fe-bearing minerals of the quartz-sulfide veins in the Wulong reduced gold deposit. Iron isotope fractionation modeling by Zheng et al. shows that under relatively low oxygen fugacity conditions, pyrrhotite with light δ56Fe crystallized first from the initial ore-forming fluids, resulting in ore-forming fluids with elevated δ56Fe values. Due to an increase of oxygen fugacity, pyrite with heavy δ56Fe started to precipitate later. The iron isotopic compositions provide a new perspective for the initial redox conditions and evolution of the Wulong gold deposit, which are important to trace the source of ore-forming materials and further exploration.

Tungsten mineralization during the evolution of a magmatic-hydrothermal system: mineralogical evidence from the Xihuashan rare-metal granite in South China

Micas can record the magmatic-hydrothermal evolution of tungsten granite. Li et al. demonstrate that the geochemical variations and textures of zoned micas indicate magmatic fluids, rather than external fluids, were involved in greisenization. The siderite present is related to a Fe, Mn, and CO2-rich fluid under reducing conditions. The greisenization process plays a critical role in tungsten mineralization. The reducing environment and the mixture of a W-rich solution and a Fe-, Mn-rich external fluid facilitated tungsten mineralization.

Crystallization and melt extraction of a garnet-bearing charnockite from South China: Constraints from petrography, geochemistry, mineral thermometer and rhyolite-MELTS modeling

Zhang et al. investigated the Yunlu garnet-bearing charnockite as an example of the very few peraluminous magmatic charnockites around the world. The magmatic pressure-temperature-melt H2O content and associated crystallization of the charnockite was constrained quantitatively by the integration of petrography, geochemistry, fluid inclusion investigations, mineral thermo-barometry, and thermodynamic modeling. The Yunlu magma solidified at "wet" (H2O-saturated) and "cold" (~630 °C) conditions, which is different from metaluminous charnockites that solidified at "dry" (H2O-unsaturated) and "hot" (>800 °C) conditions. This study indicates that the peraluminous charnockites may experience a distinct crystallization process compared to metaluminous charnockites. Meanwhile, the temperature discrepancies between mineral thermometer results and the magmatic solidus were interpreted by the "melt extraction" model. The study sheds new light on the interpretations of granite thermometry.

Reducing epistemic and model uncertainty in ionic inter-diffusion chronology: A 3D observation and dynamic modeling approach using olivine from Piton de la Fournaise, La Réunion

Modeling of Mg-Fe zonation in olivine crystals from mafic ejecta and deposits from volcanic eruptions is an often-used tool for calculating magmatic timescales, but sub-perfect diffusion profiles are often rejected. This is a bias that Couperthwaite et al. suggest should, and can, be addressed as a community. The results in this open access paper by Couperthwaite et al. unlock the majority of these profiles and, in doing so, reveal a richer view of magmatic processes than previously could be seen.


Memorial of James J. Papike .

February 2021

Effect of water on carbonate-silicate liquid immiscibility in the system KAlSi3O8-CaMgSi2O6-NaAlSi2O6-CaMg(CO3)2 at 6 GPa: Implications for diamond-forming melts

Partial melting of recycled sediments (metapelites) subducted to mantle depths is essentially controlled by the phengite/K-feldspar + clinopyroxene + carbonate assemblage. Here Shatskiy et al. show that at 6 GPa and nominally dry conditions incipient melting of this assembly occurs at 1050-1100 °C and yields a K-dolomitic melt. By contrast, in the presence of H2O two immiscible phonolite-like and K-dolomitic melts appear. The established melts resemble the carbonatitic and silicic HDFs established in diamonds worldwide.

Jasonsmithite, a new phosphate mineral with a complex microporous framework, from the Foote mine, North Carolina, U.S.A.

Kampf et al. describe a new mineral, jasonsmithite, from the Foote mine in North Carolina. It has a complex framework structure containing large channels. Its 70% void space makes it one of the most porous mineral structures known and suggests that it may have technological applications.

Titanium in calcium amphibole: Behavior and thermometry

Thermometry of high-grade metamorphic rocks is difficult due to fast cationic diffusion during slow cooling. Liao et al. propose to use Ti content of amphibole (Ti-Amp) as thermometric tools for igneous and high-grade metamorphic rocks.

Phase relationships in the system ZnS-CuInS2: Insights from a nanoscale study of indium-bearing sphalerite

Xu et al. conducted micrometer- to nano-scale characterization on sphalerite containing 17-49 mol% CuInS2 that occurs in a pyrrhotite-dominant matrix. Results provide new insights into phase relationships in the system ZnS-CuInS2. Metal ordering is modeled as mixed sites in a sphalerite-type structure, [(Cu,In,Zn)3(Zn0.5Fe0.5)]4S4, with P4-3m symmetry. This modification is distinct from the reported cubic-tetragonal phase transition. Different degrees of fluid percolation explain the textures shown.

Major and trace element composition of olivine from magnesian skarns and silicate marbles

Nekrylov et al. analyzed olivine from magnesian skarns and silicate marbles, which could be easily distinguished from olivine formed in other processes by unusually low contents of Ni, Co, and Cr and high content of B. These features are linked to the composition of its formation environment -- dolomites and their contacts with intermediate-acidic magmatic rocks.

Decompression experiments for sulfur-bearing hydrous rhyolite magma: Redox evolution during magma decompression

Understanding redox evolution during magma ascent is important for exploring oxygen fugacity of magma and mantle and for modeling and predicting the chemical species of volcanic gases emitted to the surface. Okumura et al. experimentally investigated redox evolution during magma ascent to the surface. Results showed that sulfur-bearing hydrous rhyolite magma is slightly reduced during the ascent with decompression rates corresponding to explosive and effusive volcanic eruptions.

On the crystal chemistry of sulfur-rich lazurite

Sapozhnikov et al. report that dark-blue lazurite from Malo-Bystrinskoe deposit (Russia) contains high sulfur. In addition to SO42-, the S3- radical-ion occupies beta-cages within the structure. The idealized formula of the mineral is Na7Ca[Al6Si6O24](SO4)2–(S3)–·H2O. The structure of studied lazurite contains incommensurate modulations.

Experimental evaluation of a new H2O-independent thermometer based on olivine-melt Ni partitioning at crustal pressure

Pu et al. demonstrate that the partitioning of Ni between olivine and basaltic melt (DNi) is independent of dissolved water (up to 4.3 wt%), unlike the partitioning of Mg (DMg). Olivine-melt equilibrium experiments were performed at 1 bar (anhydrous) and 0.5 GPa (anhydrous and hydrous). The results confirm that an olivine-melt thermometer based on DNi can be applied to hydrous natural basalts from subduction zones without any correction for H2O content.

Contrasting compositions between phenocrystic and xenocrystic olivines in the Cenozoic basalts from central Mongolia: Constraints on source lithology and regional uplift

Two Cenozoic prominent features are spatio-temporally associated in central Mongolia, i.e., the continental basalts and regional uplift, but their causes and relationship remain unclear. To solve these issues, Zhang et al. conducted analyses of major and trace element compositions for olivine phenocrysts and xenocrysts in the Cenozoic basalts. They conclude that mass deficit in the lithosphere could have caused isostatic uplift of central Mongolia in the Cenozoic.

The composition of garnet in granite and pegmatite from the Gangdese orogen in southeastern Tibet: constraints on pegmatite petrogenesis

Yu et al. report two stages of garnet growth in granite and pegmatite from the Gandese orogen in Tibet. The first generation of garnet (Grt-I) grew in the pegmatite from early evolved magmatic-hydrothermal fluids, and the second generation (Grt-II) crystallized after dissolution of the preexisting pegmatite garnet in the presence of a granitic magma. Both granite and pegmatite originate from partial melting of the same juvenile crust in the Mesozoic continental arc prior to the Cenozoic continental collision forming the Himalayan orogen.

Formation of metasomatic tourmalinites in reduced schists during the Black Hills Orogeny, South Dakota

Boron is an important component of granites, pegmatites, and metamorphic rocks in many geologic settings, including collisional orogens. Boron is a highly fluid-mobile element and thus it exchanges easily between these geologic materials. Nabelek describes mineral chemistry of tourmalinite in the Black Hills orogen and proposes ion-exchange reactions that led to its formation. The tourmalinite places constraints on the chemistry of the fluid that caused replacement of a schist by tourmaline and graphite.

New insights into the crystal chemistry of sauconite (Zn-smectite) from the Skorpion zinc deposit (Namibia) via a multi-methodological approach

Schingaro et al. conducted a multi-methodical characterization of a sauconite (Zn-bearing trioctahedral smectite) specimen from the Skorpion ore deposit (Namibia) using X-ray diffraction, cation exchange capacity analysis, differential thermal analysis, thermogravimetry, infrared spectroscopy and transmission electron microscopy. The results have implications not only for economic geology/recovery of critical metals but also, more generally, in the field of environmental sciences.

The new mineral crowningshieldite: A high-temperature NiS polymorph found in a type IIa diamond from the Letseng mine, Lesotho

Smith et al. describes crowningshieldite, a new nickel monosulfide mineral that is the high-temperature polymorph of millerite. It was discovered within a fine-grained multi-phase inclusion in a gem quality diamond from the Letseng Mine, Lesotho. The mineral name recognizes G.R. Crowningshield, who was a central figure in research at the Gemological Institute of America for over fifty years.

Elucidating the natural-synthetic mismatch of Pb2+Te4+O3: The redefinition of fairbankite

The determination of fairbankite's structure by Missen et al. has solved the long-standing uncertainty around the natural Pb2+Te4+O3 formula. Additionally, the fairbankite structure is unique and contains a trimeric tellurite anion new to both synthetic and natural structures.

Are the Thermodynamic Properties of Natural and Synthetic Mg2SiO4-Fe2SiO4 Olivines the Same?

It is unclear whether the thermodynamic properties of some rock-forming minerals and their synthetic analogues are quantitatively the same. Olivine is an important substitutional solid-solution consisting of the two end-members forsterite, Mg2SiO4, and fayalite, Fe2SiO4. Gieger et al. undertook first low-temperature CP measurements on two natural olivines between 2 and 300 K; nearly end-member fayalite and a forsterite-rich crystal Fo0.904Fa0.096. They show that the CP behavior of the natural and synthetic crystals is similar.

January 2021

P-V-T equation of state of hydrous phase A up to 10.5 GPa

Yang et al. report the precise equation of state of Mg7Si2O8(OH)6 phase A by using the pressure-volume-temperature (P-V-T) data of synthetic pure phase A up to ~10.5 GPa and ~900 K by in situ X-ray diffraction study at the Photon Factory-Advanced Ring (PF-AR) in Japan. Because phase A is one of the important hydrous phases and the water carriers into the Earth's mantle, the precise equation of state of phase A is very important and valuable to investigate the precise thermoelastic properties (e.g., temperature dependence of bulk modulus, thermal expansion, etc.), for discussing the state of the Earth interiors.

Elastic properties and structures of pyrope glass under high pressures

Hisano et al. indicate that the structural densification of pyrope glass is closely correlated to the change in elastic properties. Compared to other silicate glasses, the variation of chemical composition has a significant effect on the properties and their pressure dependences. Especially, the influence of the magnesium cation on the aluminosilicate glass is larger than that of other cations. Considering the melting of Earth's mantle, the magnesium component is abundant in the melt at the deeper region. Therefore, the knowledge of magnesium-bearing aluminosilicate glass can help us to understand the behavior of deep magmas.

Effects of pH and Ca exchange on the structure and redox state of synthetic Na-birnessite

Elmi et al. investigate the structures of a series of synthetic Ca-birnessite analogs prepared by cation-exchange with synthetic Na-birnessite at pH concentrations from 2 to 7.5. The exchange experiments described in this paper, performed over a range of pH values (3 to 7.5) and Ca2+ concentrations that are relevant to natural environments, yielded Ca-birnessite products that all have the same triclinic structure with nearly identical unit-cell parameters. Their results reveal that the transformation of Na- to Ca-birnessite is more than a simple replacement of Na by Ca.

A systematic assessment of the diamond trap method for measuring fluid compositions in high-pressure experiments

Rustioni et al. provide tests of the diamond trap method to study fluid compositions in experimental charges in order to evaluate the reliability of this method. They also describe several experimental improvements of the method that will be useful for further studies. This paper will be quite interesting for a wide audience of experimental petrologists and geochemists.

Origin, properties, and structure of breyite: the second most abundant mineral inclusion in super-deep diamonds

Breyite is the second most abundant mineral inclusion after ferropericlase in diamonds of super-deep origin. The occurrence of breyite is widely used as a strong indication of a lower mantle (>670 km depth) or at least lower transition zone (>520 km depth) origin of both the host diamond and the inclusion suite. Brenker et al. demonstrate through different formation options that the finding of breyite alone in a diamond is not a reliable indicator for the formation depth, accompanying paragenetic phases such as ferropericlase and/or MgSiO3 are needed. As a large set of papers were published in recent years based on findings on breyite in super-deep diamonds and used to determine the chemical composition of the deep Earth and to decipher deep Earth processes, the new work presented here question some of the conclusions drawn if based on the sole finding of breyite.

Why Tolbachik diamonds cannot be natural

Litasov et al. provide comprehensive evidence that type Ib cuboctahedral diamonds and microcrystalline diamonds from Kamchatka volcanic rocks and alluvial placers cannot be natural and undoubtedly represent synthetic materials, which appear in the natural rocks by anthropogenic contamination. The major arguments provided in favor of the natural origin of those diamonds can be easily disproved. The cavitation model proposed for the origin of Tolbachik diamonds is also unreliable, since cavitation can cause the formation of nanosized diamonds only.

Deciphering the enigmatic origin of Guyana's diamonds

Bassoo et al. present a comprehensive study of Guyana's diamonds, using cathodoluminescence, UV fluorescence, Raman spectroscopy, FTIR and δ13C analyses. Inclusion suites and δ13C values indicate diamonds are derived from peridotitic lithospheric mantle at ~1120 °C. Dissolution textures suggest diamonds were hosted in a H2O rich magma. Detrital zircon geochronology and accessory mineral chemistry suggest most diamonds are likely sourced from middle to late Trans-Amazonian rocks which have since weathered into the ~1.98 Ga Roraima Supergroup conglomerates and volcaniclastics. However, provenance of a population of pristine, non-abraded diamonds is enigmatic, because cathodoluminescence and dissolution textures suggest a lesser metamorphic overprint and different source melt than abraded diamonds. This study characterizes and places Guyana's diamonds into the wider context of Guiana Shield and West African Craton evolution.

Precipitation of low-temperature disordered dolomite induced by extracellular polymeric substances of methanogenic Archaea Methanosarcina barkeri: Implications for sedimentary dolomite formation

Dolomite [CaMg(CO3)2] is a common mineral in the rock record. However, the rarity of modern dolomite and the notorious difficulty in synthesizing dolomite abiotically under Earth-surface conditions result in the long-standing enigma in sedimentary geology, known as the "dolomite problem." Anaerobic microorganisms, such as sulfate-reducing bacteria (SRB) and methanogens have been recognized to mediate dolomite precipitation. However, the mechanistic link between anaerobic bacteria and dolomite formation is still not revealed. In the present study, Zhang et al. report precipitation of disordered dolomite in Ca-Mg carbonate solutions containing purified inactive biomass of a natural consortium of the methanogen Methanosarcina barkeri. Carbonate precipitation experiments in solutions containing capsular extracellular polymeric substances (EPS) extracted from the biomass showed that EPS was the active component that triggered dolomite crystallization. The present study clearly demonstrates the catalytic role of EPS in dolomite formation. It also proves the possible involvement of fermentative bacteria in dolomite formation, which to their best knowledge has not been reported previously. This study provides significant insight into the formation mechanism of microbial-induced dolomite with heavy δ13Cvalues during diagenesis.

Atomic-scale Characterization of Commensurate and Incommensurate Vacancy Superstructures in Natural Pyrrhotites

Jin et al. report that (1) Superstructures formed by Fe vacancy ordering in natural pyrrhotites are revealed using atomic-resolution scanning transmission electron microscopy; (2) Picometer-scale shifts of individual Fe atomic columns are consistent with a model for the structure of 4C pyrrhotite derived using X-ray diffraction; and (3), 5C-like unequally-sized nano-regions joining at anti-phase-like boundaries lead to the incommensurability observed in 4.91 ± 0.02 C pyrrhotite.

Three-dimensional and microstructural fingerprinting of gold nanoparticles at fluid-mineral interfaces

Zhou et al. present the first 3D FIB/SEM tomography images and TEM microstructures of gold nanoparticles in an ore mineral, in this case from the world-class Beiya Au deposit, China. The results unravel how intrinsic and extrinsic factors drove the formation of Au nanoparticles at fluid-mineral interfaces. Non-lattice-bound gold in ore minerals is commonly interpreted to be remobilized from exsolution of earlier formed minerals. However, their work offers an alternative explanation that this gold can also be introduced by hydrothermal fluids during later mineralization stages. They also suggest that this gold in many ore minerals may reflect nano-scale permeability between nanopores that developed during dissolution-reprecipitation reactions involving earlier minerals.

Seaborgite, LiNa6K2(UO2)(SO4)5(SO3OH)(H2O), the first uranyl mineral containing lithium

Kampf et al. report on the first uranyl mineral containing essential lithium, which plays an important role in this structure. The structure is unique among both natural and synthetic phases.

Reheating and magma mixing recorded by zircon and quartz from high-silica rhyolite in the Coqen region, southern Tibet

Chen et al. report on: (1) High-silica rhyolites (HSRs) erupted at ~74 Ma from the Coqen region, southern Lhasa terrane; (2) Trace element characteristics of zircons from Nuocang HSRs indicate multi-stage magmatic processes involving magma mixing; (3) Ti geothermometers of zircon and quartz suggest reheating in the magma reservoir; and (4) Zircon and quartz with distinct internal textures provide critical insights into the formation of HSR.

Crystal-chemistry and thermal behavior of Fe-carpholite: a case of study from the Pollino Massif (southern Italy)

Mesto et al. report on: (1) new insights into the crystal chemistry of Fe-Mg carpholites; (2) new data on the thermal evolution of the species, obtained by TG-DTA as well as in situ high-temperature powder diffraction analysis; (3) new Raman data; and (4) first study on the thermal behavior of the most common carpholite composition occurring in HP/LT metasediments in collisional contexts.

New insights into the control of visible gold fineness and deposition: A case study of the Sanshandao gold deposit (Jiaodong, China)

Peng et al. report on: (1) Mineralogy and chemistry research on the gold-producing zone from –4000 m depth to surface; (2) Two generations of gold grains with different fineness show different gold-precipitating processes; (3) Three types of pyrite record the temporal and spatial evolution of ore-forming fluid; and (4) Integrated gold fineness, pyrite trace element, and sulfur isotope reveal a comprehensive genetic model.

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New Mineral Names

American Mineralogist Volume 105

December 2020

The Incompressibility of Atoms at High Pressures

How compressible is an atom? Here, Gibbs et al. report the impact of pressure on the sizes of the bonded radii of a diverse range of atoms by calculating their bonded interactions and electron density distributions as a function of pressure. The results provide novel insights about the compressibility of atoms and changes in their chemical bonding at high pressure.

Phase transitions in ε-FeOOH at high pressure and ambient temperature

Constraining the accommodation, distribution, and circulation of hydrogen in the Earth's interior is vital to our understanding of the deep Earth due to the significant influence of hydrogen on the material and rheological properties of minerals. Recently, a great deal of attention has been paid to the high-pressure polymorphs of FeOOH. These structures potentially form a hydrogen-bearing solid solution with AlOOH and phase H (MgSiO4H2) that may transport water (OH-) deep into the lower mantle. In this study, Thompson et al. re-evaluated the high-pressure evolution of FeOOH up to ~75 GPa using a combination of synchrotron X-ray diffraction, Fourier transform infrared spectroscopy, and optical absorption spectroscopy. Based on these measurements, the authors report on the phase transitions which occur in ε-FeOOH at ambient temperature and pressures up to 70 GPa.

Thermal state of the upper mantle and the origin of the Cambrian-Ordovician ophiolite pulse: Constraints from ultramafic dikes of the Hayachine-Miyamori Ophiolite

Kimura et al. demonstrated that a global thermal state of the upper mantle in the Cambrian-Ordovician time, which corresponds to the period of high production of ophiolites, called a ophiolite pulse, was similar to the present one by quantitatively estimating the mantle potential temperature from an arc ophiolite. They also showed that a slab breakoff tectonics is responsible for the magma generation and further argued that such tectonics frequently took place during the Cambrian-Ordovician time resulting in the ophiolite pulse.

Quadrivalent praseodymium in planetary materials

Praseodymium is a rare earth element (REE) that predominantly occurs in the trivalent oxidation state like most other REEs. However, in some cases it can occur as tetravalent Pr, leading to a subtle anomaly in REE patterns. It is stabilized by high oxygen fugacities and low temperatures. Anenburg et al. find Pr4+ in tetravalent Ce minerals (cerianite and stetindite), and they discuss the possibility of finding a pure tetravalent Pr mineral.

Quantitative microscale Fe redox imaging by multiple energy X-ray fluorescence mapping at the Fe K pre-edge peak

Ellison et al. describe a synchrotron-based X-ray spectroscopic approach that allows microscale quantitative mapping of Fe valence state by extending the Fe XANES pre-edge technique. An area of interest is mapped at 10 excitation energies, allowing reconstruction, baseline subtraction, and integration of the pre-edge feature to determine Fe(III)/ΣFe at every pixel. By combining the Fe redox mapping approach with hyperspectral Raman mapping, the Fe oxidation state distributions of the major mineral phases can be revealed. They provide an example of application of this method to a partially serpentinized peridotite from the Samail Ophiolite (Oman) that displays a variety of secondary mineral phases with a range of Fe redox states. Quantification of Fe(II) and Fe(III) in these phases allows for determination of the sequence of reactions and Fe transformations that produced the observed alteration assemblage.

Quantification of excess 231Pa in late Quaternary igneous baddeleyite

Initial excess protactinium (231Pa) is a frequently suspected source of discordance in baddeleyite geochronology, which limits accurate U/Pb dating, but such excesses have never been directly demonstrated. In this study, Sun et al. quantified Pa incorporation in late Holocene baddeleyite from Vesuvius (Campanian Volcanic Province, Central Italy) and Laacher See (East Eifel Volcanic Field, Western Germany) by U-Th-Pa measurements using a large-geometry ion microprobe. Measured 231Pa excesses indicate preferred Pa baddeleyite-melt partitioning relative to U equivalent to, or possibly even more severe, than for zircon. These excesses require significant corrections for 207Pb/206Pb and 207Pb/235U ages. Comparison with partitioning of other trace elements suggests that Pa is predominantly present as Pa4+, even in comparatively oxidized melts. The crystallization age of baddeleyite crystals obtained here for the first time for Somma-Vesuvius syenitic ejecta, which represent parts of the solidified magma chamber margin, is also crucial for constraining magma chamber evolution before major eruptions.

Magma oxygen fugacity of mafic-ultramafic intrusions in convergent margin settings: insights for the role of magma oxidation states on magmatic Ni-Cu sulfide mineralization

Cao et al. investigated the magma fO2 of the mafic-ultramafic intrusions in typical convergent margin settings to examine the correlation of magma fO2 and Ni-Cu sulfide mineralization. They calculated the magma and mantle fO2 for a number of Ni-Cu-sulfide-bearing mafic-ultramafic intrusions in the central Asian orogenic belt (CAOB), based on the olivine-spinel oxygen barometer and the newly established method of modeling the partition coefficient of V between olivine and melt. The results show that the high magma fO2 for the intrusions in the CAOB is likely related to the fractionation of hydrous magmas derived from slightly oxidized metasomatized mantle. This may be a fundamental feature to distinguish the magmas produced in the subduction zones from those in the mid-ocean ridges. They also discussed the relationship between the redox conditions of basaltic magmas and formations of Ni-Cu sulfide deposits in various settings, and proposed that the high fO2 of mantle-derived magmas may be able to dissolve enough mantle-derived sulfur to form economic Ni-Cu sulfide deposits in convergent margin settings.

Investigation of the crystal structure of low water content hydrous olivine to 29.9 GPa: a high-pressure single-crystal X-ray diffraction study

Xu et al. conducted high-pressure single-crystal synchrotron X-ray diffraction experiments on a synthetic low water content hydrous Mg-rich olivine (Fo95; 1538 ppm water) to determine the effects of minor hydration associated with the Si sites on the structural evolution under compression. Previous studies on hydrous olivine indicated that the incorporation of water affects the structure of olivine at high pressure; however, the water contents of the olivine are much higher than expected in natural mantle olivine. Field observation and petrological experiments suggest that the water storage capability of olivine in peridotite in the upper mantle is lower than 2000 ppm. In this study, the authors investigated the structural evolution of a hydrous olivine whose water content is more accessible in natural olivine. The results indicate that low water content (less than 2000 ppm) has negligible effects on the EoS of olivine, though the incorporation of water softens the Si-O1 and Si-O2 bonds.

Ferric-ferrous iron ratios of experimental majoritic garnet and clinopyroxene as a function of oxygen fugacity

Rzehak et al. compared the Fe3+/ΣFe contents of experimental majorites and clinopyroxenes at 10 GPa with two analytical methods: EPMA flank and TEM-EELS analysis on the same samples. They used a FeO content of 8 wt% that is considered to be the natural bulk FeO content of the Earth's upper mantle. Previous studies have used much higher FeO contents (~25 wt%) to facilitate Fe3+/ΣFe measurements. Although results from the flank method are systematically lower than the EELS measurements, Fe3+/ΣFe obtained with both methods agree well within 2θ errors. The authors used their experimental results to compare four different geobarometers by Collerson et al. (2010), Wijbrans et al. (2016), Beyer and Frost (2017), and Tao et al. (2018) and found that all of them are recommendable for different settings.

The origin of Ti-oxide minerals below and within the eastern Athabasca Basin, Canada

Titanium oxides along a major fault within and below the eastern Athabasca Basin record a protracted geological history of the area. Early rutile records regional metamorphism of the basement and reducing hydrothermal activity prior to the deposition of the Athabasca sandstones. Anatase in the sandstones and basement formed from oxidizing, acidic basinal fluids. Anatase in the basement records the incursion of basin fluids into the basement, which marks the onset of hydrothermal activity related to the formation of unconformity-type uranium deposits. This study by Adlakha et al. confirms that Ti-oxides are useful in unraveling the geological history of an area that underwent prolonged hydrothermal alteration.

Partition behavior of platinum-group elements during the segregation of arsenide melts from sulfide magma

Pina et al. determined experimentally the partition behavior of platinum group elements (PGE) between arsenide and sulfide melt when an arsenide melt segregates by immiscibility from a sulfide melt after As oversaturation. The results show that PGE are strongly compatible into arsenide melt supporting empirical observations in several Ni-Cu-PGE sulfide deposits where PGE occur preferentially concentrated in the most As rich zones. The implications for exploration are considerable: the formation of arsenide melts in PGE-bearing natural sulfide systems can lead to the formation of As-PGE-rich orebodies associated with S-rich and As-poor mineralized zones.

Vapor-bubble growth in olivine-hosted melt inclusions

Olivine-hosted melt inclusions (MI) are the best tool for studying magmatic volatiles. Recent studies have shown that vapor bubbles, commonly found in MI, contain up to ~90% of entrapped CO2 contents. This revelation has put much of the last 30 years of MI work into question because many studies required knowledge of entrapped CO2 contents but did not account for bubble growth. CO2-reconstruction methods exist, but their accuracy has not been tested. Rasmussen et al. describe bubble growth, evaluate existing corrections, and develop improved experimental and computational (MIMiC program) methods.

November 2020

Parageneses of TiB2 in corundum xenoliths from Mt Carmel, Israel: Siderophile behavior of Boron under reducing conditions

Natural titanium diboride (TiB2) occurs rarely on Earth, but is widespread in mantle-derived aggregates of corundum found in the volcanic ash of small Cretaceous volcanoes on Mt Carmel, Israel. Petrographic studies by Griffin et al. show that the highly reduced igneous melts trapped in these aggregates during their growth later separated into immiscible metallic (Fe-Ti-Si) and silicate (Ca-Mg-Al-Si-O) melts. TiB2 crystallized mainly from the metallic melts, and less commonly from the silicate melts. This indicates that boron, though usually a lithophile element, becomes a strongly siderophile element under reducing conditions, like those expected in the deep mantle. Metallic melts in the mantle may be the major reservoir for boron below the crust.

Crystal structure and Raman spectroscopic studies of OH stretching vibrations in Zn-rich fluor-elbaite

Pieczka et al. present results of their study of OH stretching vibrations in the Raman spectrum of Zn-rich fluor-elbaite. Deconvolution of the bands provides new insights into crystal chemical and structural details of the studied crystals, even at the nano-scale, and direct evaluation of Li and OH (Li2O and H2O) concentrations, two components not measured in microprobe analysis and usually only calculated.

Crystal structure of Ag-exchanged levyne intergrown with erionite: single crystal X-ray diffraction and molecular dynamics simulations

Cametti and Churakov report the crystal structure characterization of a Ag-exchanged levyne intergrown with erionite. Ag-exchanged zeolites are particularly interesting due to their improved catalytic, photocatalytic, adsorption, and luminescent properties. To resolve the extreme disorder of Ag ions within the zeolitc pores we combined experimental (X-ray diffraction) and theoretical approachs (Molecular dynamics).

Br diffusion in phonolitic melts: Comparison with fluorine and chlorine diffusion

Balcone-Boissard et al. investigated the diffusion of Br in phonolitic melts with differing Na/K ratios, which had previously been used to study Cl and F diffusion, and compared their results to published studies of Br diffusion in magmatic melts. Br diffusion may be affected by the Na/K ratio of phonolitic melts, as previously seen for Cl, but not for F, diffusion. Similarly to noble gases, halogen diffusivity at a given temperature in the phonolitic melts appears related to the ionic porosity of the silicate structure, and Br diffusion appears to be at least partially decoupled from melt viscosity.

Crystal-chemistry and microfeatures in gadolinite imprinted by pegmatite formation and alteration evolution

Alteration patterns observed by Tomašić et al. in gadolinite from pegmatites of southern Norway point to late magmatic and metasomatic changes of local mineral chemistry. The general alteration path suggests a shift to a more Ca-rich mineral chemistry within the gadolinite group. The changes induced by alteration mechanisms are related to the metamictization-inherited structural properties of gadolinite, thus promoting gadolinite as one indicator of late magmatic processes in these pegmatite systems.

A new occurrence of corundum in eucrite and its significance

Corundum is an important indicator of Al-rich and Si-poor regions and/or lithologies. It is rarely observed in samples from extraterrestrial differentiated celestial bodies, although shock-induced corundum has recently reported by Li et al. In this study, Li et al. report the presence of non-shock-induced corundum in a eucrite. The petrographic texture of corundum and its associated pyroxenes and their compositions indicate that corundum has an indigenous origin and was captured during the ascent of a basaltic melt. Its presence suggests a hidden Al-rich and Si-poor region or lithology in the interior of Vesta and further implies that this asteroid’s internal evolution could be much more complicated than previously thought.

Zircon survival in shallow asthenosphere and deep lithosphere

Borisova et al. experimentally investigated the behavior of zircon in mafic/ultramafic melts to predict zircon survival in the terrestrial lithosphere and shallow asthenosphere. They performed high-temperature experiments on zircon dissolution in natural mid-ocean ridge basaltic and synthetic haplobasaltic melts at temperatures of 1250 to 1300 degrees Celsius and pressures from 0.1 MPa to 0.7 GPa coupled with electron probe microanalyses of the experimental products. For the first time, the zirconium diffusion coefficient (2.87 x 10-12 m2/s) at 1300{degree sign}C and 0.5 GPa pressure in a natural tholeiitic basaltic melt has been estimated. The experimental data raise questions about the origin of zircons in mafic and ultramafic rocks, in particular, in shallow oceanic asthenosphere and deep lithosphere, as well as the meaning of the zircon-based ages estimated from these minerals. Large zircon megacrysts in kimberlites, peridotites, alkali basalts, and other magmas suggest the fast transport and short interaction durations between zircon and melt.

Reconsidering initial Pb in titanite in the context of in situ dating

The mineral titanite has become a popular U-Pb geochronometer, but determination of dates requires a correction for non-radiogenic (initial) Pb. This contribution by Bonamici and Blum reviews different methods for handling initial Pb in titanite, specifically when U-Pb data are collected by in situ mass spectrometry techniques that sample small domains within grains. Their results show that initial Pb is heterogeneous both between and within the studied titanite grains. In addition, analysis of U-Pb data patterns suggests that some titanite grains have lost Pb by diffusion during a high-temperature metamorphic event. They demonstrate that even when U-Pb data are scattered and precise ages cannot be determined with in situ techniques, analysis of U-Pb data patterns can provide information about the processes that caused the scatter.

Solubility of Na2SO4 in silica-saturated solutions: Implications for REE mineralization

The solubility of Na2SO4 is traditionally considered to display retrograde behavior in aqueous solutions. Chen et al. show experimentally that the solubility of Na2SO4 changes from retrograde to prograde in the presence of silica. The authors show that sulfate-rich fluids are capable of transporting large quantities of REE’s and that mass balance calculations demonstrate that hidden, deep-seated magma chambers are not necessary for the Maoniuping carbonatite-related rare earth element (REE) deposit (SW China).

Vanadium micro-XANES determination of oxygen fugacity in olivine-hosted glass inclusion and groundmass glasses of martian primitive shergottite Yamato 980459

Nakada et al. focuses on the oxidation states of redox-sensitive elements in an olivine-hosted glass inclusion and groundmass glasses in a Martian meteorite; these glass phases reflect the earliest and latest stages of formation of basaltic rocks on Mars. The study shows that the oxygen fugacity (fO2) was almost equal to the Iron-Wüstite (IW) buffer (IW-0.07{plus minus}0.32) for the glass inclusion, whereas the groundmass glass is 0.9 log units more oxidized than the IW buffer (IW+0.93{plus minus}0.56). This fact suggests that the fO2 of the parent magma of Yamato 980459, which is believed to have evolved in a closed system, increased during magma ascent and emplacement. Our findings show that the µ-XANES technique is useful to investigate the redox conditions of volcanic rocks, when they do not possess mineral phases applicable to conventional oxybarometers, and surely contributes to better understanding of the redox evolution in the Martian interior.

Donwilhelmsite, [CaAl4Si2O11], a new lunar high-pressure Ca-Al-silicate with relevance for subducted terrestrial sediments

Donwilhelmsite (IMA 2018-113) is a new lunar high-pressure mineral identical to the calcium aluminum silicate, CAS, phase first reported from static pressure experiments. This paper by Fritz et al. details the first extraterrestrial mineral with a crystal structure solved by three dimension electron diffraction (3D ED). The name honors the lunar geologist Don E. Wilhelms. In the feldspathic lunar meteorite Oued Awlitis 001, donwilhelmsite crystallized as micrometer-sized needles in shock melt pockets at high-pressure and temperature conditions mimicking the conditions in the Earth's mantle. On Earth, donwilhelmsite is an important mineral in continentally derived sediments subducted into the deep mantle (460 to 700 km) and likely associated with the enriched mantle components EM1 and EM2.

Magnetite texture and trace-element geochemistry fingerprint of pulsed mineralization in the Xinqiao Cu-Fe-Au deposit, Eastern China

Zhang et al. use magnetite to investigate the Cu-Fe-Au mineralization of the Xinqiao ore deposit. They find 6 different generations of magnetite, including veins that formed later than pyrite and are genetically linked to elevated fO2 resulting from fracturing. Furthermore, the magnetite textures and chemistry imply that multiple pulses of fluids from a magmatic-hydrothermal system played an important role in the generation of the ore deposit.

Magmatic haggertyite in olivine lamproites of the West Kimberley region, Western Australia

Jaques et al. report the first occurrence of the alkali titanate mineral haggertyite (BaTi5Fe6MgO19) as a magmatic phase in two individual olivine lamproite bodies from the classic West Kimberley lamproite province in Western Australia. This is only the second report of haggertyite. In the original description of haggertyite (American Mineralogist 1996) from the Prairie Creek lamproite, the haggertyite was inferred to be of metasomatic origin. The haggertyite in the West Kimberley lamproites varies in composition (Fe, Ti) and in the calculated Fe3+ and Fe2+ depending on the prevailing fO2 as a result of the coupled substitution between Ti + Fe2+ on one side of the reaction and and 2 Fe3+ On the other. They show that the haggertyite crystallized at relatively low temperatures (650 to 800 degrees Celsius) and that the haggertyite in one of the lamproite pipes crystallized under increasing fO2 conditions whereas in the other it formed under more reducing conditions and decreasing fO2. Jaques et al. present major and trace element analyses of the haggertyite and co-existing titanate and alkali titanate minerals. Their new data show that haggertyite is not an isolated occurrence, but one of an increasing number of new minerals in upper mantle rocks, and volcanics derived from the upper mantle, hosting large-ion-lithophile and high field strength cations. They suggest that haggertyite may be more widespread than thought, perhaps having been overlooked or misidentified.

Trace elements in sulfides from the Maozu Pb-Zn deposit, Yunnan Province, China: Implications for trace element incorporation mechanisms and ore genesis

To understand the genesis of Pb-Zn deposits in the Sichuan-Yunnan-Guizhou Metallogenic Province (SYGMP), the trace elements in different sulfides (sphalerite and galena) from the Maozu Pb-Zn deposit, which is a representative Pb-Zn deposit in the SYGMP, were analyzed by Li et al. via LA-ICP-MS. Compared with trace elements in sulfides from the different genetic types of Pb-Zn deposits, the Maozu Pb-Zn deposit is characterized by enrichment of Ge, Fe, Mn, and Co in sphalerite and Ag, Sb, Cd, and Se in galena, which is similar to the composition of typical Mississippi Valley Type (MVT) deposits and different from those of SEDEX, VHMS, and skarn type Pb-Zn deposits, suggesting that the Maozu deposit is a MVT deposit.

New pressure-induced phase transition to Co2Si-type Fe2P

The phase relations and elastic properties of Fe alloys with light elements under relevant high-pressure and high-temperature conditions are the keys to understanding the nature of planetary metallic cores. Phosphorous is one of the candidates for core light elements. Here, Nakajima et al. discovered a new phase transition in Fe2P from Co2P-type (C23) to Co2Si-type (C37) structure at 42 GPa based on in-situ high pressure-high temperature X-ray diffraction measurements. The results indicate that the new C37-type Fe2P phase can be stabilized as the most iron-rich phosphide phase under planetary core conditions where the pressure is higher than 42 GPa. Moreover, the present study reveals that (Fe,Ni)2(S,Si,P) could have wide solid solution and constitute planetary cores.

Effects of small crystallite size on the thermal infrared (vibrational) spectra of minerals

The infrared spectra of minerals change when their crystallite sizes approach the wavelength of light. Hamilton et al. show that these changes can be used to recognize small crystallite sizes in samples lacking other indicators of this characteristic. Such small crystallite sizes are commonly associated with abbreviated crystal growth and reflect a potentially important aspect of the sample's geologic history.

October 2020

The effects of solid-solid phase equilibria on the oxygen fugacity of the upper mantle

Terrestrial magmas and peridotites display large variations in oxygen fugacity, typically attributed to differences in the redox state of multivalent elements in the mantle or to processes acting on segregated melts. Using thermodynamic models, Stolper et al. show that the oxygen fugacity of subsolidus mantle peridotite of a fixed composition can vary by 1.5 log units in the upper ~100 km of the mantle due to the same reactions that produce transitions from plagioclase-spinel-garnet lherzolite and variations in Al in pyroxenes. These effects of phase equilibria on peridotite oxygen fugacity are likely to be superimposed on variations due to bulk composition and should be considered in efforts to understand variations in the oxygen fugacities of magmas and their mantle sources.

Structural and spectroscopic study of the kieserite-dwornikite solid solution series, at ambient and low temperatures, with cosmochemical implications for icy moons and Mars

Talla et al. present new detailed data on the behavior of the kieserite-Ni-kieserite solid solution series. The study documents changes in the crystal structure along the binary join, as well as variations in the IR and Raman spectral band positions in relation to the changing chemical and structural properties. These sulfate phases are of cosmochemical importance, as they occur in considerable amounts on some planetary objects in our solar system.

Mineral compositions and thermobarometry of basalts and boninites recovered during IODP Expedition 352 to the Bonin forearc

IODP Expedition 352 to the Bonin forearc recovered tholeiitic forearc basalt (FAB) and slightly younger basaltic, low-Si and high-Si boninite. Using mineral compositions and thermobarometry, Whattam et al. determined the conditions of crystal growth and differentiation for Expedition 352 lavas and compared these conditions with those recorded in lavas from mid-ocean ridges, forearcs, and ophiolites.

An evolutionary system of mineralogy, Part II: Interstellar and solar nebula primary condensation mineralogy

The evolutionary system of mineralogy relies on varied physical and chemical attributes, including trace elements, isotopes, solid and fluid inclusions, and other information-rich characteristics, to understand processes of mineral formation and to place natural condensed phases in the deep-time context of planetary evolution. This paper by Hazen and Morrison considers the formation of primary crystalline and amorphous phases by condensation in interstellar molecular clouds and the earliest period of solar nebula evolution-environments that increased mineralogical diversity and distribution prior to the accretion of planetesimals greater than 4.5 billion years ago.

Swelling capacity of mixed talc-like/stevensite layers in white/green clay infillings ('deweylite'/'garnierite') found in serpentine veins of faulted peridotites (New Caledonia)

Fonteneau et al. describe the occurrence of stevensite in vein-infillings of reactivated faults from peridotitic formations, New Caledonia. Five samples of white (deweylite) and bluish green (garnierite) clay infillings were selected from a large set of vein infillings to investigate their swelling ability and crystal chemistry. They belong to the ultimate phases of clay infillings, which are predominantly made of 2:1 layer silicates. Different treatments were used to link the gradual increase of the swelling capacity to charge deficiency and structural defects in mixed S and TL phases.

Experimental observations of TiO2 activity in rutile-undersaturated melts

Over the past decade, trace-element thermobarometry (e.g., Ti-in-quartz, Ti-in-zircon) has emerged as a powerful tool to determine the thermal and barometric histories of igneous and metamorphic rocks. In many instances, the application of these tools requires an accurate estimate of Ti activity when rutile is not present. In this study, Ackerson and Mysen utilized two methods (a rutile-saturation model and a Ti-in-tridymite solubility calibration) to calculate Ti activity in rutile-undersaturated melts, and demonstrate that rutile-saturation model estimates consistently over-predict Ti activity. When applied to natural systems, this over-prediction will lead to an under-estimation of magmatic temperatures.

Direct evidence for the source of uranium in the Baiyanghe deposit from accessory mineral alteration in the Yangzhuang granite porphyry, Xinjiang Province, Northwest China

Zhang et al. provide new insights into ore sources and enrichment processes of post-magmatic hydrothermal uranium deposits. This study also provides an example illustrating the application of primary versus altered U-bearing mineral-assemblages and their trace element abundances and textures to trace the source of uranium for uranium deposits. The results highlight the potential of element mapping in obtaining direct evidence for uranium leaching from source rocks and in tracing the source of uranium in ore deposits.

Extraction of high-silica granites from an upper crustal magma reservoir: insights from the Narusongduo magmatic system, Gangdese arc

Although the magma dynamics for the genesis of crystal-poor high-SiO2 rhyolites has been a subject of many studies, the dynamics for the formation of high-silica granites are poorly understood. Yang et al. provide a scarce case that kilometer-scale high-silica granite bodies are the extracted products from a shallow magma reservoir. These high-silica granites can be regarded as failed eruptions of high-SiO2 rhyolites. The work addresses a broad range of issues concerning silicic magmatism, such as the behaviors of shallow magmatic systems, the volcanic-plutonic connections, and, particularly, the capability and efficiency of crystal-melt separation in upper crustal reservoirs.

Synthesis and crystal structure of Pb-dominant tourmaline

Vereshchagin et al. synthesized a Pb-dominant tourmaline (up to 14.7 wt% PbO) at 700 °C and 200 MPa using a hydrothermal method. Structural analysis shows that tourmaline can incorporate significant amounts of Pb2+ (up to 0.71 apfu) in its X-site, which means that natural Pb-rich tourmalines belong to a Ca-dominant group. Similarities between (1) the paragenesis of Minh Tien tourmaline and (2) the observed, final experimental phase assemblage indicate comparable P-T conditions of formation.

Element loss to platinum capsules in high-temperature-pressure experiments

Wang et al. investigate the loss behavior of 45 elements in high-temperature-pressure experiments and find (1)15 elements including V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Cd, In, Sn, W, and Mo lose significantly from the sample by alloying with Pt at reducing condition; (2) Graphite- and Re-lined Pt capsules can prevent the loss of V, Cr, Mn, Fe, Zn, Ga, Ge, Cd, In, Sn, W, and Mo, but can barely reduce the loss of Ni and Cu; (3) Element loss can be reduced effectively at the oxidizing condition, and all the elements except Cu are retained at Ru-RuO2 buffered condition. The results provide several viable capsule assemblies that are capable of preventing or reducing element loss, which may prove useful in determining precise phase diagrams and accurate partition coefficients in HTP experiments.



New Mineral Names

September 2020

How American Mineralogist and the Mineralogical Society of America influenced a career in mineralogy, petrology, and plate pushing, and thoughts on mineralogy’s future role

This reflection by W.G. Ernst was a part of the recent yearlong centennial celebration of the MSA that took place at the 2019 GSA annual meeting in Phoenix. One day’s activities featured invited talks by extant MSA presidents. Most speakers described exciting new mineralogic studies in progress, whereas Dr. Ernst took a “walk down memory lane” -- a chronicle of 65 years of scientific studies and a few lessons learned from them, as well as concerns regarding the future habitability of the Earth. Dr. Ernst’s goals involved the integration of mineralogy, petrology, and geochemistry with regional geology and plate tectonics.

Petrographic and spectral study of hydrothermal mineralization in drill core from Hawaii

This is the first analysis of alteration in basaltic materials obtained from the subsurface that can provide a unique analog for alteration environments on Mars. The paper by Calvin et al. describes the initial analysis of spectra taken of drill core in the field used to investigate a set of cut sections that represent the mineralogical diversity present. Petrography and infrared spectroscopy identify similar minerals and show that field spectra can be a useful reconnaissance tool for subsequent higher resolution and more time-consuming analysis in the lab.

Characterizing low-temperature aqueous alteration of Mars-analog basalts from Mauna Kea at multiple scales

Rasmussen et al. characterized the low-temperature aqueous alteration of core samples documenting the shield-building phase of Mauna Kea’s formation. Trioctahedral Mg-Fe smectites and multiple zeolites make up the bulk of the alteration mineralogy, providing an opportunity to spectrally characterize these minerals in geologically realistic mixtures. Spectral characterization of these minerals and understanding their environment of formation are fundamental in the pinpointing of Martian alteration conditions.

Archean to Paleoproterozoic seawater halogen ratios recorded by fluid inclusions in chert and hydrothermal quartz

Halogens are critical elements for sustaining life on Earth and control the salinity of the oceans. Changes in past ocean halogen compositions are largely unknown, especially for the least abundant halogen, iodine. Burgess et al. determine Cl, Br, and I abundances in chert from the 2.5 Ga Hamersley Banded Iron Formation and hydrothermal quartz from the 3.5 North Pole area, both in Australia. Comparable results from both imply that ancient oceans had Br/Cl and I/Cl about 30% and 30 times higher, respectively, relative to modern seawater. The higher I/Cl indicates a smaller organic reservoir in the ancient oceans.

Metasomatism-controlled hydrogen distribution in the Spitsbergen upper mantle

This work by Tang et al. reports the hydrogen concentrations in olivine and pyroxene of Spitsbergen mantle xenoliths. The correlations between hydrogen and incompatible trace elements in xenoliths suggest that hydrogen distribution in the Spitsbergen upper mantle is controlled by metasomatism. The hydration of mantle was achieved by two different processes in this metasomatic event. Furthermore, the metasomatic melt could have derived from an OIB-type source, consistent with the Sr-Nd isotope compositions of the xenoliths.

Phase transformation of hydrous ringwoodite to the lower-mantle phases and the formation of dense hydrous silica

Chen et al. performed new high-pressure experiments that showed that hydrous ringwoodite can break down to form bridgmanite, ferropericlase, and a dense hydrous silica phase. The new observation of hydrous silica formation may explain the coexistence of stishovite and ferropericlase inclusions in lower-mantle diamonds, which cannot be explained by the equilibrium phase equilibria of dry phases.

Density and sound velocity of liquid Fe-S alloys at Earth’s outer core conditions

The Earth’s outer core is mostly composed of liquid iron (Fe) alloy with a small amount of light elements, such as S, O, C, Si, and H. Among these light elements, S stands as a key candidate element, but its concentration is very controversial. In order to constrain the S concentration, Fu et al. developed a thermal equation of state for liquid Fe-S alloys by first-principles molecular dynamics simulations. Comparing with Preliminary Reference Earth Model, they clarify that the S concentration range is from 10 ~ 14 wt%, assuming S is the only light element. Taking into account the geophysical and geochemical constraints, they propose the outer core contains no more than 3.5 wt% S, 2.5 wt% O, or 3.8 wt% Si.

Some geometrical properties of fission-track-surface intersections in apatite

Apatite fission-track dating conventionally relies on counting the etched damage trails from uranium fission in apatite prism faces, which are considered to have high counting efficiencies. This presents a severe practical limitation. The work by Jonckheere et al. aims to lay the basis for extending fission-track counts to other apatite faces. It presents a model calculation of the orientations, lengths, and widths of the intersections of etched fission tracks with the prism and non-prism faces of apatite. The model predictions are consistent with extensive measurements. Their implications section discusses future practical uses of such measurements. Their contribution is a step toward understanding which tracks are counted and which are not in fission-track dating, and toward using other faces than prism faces for dating.

Thermal equation of state of post-aragonite CaCO3-Pmmn

Lv et al. investigated the stability and physical properties of post-aragonite CaCO3-Pmmn, a high-pressure polymorph of CaCO3, at lower mantle pressure and temperature conditions. Using synchrotron X-ray diffraction measurements in a laser-heated diamond-anvil cell, they confirm the stability of CaCO3-Pmmn at pressures and temperatures corresponding to the middle of the lower mantle, and provide the first characterization of the thermal equation of state of this phase. The newly determined thermodynamic parameters of CaCO3-Pmmn are used to quantitatively model the density and seismic velocity of CaCO3 and carbonated eclogite. With the assumption that carbonates are homogeneously mixed into the slab, this work determines that the presence of carbonates in the subducted slab is unlikely to be detectable through seismic observations, and the buoyancy of carbonates will have a negligible effect on slab dynamics.

Structure of NaFeSiO4, NaFeSi2O6, and NaFeSi3O8 glasses and glass-ceramics

This paper by Ahmadzadeh et al. studies the structure of melts of geologically relevant Na-Fe-silicates. Both as-quenched and heat-treated powders of three Na-Fe-silicates with compositions analogous to well-known aluminosilicate minerals were studied. The main focus of the paper is on the role of Fe and the effects of increasing Si/Fe ratio. It is found that most of the Fe occur as 4-coordinated Fe3+ in the glasses. The crystallization behavior upon both quenching and heat-treating varies depending on Si/Fe ratio. The correlation between Si/Fe ratio and Fe structural role, as well as its oxidation state in such glasses, is important for geosciences and nuclear waste management. The study of these simplified compositions can provide fundamental insight into the physical behavior of silicate melts as well as high-Fe nuclear waste glass crystallization, which is related to their aqueous alteration behavior.

Raman spectroscopic studies of OH stretching vibration in Mn-rich apatites: A structural approach

In this manuscript, Pieczka et al. present results of their studies of the OH-stretching vibration band in Raman spectra of Mn-bearing apatites. Deconvolution of the complex band gives an opportunity to gain insight into the distribution of Mn among the M1 and M2 structural sites in the apatite structure. Thus, the data can, at least partly, verify (or complete) structural data obtained by single-crystal X-ray diffraction and structure refinement, and in the case of very small crystals or their zoned texture, the described method may be the only tool that is able to yield structural data.

Characterization of modified mineral waste material adsorbent as affected by thermal treatment for optimizing its adsorption performances

In the paper by Su et al., physiochemical properties of mineral waste material (MMWM) can be modified by thermal treatment. Increase in the adsorption capacity of MMWM is attributed to the enlarged surface area and disappearance of -OH groups. Adsorption of Pb on MMWM could be assigned to monolayer coverage and chemisorption. Thermally treated MMWM is a good adsorbent for Pb in solution.

Morin-type transition in 5C pyrrhotite

Haines et al. report the discovery of a low-temperature spin-flop transition in 5C pyrrhotite at ~155 K that is similar to those seen in hematite at 260 K and FeS (troilite) at 440 K. The 5C crystal was produced by annealing a 4C pyrrhotite crystal at 875 K to produce a change in the vacancy-ordering scheme that developed during cooling. The 5C structure is confirmed by single-crystal x-ray diffraction and the stoichiometry and homogeneity by electron microprobe and SEM BSE mapping. Resonant ultrasound spectroscopy, heat capacity, and magnetization measurements from room temperature down to 2 K are reported. The transition is marked by a steep change in elastic properties at the transition temperature, a peak in the heat capacity, and weak anomalies in measurements of magnetization. Magnetic hysteresis loops and comparison with the magnetic properties of 4C pyrrhotite suggest that the transition involves a change in orientation of moments between two different antiferromagnetic structures, perpendicular to the crystallographic c-axis at high temperatures and parallel to the crystallographic c-axis at low temperatures. The proposed structures are consistent with a group theoretical treatment that also predicts a first-order transition between the magnetic structures.

The formation of marine red beds and iron cycling on the Mesoproterozoic North China Platform

Tang et al. makes three points in this paper. First, marine red beds (MRBs) have been identified in the 1.4 Ga Xiamaling Formation, North China. Second, continentally sourced iron reactivated by dissimilatory iron reduction and distal hydrothermal fluids have supplied Fe(II) for the MRBs Third, Xiamaling MRBs record a moderate oxygenation event and a long-lasting ferruginous deep water mass.

A multi-methodological study of kernite, a mineral commodity of boron

Gatta et al. investigated the chemical composition and the crystal structure of kernite [Na2B4O6(OH)2·3H2O], one of the most important mineral commodities of B, by a series of analytical techniques, including single-crystal neutron diffraction. The general experimental formula of the kernite sample used in this study is Na1.99B3.99O6(OH)2·3.01H2O; the fraction of other elements (measured for more than 50 elements) is, overall, insignificant (<100 wt ppm); excluding B, kernite does not act as geochemical trap of other technologically-relevant elements (e.g., Li, Be, or REE). The H-bonding network in the structure of kernite is complex, pervasive, and plays a primary role on its structural stability. The potential utilizations of kernite, as a source of B (B2O3 ~ 50 wt%), are discussed on the basis of the experimental findings of this study.

Si-rich Mg-sursassite Mg4Al5Si7O23(OH)5 with octahedrally coordinated Si

The crystal structure of a new high-pressure hydrous phase, Si-rich Mg-sursassite, of composition Mg4Al5Si7O23(OH)5 that was produced by sub-solidus reaction at 24 GPa and 1400 degrees in an experiment using a model sedimentary bulk composition has been determined by Bindi et al. using single-crystal X-ray diffraction. The reactions defining the stability of Si-rich Mg-sursassite are unknown, but are likely to be fundamentally different from those of Mg-sursassite, and involve other ultra-high-pressure dense structures such as Phase D, rather than Phase A.

Inherited Eocene magmatic tourmaline captured by the Miocene Himalayan leucogranites

Tourmaline, which is very common in the Himalayan leucogranites and typically the dominant reservoir of B in the rocks, is stable in various P-T-X conditions and could record the physical and chemical conditions of its formation. Thus, tourmaline chemistry has been frequently used to investigate the petrogenesis of its host magmatic rocks. However, these studies relied on the assumption that the tourmalines formed cogenetically with their magmatic host rocks, as is widely interpreted in most of the global tourmaline occurrences. Han et al. applied the 40Ar/39Ar dating method to coarse-grained tourmalines from the Miocene Cuonadong leucogranite in the Tethyan Himalayan, Southern Tibet, which yielded Eocene ages. The results, together with textural observations and geochemical studies, clearly suggest that the coarse-grained tourmalines found in pegmatites were inherited. The identification of inherited tourmalines not only contributes new insights into the Himalayan collisional orogeny, but also provides constraints for the application of tourmaline chemistry to petrological studies. The capture of Eocene tourmaline by the Miocene leucogranites at Cuonadong suggests that the crust-derived Eocene magmatism may have occurred in the southern Tethyan Himalaya together. Identification of the inherited magmatic tourmaline, although not common, challenges the current application of tourmaline chemistry to the investigation of magmatic-hydrothermal systems.

Book Review: Geology of the Lassen Country: The Geologic Story of Lassen Volcanic National Park and Vicinity. (2019)

Memorial of Don Bloss

August 2020

Are quasicrystals really so rare in the Universe?

Until 2009, the only known quasicrystals were synthetic, formed in the laboratory under highly controlled conditions. Conceivably, the only quasicrystals in the Milky Way, perhaps even in the Universe, were the ones fabricated by humans, or so it seemed. Then came the report that a quasicrystal with icosahedral symmetry had been discovered inside a rock recovered from a remote stream in far eastern Russia, and later that the rock proved to be an extraterrestrial, a piece of a rare CV3 carbonaceous chondrite meteorite (known as Khatyrka) that formed 4.5 billion years ago in the presolar nebula. At present, the only known examples of natural quasicrystals are from the Khatyrka meteorite. Does that mean that quasicrystals must be extremely rare in the Universe? In this speculative essay, Bindi et al. present a number of reasons why the answer might be no. In fact, quasicrystals may prove to be among the most common minerals found in the Universe.

Reaction between Cu-bearing minerals and hydrothermal fluids

Qi et al. study the reaction between native Cu and NaCl solution that leads to the coexistence of fluid inclusions and Na-bearing silicate melt inclusions. Micrometer- to submicrometer-sized cuprite (Cu2O) crystals have been observed in both types of the inclusions, and they are formed most probably due to the dissociation of CuOH. When CuO reacts with HCl and CuCl solutions, or Cu+ reacts with NaCl solution, nantokite (CuCl) has been found in the fluid inclusions that were formed due to oversaturation. Cu solubility in pure water and in 1.5 m NaCl solutions are 0.004 ± 0.002 m and 0.16 ± 0.07 m, respectively. The main responsible Cu-bearing complexes are CuOH(H2O)x in water, NaCuCl2 in NaCl solutions, and HCuCl2 in alkali-free solutions.

Evaluation and application of the quartz-inclusions-in-epidote mineral barometer

Epidote is one of the most common minerals found in igneous and metamorphic rocks. This study by Cisneros et al. introduces a new barometer that has the potential to be widely used among petrologists and other geoscientists. The barometer utilizes elastic modeling of quartz inclusions in epidote to constrain the pressure conditions of epidote growth. Modeling, pressure estimates, and applications of the technique are discussed.

Let there be water: how hydration/dehydration reactions control Earth and life key properties

This paper by Brovarone et al. highlights some fundamental aspects of hydration and dehydration reactions in the solid Earth, in biology, and in modern society, as well as their connections to carbon cycling on our planet.Origin of corundum within anorthite megacrysts from anorthositic amphibolites, Granulite Terrane, Southern India

Karmaker et al. describe the occurrence of and offer an explanation for the genesis of corundum in anorthositic amphibolites from ~2.5 Ga old basement of the Granulite Terrane of Southern India (GTSI). The studied amphibolites from the Manavadi (MvAm) and Ayyarmalai (AyAm) localities contain anorthite lenses (An90-99) with euhedral to elliptical outline set in a finer grained matrix of calcic plagioclase (An85-90) and aluminous amphibole (pargasite-magnesiohastingsite). The lenses, interpreted as primary magmatic phenocrysts, and the matrix are both recrystallized under static conditions presumably during the regional high pressure (HP) metamorphism (~800 °C, 8 to 11 kbar) at ~2.45 Ga. Combined petrological data and computed phase relations are consistent with metasomatic growth of corundum in an open system during infiltration-driven regional metamorphism. This study thus presents a new viable mechanism for the origin of corundum in anorthositic amphibolites, and basic-ultrabasic rocks in general, which should provide new insight into lower crustal processes.

Raman spectroscopy study of manganese oxides -- tunnel structures

With the comprehensive Raman database of well-characterized Mn oxide standards, reported here (and as supplementary data) by Post et al., and use of appropriate data collection conditions, micro-Raman is a powerful tool for identification and characterization of biotic and abiotic Mn oxide phases from diverse natural settings (including on other planets) and thereby can provide new insights into the roles of these phases in our environment.

Experimental constraints on the partial melting of sediment-metasomatized lithospheric mantle in subduction zones

Zhang et al. performed high-pressure experiments in sediment-harzburgite systems at 1.5-2.5 GPa and 800-1300 °C to investigate the partial melting behavior of mixed sediment-harzburgite. The results demonstrate that bulk sediment diapirs, in addition to sediment melt, may be another possible mechanism to transfer material from a subducting slab to an upper mantle wedge or lithospheric mantle. On the other hand, the breakdown of phlogopite may play an important role in the mantle source that produces potassium-rich arc lavas in subduction zones.

Interlayer energy of pyrophyllite: Implications for macroscopic friction

The origin of macroscopic friction of clay minerals is one of the most crucial problems to be solved in the geoscience to understand the fault behavior in the crust. Low frictional strength of clay minerals seems to originate from the crystal structure; however, the underlying physics remains unclear. Here, Sakuma et al. succeed in bridging the gap by conducting the ab initio calculations and developing a simple model of randomly oriented pyrophyllite particles. This result would be a basis for future studies on the frictional properties of clay minerals in natural faults.

Thermodynamic and thermoelastic properties of wurtzite-ZnS by Density Functional Theory

Since wurtzite is an important mineral in scientific and technological fields spanning from geology/mineralogy to materials science, this work by Ulian et al. aims at providing deep insights into its thermodynamic, thermomechanic, and electronic properties at pressures up to 20 GPa and temperatures between 0 to 2000 K for various applications. Several properties, such as phonon dispersion relations, elastic and piezoelectric constants, thermodynamic and thermoelastic behaviors were calculated and reported.

The nature of Zn-phyllosilicates in the nonsulfide mina grande and cristal zinc deposits

Balassone et al. conducted a detailed TEM-HRTEM and AEM study of Zn-phyllosilicates in Mina Grande and Cristal Zn-sulfide and nonsulfide deposits of Bongara district, Amazonas region, Peru. They determined the amount/mode of metal incorporation in the lattices of Zn-phyllosilicates and the relationships of natural occurring clay-rich complex associations, which can act as models for possible synthetic counterparts.

Orthovanadate wakefieldite-(Ce) in symplectites replacing vanadium-bearing omphacite in the ultra-oxidized manganese deposit of Praborna (Aosta Valley, Western Italian Alps)

This study by Tumiati et al. focuses on the occurrence of Ca-bearing REE orthovanadate wakefieldite in the Italian Western Alps. The authors identified wakefieldite and refined its structure following a cutting-edge approach, using a single ~10 micrometer-sized crystal and synchrotron X-ray microdiffraction on a fragment of thin section. In addition, they performed WDS-microprobe analyses, micro-Raman spectroscopy, and thermodynamic modeling.

A simple and effective capsule sealing technique for hydrothermal experiments

Capsule sealing has always been a key procedure in hydrothermal experiments to explore the composition and properties of geo-fluids and their influence on various geological processes. Previously reported capsule sealing techniques have primarily focused on either weld-sealing or cold-sealing methods, which have some disadvantages and limitations. Here, Li et al. report on a newly developed, simple, and effective capsule sealing technique incorporating operations from the cold-sealing and weld-sealing techniques. The sealed capsules are suitable not only for piston cylinders but also for multi-anvil presses and other gas-media or hydrothermal-media apparatuses, such as autoclaves and pressure vessels.

Metamorphic amphiboles in the Ironwood Iron-Formation, Gogebic Iron Range, Wisconsin

This paper by Green et al. characterizes the metamorphic amphiboles in the Ironwood Iron-Formation in northern Wisconsin. The Ironwood is one of the largest iron ore resources in the U.S., but the lack of definitive, unbiased information regarding the presence of amphiboles and their potential human-health impacts has been a source of great controversy. The authors present data that will allow further mineralogical, medical, and biological work to aid in understanding the potential impacts of developing iron resources affected by metamorphism.

The chlorine-isotopic composition of lunar KREEP from magnesian-suite troctolite 76535

To characterize the isotopic composition of KREEP, a primordial geochemical reservoir enriched in incompatible lithophile elements, McCubbin and Barnes conducted in-situ Cl isotopic measurements of apatite within intercumulus regions and within a holocrystalline olivine-hosted melt inclusion in magnesian-suite troctolite 76535 from Apollo 17. The isotopic composition of Cl from this reservoir indicates volatile-loss from the Moon occurred prior to 4.31 Ga.

New Mineral Names

July 2020

Experimental determination of solubility constant of kurnakovite

Xiong et al. first highlight that the solubility constant for Kurnakovite has been experimentally determined. Second, they note that a precise model describing the interactions between borate and sulfate is established.

Elastic properties of majoritic garnet inclusions in diamonds and the seismic signature of pyroxenites in the Earth’s upper mantle

Koemets et al. report seismic velocities of natural majoritie garnets and use this information to evaluate the seismic signature of a possible pyroxenitic garnet layer in Earth’s mantle.

Establishing a protocol for the selection of zircon inclusions in garnet for Raman thermobarometry

Campomenosi et al. present a systematic study of zircon inclusions coupling structural and chemical information by combining a wide range of analytical techniques (Raman spectroscopy, CC-imaging, and LA-ICP-MS) to define how the degree of metamictization, their inherent structural heterogeneity, chemical zonation, and metamorphic recrystallization processes can influence the Raman spectra of zircon. The experimental results allowed them to establish a protocol devoted to the selection of reliable buried zircon inclusions to use for elastic barometry applications relying only on Raman spectroscopic measurements.

Reversely zoned plagioclase in lower crustal meta-anorthosites: An indicator of multistage fracturing and metamorphism in the lower crust

Soda et al. describe the formation mechanism of reversely zoned plagioclase, which has been observed frequently in lower crustal shear zones, by studying the microstructural and chemical characteristics of plagioclase from the Eidsfjord shear zone, northern Norway. The reversely zoned plagioclase is an indicator of multistage, brittle fracturing and subsequent hydration metamorphism during exhumation, providing information relevant to understanding the deep rupture process caused by repeated seismicity alternating with aseismic creep below the seismogenic zone.

High-pressure silica phase transitions: Implications for deep mantle dynamics and silica crystallization in the protocore

Das et al. improved the constraints on the pressure-temperature locations and slopes by first-principles atomistic computations of two silica phase transitions occurring at the lowermost mantle and outer core conditions: (1) beta-stishovite (CaCl2-structure) to seifertite and (2) seifertite to pyrite-structured silica. Seifertite has a wide stability range, covering the lowermost 470 km of the mantle. The resulting incremental density increase for recycled oceanic crust might help to segregate and stabilize basaltic materials in the two thermochemical piles beneath Africa and the Pacific.

Cr-Zr-Ca armalcolite in lunar rocks is loveringite: Constraints from electron backscatter diffraction

The compositional and EBSD study by Zhang et al. reveals that Cr-Zr-Ca armalcolite in lunar rocks should be loveringite, which could be an important mineral indicator and REE carrier of lunar Mg-suite rocks.

Effects of composition and pressure on electronic states of iron in bridgmanite

The amount, valence, spin state, and site occupancy of iron in Earth’s most abundant mineral, bridgmanite, potentially controls dynamics and physical properties of heterogeneities in the lower mantle. Electronic states of iron in bridgmanite were probed by Dorfman et al. via Mossbauer spectroscopy for a wide range of compositions. The results provide constraints on the effects of pressure and composition on spin state, valence state, and charge transfer in bridgmanite in Earth’s mantle.

Ti Diffusion in feldspar

These data by Cherniak and Watson indicate that Ti diffuses more slowly than most other impurities in feldspar, including Sr. For plagioclase, there is a dependence of Ti diffusion on feldspar An content, with more anorthitic feldspars having slower Ti diffusivities. Characterization of diffusivities for this trace impurity, which is relatively slow-diffusing and ubiquitous in feldspars, has the potential to expand the scope and applicability of modeling of time-temperature conditions for crustal rocks.

Radiation-induced defects in montebrasite: An electron paramagnetic resonance study of O- hole and Ti3+ electron centers

Toledo et al. detail color improvements of the rare gemstone montebrasite and related minerals that contain hydroxyl ions in their crystalline structure. From detailed analysis of the EPR angular rotation patterns, microscopic models for the O− hole and Ti3+ electron centers are presented, as well as their role in the formation of color centers is discussed and compared to other minerals.

New IR spectroscopic data for determination of water abundances in hydrous pantelleritic glasses

In view of the scarcity of data on peralkaline rhyolitic compositions compared with metaluminous rhyolites, new experimental data are provided by Stabile et al. that allow the use of IR spectroscopy to measure water abundance in melt inclusions in natural glasses and water content in hydrous experimental glasses of pantelleritic composition. Such compositions, once thought to be relatively H2O-poor, may have water contents as high as 5-6 wt% H2O, which needs to be considered to better gain insights into pantelleritic origins and eruption dynamics. This is of importance for such compositions because, despite the low viscosity, they can experience vastly different eruptive styles, including Plinian events involving large magma volumes; this study can also help in eruptive process modeling.

Experimental Investigation of the effect of nickel on the electrical resistivity of Fe-Ni and Fe-Ni-S alloys under pressure

It is commonly assumed that nickel does not significantly affect the chemical and physical properties of core analogues. However, the effect of Ni on electrical and thermal resistivity is not well-constrained under temperature and pressure conditions, as the very few previous electrical studies on Fe-Ni alloys were conducted either at room temperature or at atmospheric pressure. Pommier shows experimentally that at defined temperature, Fe-Ni(-S) alloys are more resistive than Fe (by a factor of ~3) and Fe-Ni alloys containing 5 and 10 wt% Ni present comparable electrical resistivity values. A comparison is made with data on pure iron and Fe-5S samples. Pommier also provides estimates of the lower and upper bounds of thermal conductivity. These estimates suggest that the thermal conductivity values used as part of recent numerical modeling of the cores of Ganymede and the Moon are too high, and thus, this paper has the potential to better constrain future thermochemical models of planetary cores. Pommier shows that a similar amount of heat is conducted at any depth along the adiabat gradient of a Fe-Ni(-S) core, whereas less heat is conducted down this gradient at shallow depth in a Ni-free core. Because variation in heat conduction is critical for driving convection, this implies that it may be easier to drive convection in a Ni-free core than in a Ni-bearing core.

An experimental approach to examine fluid-melt interaction and mineralization in rare-metal pegmatites

McNeil et al. establish an experimental technique to investigate solubilities of minerals in melts via fluid-melt interactions, through the interactions of a melt enriched in HFSEs with a hydrothermal fluid enriched in a fluid mobile element. HFSE minerals that crystallize from fluid-melt interactions texturally occur as euhedral crystals as phenocrysts in glass, i.e., are purely magmatic textures. Therefore, crystallization of HFSE minerals from fluid-melt interactions in rare metal granites and pegmatite deposits may be more widespread than previously recognized. This is significant because the formation of these deposits may require magmatic-hydrothermal interaction to explain the textures present in deposits worldwide, rather than always being the result of a single melt or fluid phase.

Crystal-chemistry of sulfates from the Apuan Alps (Tuscany, Italy). VI. Tl-bearing alum-(K) and voltaite from the Fornovolasco mining complex

Sulfates play an important role in determining the dispersion in the environment of acids and potentially toxic metals related to the weathering of ore deposits, coals, and mine wastes. Alum-(K) and voltaite are two common phases in sulfate assemblages, and they may be able to host high contents of the toxic element Tl. Through a multi-technique study of these minerals, Biagioni et al. found that it was possible to highlight their role as scavengers of Tl in acid mine drainage systems.

First-Principles Modeling of X-ray absorption spectra enlightens the processes of scandium sequestration by iron oxides

Chassé et al. investigate the mechanisms of scandium sequestration by iron oxides combining first-principles calculations with X-ray absorption near-edge structure spectroscopy. Their results show the specificities of scandium sorption processes, explaining its concentration in iron-oxide rich surficial environment, with implications for mineral processing. This work demonstrates the relevance of the approach to study the speciation of trace metals in the environment.

Effects of the dissolution of thermal barrier coating materials on the viscosity of remelted volcanic ash

The chemical interaction between remelted volcanic ash and ceramic coatings of yttria-stabilized zirconia (YSZ) and/or gadolinium zirconate (GZO) is of special importance for the design of volcanic ash melt-resistant thermal barrier coatings (TBCs) for aviation turbine technologies. Müller et al.’s investigation of the high-temperature viscosity of five volcanic ash samples of basaltic, andesitic, rhyolitic, and phonolitic compositions, each doped with 6.5 wt% YSZ or GZO, revealed a reduction of viscosity for all samples compared to their natural counterparts. With respect to thermal barrier coatings, it can be concluded that once the dissolution of the YSZ or GZO coating material in contact with a silicate melt starts, the viscosity will decrease, enabling an enhanced spreading on the surface and/or infiltration in the coating. A simple parameterization of the effects of YSZ and GZO on the viscosity of melts of volcanic ash samples can be expressed as a linear relationship. This parameterization should be employed in any future modeling of the dynamics of CMAS or natural ash melt interaction with TBCs.

New Mineral Names

June 2020

Halogens in amphibole and mica from mantle xenoliths: Implications for the halogen distribution and halogen budget of the metasomatized continental lithosphere

Hecker et al. conducted a study on the halogen contents (F and Cl) in amphibole and phlogopite in mantle xenoliths of variable modal compositions (dunites, harzburgites, lherzolites, wehrlites, olivine websterites, websterites, and clinopyroxenites) complemented by amphibole and phlogopite megacrysts from several localities and samples from the metasomatized Finero peridotite massif in northern Italy. For comparison, the authors also analyzed amphibole from several mafic magmatic cumulates (hornblendites). Totally, data for 12 localities in Europe and Africa are presented. The data provide insight into halogen storage and redistribution in the mantle and demonstrate the importance of amphibole and mica for the total halogen budget of the lithospheric mantle and their potential role as a halogen source for mantle-derived melts. The data also record large differences in Cl concentrations (which correlate negatively with F/Cl ratios) between samples of the same region, recording desiccation processes that may cause very large variations in mantle halogen contents on a regional scale.

New insights on Br speciation in volcanic glasses and structural controls on halogens degassing

Despite their relatively low concentrations compared to H2O and CO2, halogens (F, Cl, Br, and I) are considered key actors in magmatic processes, because they may have a significant effect on melt properties. Their volcanic degassing may also lead to considerable perturbations of atmospheric chemistry, with the rapid transformation of HBr to reactive BrO during explosive eruptions notably known to trigger ozone depletion on various temporal and spatial scales. While recent analytical developments enable a better detection and characterization of their concentrations in magmas and volcanic gases, current understanding of incorporation mechanisms in silicate melts is scarce and potentially limits the interpretation of elemental ratios and modeling of degassing behavior. Here, Louvel et al. take advantage of the development of high-energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) to study Br speciation in natural volcanic glasses that contain 100-3000 ppm Br. The measurements reveal that Br speciation in basalt, andesite, and rhyodacite differs significantly from that previously recorded in haplogranite laboratory analogs, with Br being incorporated in at least three distinct sites, surrounded by Na, K, or Ca. The reported structural environment for natural glasses are similar to those found for Cl in silicate and borosilicate glasses, suggesting that melt composition may not play a significant control on Cl and Br incorporation in natural melts, and further supporting their coupled degassing in volcanic systems.

Decoupled water and iron enrichments in the cratonic mantle: A study on peridotite xenoliths from Tok, SE Siberian Craton

Doucet et al. provide new data of water contents of olivine and pyroxenes from peridotite xenoliths that represent the cratonic lithospheric mantle at the edge of the Siberian Craton. The peridotite xenoliths display significant iron enrichment due to complex metasomatic history that might have led to the destruction/delamination of the cratonic root beneath the Anabar shield in the southeastern part of the Siberian Craton. This study addresses the fundamental question of the role of water and iron on the stability and longevity of the cratonic lithosphere and describes, in particular, the role of “wet” and “dry” metasomatism on the physical properties of the lithospheric mantle.

Deconvolution of the composition of fine-grained pyrite in sedimentary matrix by regression of time-resolved LA-ICP-MS data

Sedimentary pyrite is an important host of many trace elements. The LA-ICP-MS analysis of sedimentary pyrite by Stepanov et al. results in variable, mixed data. Pyrite composition is calculated from the mixed data using an algorithm based on linear regression. The algorithm involves segmentation of the time-resolved signal, normalization to total, calculation of the regression equations, and estimation of explanatory variable normalizing to total. Regression analysis of time-resolved mixed LA-ICP-MS data is a powerful technique for the analysis of complex substances.

A multi-method characterization of natural terrestrial birnessites

Ling et al. investigate a variety of natural birnessites collected from freshwater environments. Birnessite exists in two varieties, triclinic and hexagonal birnessite, which can determine the metal cycling and redox reactions in which they are involved. Laboratory experiments suggest that biotically produced birnessite is hexagonal, although little is known about the varieties of birnessite that exist in freshwater environments. From the analyses of 11 non-marine birnessite samples, they found that the birnessite samples span the entire spectrum of triclinic to hexagonal birnessite, including intermediate structures.

REE redistributions during granite weathering: Implications for Ce anomaly as a proxy for paleoredox states

Different response of Ce to redox state from those of the other light rare earth elements (LREEs) can be used to understand paleo redox states. In order to establish the possibility of using Ce anomaly as a proxy for paleo-environments, Ichimura et al. examined the mineralogical and chemical characteristics of bulk samples and REE-bearing minerals of a modern weathering profile developed on granite using a number of experimental techniques. They have classified LREE redistributions in both secondary minerals and bulk weathered samples during oxic weathering and suggested that Ce anomaly can provide useful information on paleoredox states if Ce anomalies of both bulk samples and secondary REE-bearing minerals are determined.

Formation of destinezite in relationship with the acid-sulfate alteration associated to the magnetite deposit of El Laco, Chile

The giant El Laco magnetite-(apatite) deposit, Chile, has been interpreted as a complex polyphase magmatic-hydrothermal mineralization in which most of the existing magnetite and apatite formed as lava flows or (crypto)-domes. However, there are opinions that support a genesis exclusively related to an intense hydrothermal replacement of preexisting andesitic rocks. Among the geological evidences that support the first explanation has been argued the presence of unconsolidated Fe-P-oxide materials ejected as tephra (volcanic bombs) by the El Laco volcano, currently replaced by destinezite forming enigmatic lumps. However, these rare (almost) monomineralic rocks, affected by hydrothermal alteration and weathering, have received little attention. In this study, Velasco et al. use conventional geological information and mineralogical identification of the lumps, besides thermal decomposition of pure destinezite to derive its thermodynamic properties. These new data have been used to discuss the extension of fields of stability of this mineral and predict the favorable conditions for destinezite formation. Finally, they propose that quoted destinezite formed at low-temperature hydrothermal conditions, an origin very different from the traditionally proposed one for this mineral (i.e., supergene).

Implications for anisotropy at the slab–mantle interface due to Si-metasomatism

Nagaya et al. carried out EBSD measurements of talc grains and obtained the crystallographic preferred orientation (CPO) of talc from talc schists formed due to Si-metasomatism by subduction zone fluids into ultramafic rocks. Talc CPO shows a strong concentration of the pole to the (001) plane, which is normal to the foliation. The strongest concentration of the [100] direction is parallel to the lineation. The combination of TEM and EBSD observations of talc grains implies that the use of EBSD orientation data of talc grains with MAD values in the range of less than 1.3° to less than 0.7° enables determination of a relatively accurate talc CPO when using the thin-section parallel to the foliation of talc schist. As with talc, CPO measurements of most clay minerals have not been reported. The methods used here may be applicable to other mechanically-weak minerals. Their calculations of seismic anisotropies based on the talc CPO obtained demonstrate that Vp and AVp of talc schist can show slower and stronger values, respectively, as compared with antigorite schist at deeper domains. Therefore, if S-waves with different ray paths through the same anisotropic domain show a larger variation in Vp values than that predicted in antigorite schist, this anisotropic domain may be composed of talc schist. The strong talc CPO and the significantly weak layer developed in the slab-mantle interface in the subduction zone can promote spatial expansion of the slip area during an earthquake in the wedge.

The occurrence, origin, and fate of water in chromitites in ophiolites

Su et al. presented direct petrographic, mineralogical, and geochemical evidence confirming the occurrence, origin, and fate of water in podiform chromitites. They provided the petrological evidence in clinopyroxene-bearing chromitites for the presence of water, self-alteration features in podiform chromitites, chemistry of clinopyroxene associated with chromite, and water contents and Li isotopic ratios of olivine and clinopyroxene. Additionally, the fate of surface fluids on chromite grains and the importance of water in the formation and evolution of chromite deposits, as inferred by earlier experimental studies, were described.

Thermoelasticity of tremolite amphibole: Geophysical implications

Peng and Mookherjee evaluate whether anomalously low seismic velocities observed in the mid lithospheric discontinuities (MLD) could be caused by mantle metasomatism i.e., hydrated layer consisting of amphibole and phlogopite. In order to do so, they determine thermoelastic parameters of tremolite using density functional theory calculations. Then they calculate the velocity depth profile for hydrated lithologies with varying amphibole and phlogopite contents. They compare the velocity of hydrated lithology with that of the dry lithology and deduce that while the presence of amphibole and phlogopite in metasomatized mantle might reduce velocity, it may not be the sole mechanism to explain MLD.

Stability of fcc phase FeH to 137 GPa

Kato et al. propose a revised phase diagram of iron hydride, FeH, which is a candidate component of the Earth’s core. Contrary to previous studies, the experimental results suggest that face-centered cubic (fcc) FeH has a wide stability field at high pressure and temperature instead of a double-hexagonal closed packed (dhcp) FeH. They also examined the compression behavior of fcc FeH and observed a change in compressibility at about 60 GPa, which could be due to a magnetic transition, as suggested by ab initio computation. Fcc FeH could be an endmember in the Fe-FeH system at the Earth’s core conditions.

Partition coefficients of trace elements between carbonates and melt and supra-solidus phase relation of Ca-Mg-Carbonates at 6 GPa

The presence of Ca-Mg-carbonates affects the melting and phase relations of peridotites and eclogites in the mantle and (partial) melting of carbonates liberates carbon from the mantle to shallower depths. The onset and composition of incipient melting of carbonated peridotites and carbonated eclogites are influenced by the pure CaCO3-MgCO3-system, making understanding of the phase relations of Ca-Mg-carbonates fundamental in assessing carbon fluxes in the mantle. By performing high pressure and temperature experiments, Sieber et al. clarify the supra-solidus phase relations of the nominally anhydrous CaCO3-MgCO3-system at 6 GPa (~200 km deep) showing that Ca-Mg-carbonates will (partially) melt at temperatures above ~1300 °C. A comparison with data from thermodynamic modelling confirms the experimental results. Furthermore, partition coefficients for Li, Na, K, Sr, Ba, Nb, Y and rare earth elements between calcite and dolomitic melt, Ca magnesite and dolomitic melt and magnesite and dolomitic melt are established.

Systematics of H2 and H2O evolved from chlorites during oxidative dehydrogenation

Lempart et al. performed thermogravimetric analysis combined with quadrupole mass spectrometry to give a complete picture of the thermal decomposition for Fe(II)-containing phyllosilicates. The evolved gas analysis of chamosties, Fe-clinochlores, and biotite revealed that H2 was generated along with H2O under inert atmosphere conditions in the temperature range 500-1000 °C. The higher the Fe(II) content in the chlorites, the more intense the H2 evolution, which results in the increased oxidation of structural Fe(II). H2O and H2 gas are, respectively, indicators of dehydroxylation and oxidative dehydrogenation. These molecules evolve simultaneously, but independently, as they are governed by different mechanisms and kinetics. Despite ongoing dehydrogenation, under oxidizing gas conditions, no H2 was detected, as it immediately combines with an external oxygen to form H2O in the carrier gas. Potential dehydrogenation of Fe(II)-bearing phyllosilicates affects the pathways of fluids released during the deep burial of sedimentary rocks and during metamorphism and melting. This changes the budget of both water and H2 gas in metamorphic and magmatic processes, not only on Earth but also on Mars. The thermal heating experiments were carried out under both an inert and oxidizing atmospheres, which closely reflect anaerobic and aerobic geological environments. The occurrence of oxidative dehydrogenation complicates the interpretations based on the valence of Fe (or Mn), because the Fe(III)/Fetot ratio does not directly reflect oxygen fugacity nor temperature of formation.

Texture and geochemistry of multi-stage hydrothermal scheelite in the Tongshankou porphyry-skarn Cu-Mo (-W) deposit, eastern China: Implications for ore-forming process and fluid metasomatism

SEM and CL imaging of scheelit can reveal textures that are not seen under transmitted light imaging and shed light on the dissolution-reprecipitation process in scheelites. During this process, the REE patterns and Sr isotopes of the scheelites were modified without significantly modifying their morphology and appearance. Therefore, it is crucial to evaluate such a process according to the textures and geochemical characteristics before using scheelite as an indicator mineral for hydrothermal ore deposits. This study by Han et al. provides a good example of the use of scheelite textures and geochemistry to reveal the nature and source of ore-forming fluids. The scheelite-precipitating fluids were not depleted in Mo based on the coexistence of molybdenite and chalcopyrite and the coexistence of scheelite and chalcopyrite. The Mo contents in the scheelites reflect the fluid oxygen fugacity and, together with the observed Eu anomalies, suggest that the scheelite-fluid partition coefficient of Eu2+ may be greater than that of Eu3+.

EXCALIBR to EXCELIBR and the optical orientation of minerals: Correcting the optical orientation of clinoamphiboles

Steven and Gunter revise the crystallographic setting depicted in crystal form drawings of clinoamphiboles. The crystal form drawings were originally made for a body-centered setting for clinoamphiboles in contrast with the standard C-centered lattice. They also present a methodology for relating the optical geometry of a crystal with its crystallographic setting for orientation-dependent analytical methods, and for future characterization of minerals.

A refined zirconium-in-rutile thermometer

The zirconium-in-rutile thermometer is widely applied to natural metamorphic rocks, but its accuracy has not been evaluated. Here, Kohn refined the thermometer using a combination of experimental data and data from natural rocks whose pressures and temperatures of formation are well known. The new calibration reduces thermometer uncertainty by a factor of 2-3, and improves retrieval of thermodynamic properties, but also points to as-yet poorly understood causes of compositional variability in natural rocks.

New Mineral Names

Book Review

High Temperature Gas-Solid Reactions in Earth and Planetary Processes

RiMG volume 84, editors: P.L. King, B. Fegley, Jr., and T. Seward

May 2020

Texture constraints on crystal size distribution methodology: An application to the Laki fissure eruption

Crystal size distributions (CSDs) in igneous rocks are used to tease out evolving environments of crystallization. Kinetic information, such as residence times and average crystal sizes in a sample, is extracted from CSDs. However CSDs are produced from assessments of crystal length measurements, which Cone et al. find vary based on the method and type of image used. They use a combination of techniques ranging from traditional manual approaches and polarized optical microscopy to high-resolution automated mineralogy (automated scanning electron microscopy that relies on elemental information in a mineral) to demonstrate that the resulting CSDs do indeed differ. Even localized fabric can affect the interpretation of crystal lengths. A protocol should be developed that focuses on the textural effects on crystal length determinations, as each image type has the potential to skew measurements.

Hydrogenation reactions of carbon on Earth: Linking methane, and margarine, and life

Hydrogenation reactions involve the coupled movement of protons and electrons. These reactions are responsible for numerous molecular transformations deep in the Earth, in biological systems, and also in the atmosphere. Here McGlynn et al., in this open-access paper, survey hydrogenation reactions with a focus on those involving carbon.

Abiotic and biotic processes that drive carboxylation and decarboxylation reactions

Sheik et al., in their open-access paper, explain that carboxylation and decarboxylation are an important class of chemical reactions that have helped shape life on the surface of Earth for billions of years. These reactions can be driven through biotic and abiotic processes at different locations in and on Earth’s crust. It is likely that prebiotic synthesis of carboxylic acids was critical for the origin and development of life, which has been using this class of reactions for energy generation and reproduction for billions of years. It is apparent that the location of where these processes are occurring in/on the crust (i.e., subduction zone vs. coal deposit vs. aerobic water column vs. tropical forest) will greatly dictate the rates of reaction and the diversity of carboxylic acids that may be produced. Whether produced biotically or abiotically, these individually mundane reactions are at the heart of complex and even elegant pathways that are extremely important on Earth and potentially on extraterrestrial planets.

In-situ measurements of magmatic volatile elements, F, S, and Cl, by electron microprobe, secondary ion mass spectrometry, and heavy ion elastic recoil detection analysis

F and Cl measurements by EPMA and SIMS have a generally good agreement on standard glasses available to the scientific community (MPI-DING, Jochum et al. 2006). These are the two most common instruments for in situ halogen measurements and their performance has now been analyzed and compared by Rose-Koga et al. Elastic Recoil Detection Analysis (ERDA) of F and Cl on three standards independently anchors the EPMA-SIMS comparison curves and gives absolute F and Cl concentrations for these standards. The ERDA results also highlight the fact that there is a matrix effect on SIMS measurements of F and Cl in the high-SiO2 standard. Rose-Koga et al. propose a new equation to correct this matrix effect in the SIMS F and Cl data of high-SiO2 samples. We can now use, and rely upon, EPMA or SIMS for F and Cl measurements of glasses on a large SiO2 range covering most of the geological samples.

An evolutionary system of mineralogy. Part I: Stellar mineralogy

Hazen and Morrison introduce an “evolutionary system of mineralogy” — an approach to classification that links mineral species to their sources and modes of origin. Part I of this system examines stardust, including 41 kinds of minerals that originate in three very different types of stars: classic novae, Type II supernovae, and asymptotic giant branch stars, each of which forms stellar mineral grains under different physical conditions and contributes distinctive isotopic signatures to those grains. They construct a “bipartite network graph” that displays 41 links between these three types of stars (the three larger star-shaped symbols) and 27 different kinds of condensed phases (the smaller diamond-shaped symbols).

A structural study of size-dependent lattice variation: In situ X-ray diffraction of the growth of goethite nanoparticles from 2-line ferrihydrite

Heaney et al. explore an important but poorly understood nanoparticle behavior — that lattice parameters change systematically for a given metal oxide when crystal diameters fall below 100 nm. This paper documents nanoscale-induced lattice expansion in goethite (FeOOH) through synchrotron X-ray diffraction of ferrihydrite gels that transform to nanogoethite. They demonstrate that the crystallographic changes that accompany expansion due to nanodimensionality in goethite are identical to those operative during heating of goethite.

Cassiterite crystallization experiments in alkali carbonate aqueous solutions using a hydrothermal diamond-anvil cell

This paper by Liu et al. demonstrates that alkali carbonate can enhance the solubility of SnO2 in aqueous solutions and investigates cassiterite crystallization conditions and the tin-transport species in alkali carbonate aqueous solutions. They find that alkali carbonate-rich aqueous solution or hydrous melt can be a favorable transport medium for tin, and that carbonate or CO2 could be fluxes that promote the formation of rare metal pegmatites.

New insights into the nature of glauconite

The work by Lopez-Quiros et al. shows that “glauconite” must be assessed as a mica-rich, mica-smectite (R3 ordered), interstratified mineral. Furthermore, they argue that the 100%-mica extreme term (i.e., the glauconitic mica) should not have been described in nature (at least in the usual glauconitic genetic contexts) and that its K+ content would be ~0.8 a.p.f.u. In addition, their new findings give insights into the glauconitization process and, at the same time, investigate the K-deficient character of the dioctahedral mica glauconite.

Kaolinization of 2:1 type clay minerals with different swelling properties

Li et al. demonstrate that montmorillonite, illite, and rectorite in acidic Al3+-containing solutions can be transformed into kaolinite. Interstratified kaolinite-smectite (K-S), kaolinite-illite (K-I), and kaolinite-rectorite (K-R) form as the intermediate phases during the transformations. The kaolinization of 2:1 type swelling and non-swelling clay minerals is most likely via a local dissolution-crystallization mechanism, mainly from the layer edges rather than from the swelling interlayers. Sizes and stacking order of the newly formed kaolinite strongly depend on those of the precursor minerals. The findings in the present study by Li et al. provide new insights for understanding mineral-water interactions that are central to all geochemical processes.

The quintet completed: The partitioning of sulfur between nominally volatile-free minerals and silicate melts

Sulfur partitioning between clinopyroxene crystals and melts of compositions from basaltic to dacitic was measured by Callegaro et al. using synchrotron X-ray fluorescence. Crystal chemistry, oxygen fugacity, and meltwater concentration influence the partitioning. These measurements can be applied to natural crystals to determine sulfur concentrations in coexisting melts and estimate sulfur concentrations in magmatic systems in the absence of melt inclusions.

222Rn and 220Rn emanations from powdered samples of samarskite as a function of annealing temperature

Malczewski and Dziurowicz measured variations in the 222Rn and 220Rn emanation coefficients with a temperature of fully metamict samarskite-(Y) ground to a five-micrometer grain size fraction and annealed from 473 to 1373 K, which cross the structural conversion from the low- to high-temperature samarskite phase. The same mineral species showed noticeably different emanation coefficients of radon and thoron depending on the crystallographic system induced by annealing in an argon atmosphere. The 222Rn emanation coefficients obtained both for 1 h and 24 h annealing were significantly lower than the values reported in the literature for comparable metamict minerals. The results indicate that samarskite-(Y) behaves as a closed system for radon retention across a very broad temperature range, from an untreated sample to a sample annealed at 1373 K, despite high concentrations of uranium and unusual structural complexity.

Polymerization during melting of ortho- and meta-silicates: Effects on Q species stability, heats of fusion, and redox state of MORBs

The fusion of crystalline orthosilicates and metasilicates produces melts more polymerized than their precursor crystals, as discussed by Nesbitt et al. in this paper. Polymerization during melting can be rationalized from an energetics perspective. Si-NBO-M moieties are more subject to librational, rotational, and vibrational modes than Si-O-Si moieties. Conversion of the former moieties to the latter results in stabilization of melts subject to strong thermal agitation.

Formation of native arsenic in hydrothermal base metal deposits and related supergene U6+-enrichment: The Michael vein near Lahr, SW Germany

Arsenic is a common constituent of upper crustal fluids, but native arsenic is a rare commodity in natural systems. Understanding the thermodynamically constrained formation conditions of native arsenic for base metal hydrothermal mineralization, as an example, also sheds light on the formation of other As-minerals in other types of hydrothermal deposits. Furthermore, as shown by Scharrer et al., the presence of native arsenic in combination with base metal sulfides produces a unique uranium trap under oxidized weathering conditions.

Lingbaoite, a new silver telluride from the Xiaoqinling gold district, central China

Lingbaoite (AgTe3) is a new silver telluride discovered in Xiaoqinling gold district of central China. Full details of this new telluride by Jian et al. are in the paper. Lingbaoite probably formed through the cooling of polymetallic melts. Lingbaoite and associated minerals reveal a previously unrecognized magmatic-hydrothermal process, suggesting gold mineralization in the Xiaoqinling gold district involves multiple superimposed processes of gold enrichment.

Oxygen isotope fractionation between gypsum and its formation waters: Implications for past chemistry of the Kawah Ijen volcanic lake, Indonesia

Utami et al. determined the fractionation of oxygen isotopes between gypsum and its formation water for both water and sulfate, and applied these to gypsum from the Kawah Ijen volcanic lake in Indonesia to reconstruct the oxygen isotopic composition of the lake water during passive degassing and before the last eruption in 1817. Their study shows that gypsum can provide a historical isotopic record of water chemistry for volcanic lakes and other environments where gypsum forms.

2019 MSA Awards

April 2020

Buoyant rise of anorthosite from a layered basic complex triggered by Rayleigh-Taylor instability: Insights from a numerical modelling study

Massif type anorthosites constitute a major puzzle of Proterozoic geology. Formation of these rocks from basaltic magma by floatation of plagioclase crystals at the top of a magma chamber is not supported by observations of the major layered basic intrusions of the world. This numerical modelling study by Mukherjee et al. proposes a new genetic process for the origin of massif type anorthosites. It shows that layered basic intrusions may generate Rayleigh-Taylor instability between the anorthosite layers and the layers of denser rocks, when all these rocks behave like Newtonian or non-Newtonian power law fluids. This instability would trigger a vertical movement of the unstable anorthosite as anorthosite diapirs across the lower and middle crust. The result provides for the first time a dynamic basis and a testable hypothesis for the rise of anorthosite from the Earth's mantle into crust.

Chemically oscillating reactions in the formation of botryoidal malachite

The Belousov-Zhabotinsky reaction displays the same kind of patterns as botryoidal malachite. Arborescent organic structures commonly occur in the geometric center of malachite botryoids. Papineau interprets this in the context of chemical oscillations during the diagenetic decarboxylation of biological organic matter.

Micro- and nano-size hydrogarnet clusters and proton ordering in calcium silicate garnet: Part I. The quest to understand the nature of "water" in garnet continues

Garnet is a remarkable phase for a number of reasons. One of them is its ability to adapt its crystal structure to accommodate radically different compositions. Indeed, garnet, as both a synthetic phase and as a mineral, may well be unique in terms of its chemical variability. Geiger and Rossman explore one interesting system here, the "hydrogarnets" or loosely termed "water"-bearing garnets.

Micro- and nano-size hydrogarnet clusters in calcium silicate garnet: Part II. Mineralogical, petrological and geochemical aspects

Geiger and Rossman investigate mineralogical, petrological, and geochemical characteristics of hydrogarnet clusters in nominally anhydrous garnet. New scientific understanding is obtained because the cluster model not only explains measured infrared spectra, but also permits an atomistic interpretation of many varied experimental results obtained on various calcium silicate garnets over the years.

Petrogenetic insights from chromite in ultramafic cumulates of the Xiarihamu intrusion, northern Tibet Plateau, China

Song et al. study the geochemistry of chromite, one of the first crystallized phases in mafic melts. Although it can be altered by reaction with hosting mineral and trapped liquid, important clues of parental magma and magma evolution can be preserved. In the Xiarihamu ultramafic intrusion, northern Tibet Plateau, China, the most primitive chromites hosted in olivine with Fo > 87 have similarities with MORB chromite in TiO2 and Al2O3, depleted in Sc and enriched in Ga relative to MORB chromite. This indicates a partial melting of the metasomatized asthenospheric mantle at garnet stable pressures.

Enigmatic diamonds from the Tolbachik volcano, Kamchatka

Diamonds, found in products of the recent Tolbachik volcano eruption in Kamchatka, Russia, were studied comprehensively by Galimov et al. Their mineralogical and geochemical features demonstrate that the diamonds were formed from volcanic gases under low pressure conditions. This implies that diamond is a polygenetic mineral, which may be formed at both high- and low-pressure conditions.

Volcanic SiO2-Cristobalite: A natural product of chemical vapor deposition

Schipper et al. study the formation of vapor-phase SiO2-cristobalite in volcanic rocks, which is a natural proxy for Chemical Vapor Deposition. Although metastable, cristobalite may be the most widely occurring SiO2 polymorph in extrusive igneous rocks.

Alteration of magmatic monazite in granitoids from the Ryoke belt (SW Japan): Processes and consequences

Skrzypek et al. describe two different processes responsible for the alteration of magmatic monazite in granitic rocks from SW Japan: monazite replacement by allanite, apatite, and Th-U phases at low temperature and monazite recrystallization with the formation of U-Y-enriched domains at high temperature. The versatility of the accessory mineral monazite gives a unique chance to constrain the nature and timing of metasomatic events. The different alteration processes can be ascribed to hydrous fluid or granitic melt infiltration. In situ age dating of monazite alteration textures gives information on: the age of magmatic crystallization (primary monazite domains), the age of high-temperature recrystallization (secondary monazite domains), and the timing of low-temperature replacement (Th-U-rich phases).

Smamite, a new mineral and a possible sink for Sb during weathering of fahlore

Plášil et al. discovered smamite, a new supergene arsenate-hydrate phase that contains both calcium and antimony. Since this new mineral is rather inconspicuous, it is highly possible that it has remained overlooked by mineralogists for decades. As one of the few Sb(V) supergene minerals, it can serve as a sink for Sb during supergene weathering.

The new K, Pb-bearing uranyl-oxide mineral kroupaite: Crystal-chemical implications for the structures of uranyl-oxide hydroxy-hydrates

Plášil et al. report on the new mineral, kroupaite, which is an additional member of the schoepite-family of minerals. This family is important due to environmental issues associated with the storage of irradiated or spent nuclear fuel in geological repositories.

Mg diffusion in forsterite from 1250-1600 °C

New Mg isotopic diffusion experiments in forsterite show a clear dependence of diffusivity on silica activity, with higher Mg diffusivities at protoenstatite-buffered conditions than in experiments buffered by periclase. The new data by Jollands et al. reconciles some previous discrepant datasets.

Changes of antigorite cell parameters close to the antigorite dehydration reaction at subduction zone conditions

Shen et al. report TEM studies of natural antigorite after being equilibrated in a piston cylinder in the P-T range of 600-650 °C and 25-45 kbar. The length of the a-axis of antigorite samples (can be expressed as m value) was determined and a P-T-m diagram has been constructed based on the new experimental and previous data. The m-value of antigorite changes systematically with pressure and temperature (due to loss of water), which could be used to better constrain the formation conditions of subducted serpentinites.

The issue ends with a memorial of Edward J. Olsen (1927-2020) by Ian Steele and Nancy Hutcheon

March 2020

Heavy halogen geochemistry of martian shergottite meteorites and implications for the halogen composition of the depleted shergottite mantle source

Volatile elements (e.g., H, C, N) influence the physical and chemical properties of planets. The halogen group elements, Cl, Br, and I can provide insight into volatile distribution and transport processes due to their ability to track with water. By measuring halogens in shergottites, martian meteorites that are derived from volcanic processes, Clay et al. show that the halogen geochemistry of different shergottites indicates distinct mantle sources. They calculate that the halogen composition of bulk silicate Mars is very similar to that of bulk silicate Earth, suggesting common volatile source(s) or delivery mechanism(s) to the terrestrial planets.

The distribution and abundance of halogens in eclogites: An in situ SIMS perspective of the Raspas Complex (Ecuador)

Subduction of oceanic crust cycles F and Cl from surface reservoirs to the mantle, however the minerals responsible for hosting these halogens, and the bulk halogen content of deeply subducted oceanic crust, remains poorly understood. Urann et al. analyze halogen abundances of individual minerals and bulk rocks to better quantify the halogen content of high pressure metamorphic altered oceanic crust during subduction.

Pressure dependence of Si diffusion

Tsujino et al. investigate the pressure dependence of Si diffusion in gamma-Fe at pressures of 5-15 GPa and temperatures of 1473-1673 K using a Kawai-type multi-anvil apparatus. Even after 1 billion years, maximum of Si diffusion length at planetary and satellites' core conditions is less than ~1.2 km. On the other hand, the estimated strain of plastic deformation by Harper-Dorn creep based on Si diffusivity in gamma-Fe reaches more than 103 at a stress level of 103-104 Pa.

Seismic detectability of carbonates in the deep Earth: A nuclear inelastic scattering study

Elastic property measurements of two synthetic iron-containing carbonates reveal the extent to which carbonates are seismically detectable using geophysical methods. Chariton et al. confirm that nuclear inelastic scattering is a reliable method for the in situ study of the elastic properties of Fe-bearing systems at high pressures and temperatures, and reveals that seismic detectability of carbonates requires a high concentration of carbon in subducted slabs. Such contents may be reached in several present-day subduction trenches, which are potential targets for geophysical studies investigating carbonates in the deep Earth.

Equations of state, phase relations, and oxygen fugacity of the Ru-RuO2 buffer at high pressures and temperatures

Oxygen fugacity (fO2) is a critical variable in magmatic systems, affecting phase relations and element partitioning. At present, large uncertainties in fO2 exist in high-pressure experimental work. Armstrong et al. report in situ X-ray diffraction measurements of Ru and RuO2, from which they have calculate the absolute oxygen fugacity of the reaction at P/T conditions up to the top of the lower mantle. This allows utilization of this buffer to impose a known fO2 on experiments, reducing uncertainty in the exploration of mantle processes.

Experimental study of the Pt and Pd antimonides and bismuthinides in Fe-Ni-Cu sulfide systems between 1100 and 700 degrees C and applications to nature

Helmy and Botcharnikov present phase relations in the Pt-Sb and Pt-Bi sulfide systems in the temperature range 1100 to 750 °C and in the Pd-Sb and Pd-Bi sulfide systems in the temperature range 1100 to 700 °C. Partitioning behavior of Pt, Pd, Ni, and Cu between monosulfide solid solution and Sb-bearing and Bi-bearing sulfide melts is discussed, as well as Sb and Bi behavior in Pt and Pd-bearing sulfide systems. These results are used to investigate the behavior of Sb and Bi during mantle melting processes.

Layers stacking disorder in Mg-Fe chlorites based on powder X-ray diffraction data

Luberda-Durnas et al. investigate the effects of semi-random stacking in chlorites on powder X-ray diffraction patterns of bulk powder samples. A complete set of information about the stacking sequences in chlorite structures was determined based on XRD pattern simulation and study of a suite of natural samples. A detailed study of semi-random stacking sequences shows that simple consideration of the proportion of IIb-2 and IIb-4/6 polytypes, assuming equal content of IIb-4 and Iib-6, is not sufficient to fully model the stacking structure in chlorites. Several, more general, possible models were therefore considered. In the first approach, a parameter describing a shift into one of the ±1/3b directions (thus, the proportion of IIb-4 and IIb-6 polytypes) was refined. In the second approach, for samples with slightly distinguishable hkl reflections with k≠3n, some kind of segregation of individual polytypes (IIb-2/4/6) was considered. In the third approach, a model with rotations of 2:1 layers about 0°, 120°, 240° were shown to have the lowest number of parameters to be optimized and therefore, giving the most reliable fits.

Elasticity of single-crystal Fe-enriched diopside at high-pressure conditions: Implications for the cause of upper mantle low-velocity zones

Fan et al. report the acoustic wave velocities and density of a single-crystal Fe-enriched diopside by Brillouin light scattering combined with in situ synchrotron X-ray diffraction in a diamond anvil cell up to ~18.5 GPa at room temperature. Based on a comparison of the obtained elastic moduli of Fe-enriched diopside with those of Fe-free diopside (CaAlSi2O6) and hedenbergite (CaFeSi2O6) reported in the literature, the authors evaluate the Fe composition effect on the elasticity and seismic parameters of diopside at the upper mantle P-T conditions. The comparison shows systematic linear correlations between the Fe composition and single-crystal elastic moduli of diopside, while the modeling indicates that substitution of 20 mol% Fe in diopside can reduce VP and VS by ~1.8% and ~3.5%, respectively, along both the expected normal mantle geotherm and a representative cold subducted slab geotherm. Furthermore, modeling also shows that the VP and VS profiles of Fe-enriched pyroxenite along the cold subducted slab geotherm are ~3.2% and ~2.5% lower than AK135 model at 400 km depth, respectively. These results lead to the conclusion that the presence of Fe-enriched pyroxenite (including Fe-enriched clinopyroxene, Fe-enriched orthopyroxene, and Fe-enriched olivine) can be an effective mechanism for causing low-velocity anomalies in the upper mantle regions atop the 410 km discontinuity at cold subducted slab conditions.

XANES spectroscopy of sulfides stable under reducing conditions

Anzures et al. detail new S K-edge XANES spectra for sulfides stable under reducing conditions for comparison to potential extraterrestrial analogs. This information can be used to determine the coordination chemistry and oxidation state of S species in geologic materials. These new sulfide standards should improve studies of sulfur speciation in reduced silicate glasses and minerals with applications for the early Earth, Moon, Mercury, and enstatite chondrites.

Zircon and Apatite geochemical constraints on the formation of the Huojihe Porphyry Mo Deposit in the Lesser Xing'an Range, NE China

Northeastern China is an important Mo resource region, with more than 80 Mo deposits and occurrences. However, the major factors controlling large-scale porphyry Mo mineralization in this region are still unclear, and whether there is any inherent Mo enrichment of the source region and/or any pre-degassing magmatic processes leading to high-Mo melts remains enigmatic. Xing et al. use a combination of geochemistry and isotopic analyses of zircon and apatite from the Huojihe porphyry Mo deposit in NE China to provide insights into the characteristics of the parental magma and ore-forming mechanisms responsible for the porphyry Mo mineralization. The results suggest that pre-degassing enrichment of Mo and S in the original magma is not necessarily important in the formation of the Huojihe Mo deposit and that factors other than melt composition, including (but not limited to) a large magma chamber, may be more critical in forming a porphyry Mo deposit. The conclusions from this study might also apply to other porphyry Mo deposits worldwide.

Textural and compositional evolution of iron oxides at Mina Justa (Peru): implications for mushketovite and formation of IOCG deposits

Hu et al. provide mineralogical evidence to support that the platy magnetite in IOCG systems is mushketovite. The textural and compositional data on magnetite from the Mina Justa deposit provide new insights into evolutionary mechanisms of iron oxides in IOCG deposits, which are dominated by oxygen fugacity and temperature changes and lead to the formation of multiple generations of magnetite at Mina Justa. The primary hematite transformed into TM1-1 magnetite due to a decline in fO2 and then was replaced by TM1-2 magnetite with increased temperature. Meanwhile, granular TM2-1 magnetite directly precipitated from hydrothermal fluids. With the decrease of temperature, TM1-2 and TM2-1 magnetite were replaced by TM1-3 and TM2-2 magnetite, respectively.

Siwaqaite, a new mineral of the ettringite group from the pyrometamorphic Daba-Siwaqa complex, Jordan

Juroszek et al. focus on a new mineral, siwaqaite, from the pyrometamorphic Daba-Siwaqa complex in Jordan, which is a Cr6+-analog of ettringite. They provide a detailed description of physical properties, structural parameters and spectroscopic results for siwaqaite, as well as comparisons with other members of the ettringite group. Siwaqaite is an interesting phase because it contains Cr, a redox-sensitive transition element. Wide usage of Cr in the industry, such as the production of stainless steel, metal finishing, wood preservatives, and refractory products, causes release of high concentrations of chromates in surface water. Due to the high solubility and oxidizing potential, hexavalent Cr is a toxic, teratogenic, carcinogenic, and mutagenic element in biological systems, which can damage kidney and other tissue structures. Juroszek et al. suggest that siwaqaite, in analogy to ettringite, can be used for immobilization of toxic cations and oxyanions because the structure of ettringite has a wide ability to ions exchange. Moreover, slight changes of the chemical composition do not modify the structure. Therefore, minerals form ettringite group could be used for selected contaminant removal by incorporating toxic ions into their structures.

Negevite, the pyrite-type NiP2, a new terrestrial phosphide

Britvin et al. report the discovery of negevite, natural pyrite-type NiP2, that might open new insights into the role of phosphides as a source of phosphorus in prebiotic phosphorylation processes. They discuss the aquatic oxidation of meteoritic phosphides leading to formation of diverse phosphorus compounds, which could serve as prebiotic phosphorylation agents at the early stages of Earth's evolution. In that respect, the unique catalytic and electrochemical properties of NiP2 could provide new routes for further exploration of phosphorylation pathways.

Transjordanite, a new terrestrial and meteoritic phosphide, and natural solid solutions barringerite-transjordanite

Britvin et al. report the presence of natural hexagonal solid solutions along the join Fe2P-Ni2P. These Fe2P-Ni2P solid solutions are related to the Fe-Ni-P system that, along with the Fe-Ni-S and Fe-Ni-C ternaries, are the most significant reduced cosmochemical systems. They discuss how knowledge of phase relationships and compositional variations of Fe-Ni phosphide minerals has a broad range of implications, including the origin of the solar system, composition of deep planetary interiors, meteoritics, terrestrial processes in super-reduced environments, and processes of prebiotic phosphorylation on early Earth.

February 2020

A method to estimate the pre-eruptive water content of basalts

Accurate measurement of the original water content for subaerially erupted volcanic rocks is difficult due to the ubiquity of degassing during magma ascending. In this study, Di et al. developed a method to estimate the original water content of basalts by combining two magma thermobarometers. The estimated original water content of a continental flood basalt is as high as those of arc magmas. This finding supports the presence of hydrated deep mantle reservoirs as the sources of the large igneous provinces.

A new emerald occurrence from Kruta Balka, Western Peri-Azovian region, Ukraine

Franz et al. investigated emerald, the bright-green gem variety of beryl, from a new locality at Kruta Balka, Ukraine, and compare its chemical characteristics with those of emeralds from selected occurrences worldwide to clarify the types and amounts of substitutions as well as the factors controlling such substitutions. They applied secondary ion mass spectrometry and showed that—although beryl in general has quite variable Be-contents—emeralds are surprisingly simple. However, Li-contents, even small, are important; the new occurrence is unusual, because Li (together with Na) is substituting for Al, and not, as at most other emerald localities, for Be, and small amounts of Li can sit in a 'hollow' channel position. The green color of emerald is produced by the incorporation of small amounts of Cr and V.

Dissolution of poorly soluble uranyl phosphate Phases in the Metaautunite Supergroup

Uranyl phosphate minerals of the meta-autunite group are sparingly soluble in water under most conditions and have been targeted in remediation strategies for contaminated sites. Here Lobeck et al. demonstrated that under some aqueous peroxide-bearing conditions these minerals dissolve readily to form complex cage clusters built of uranyl hexagonal bipyramids that persist in solution as complex macro-anions.

Quartz crystals in Toba rhyolites show textures symptomatic of rapid crystallization

Barbee et al. studied quartz crystals in rhyolites erupted from Toba caldera (Indonesia) that show abundant textural signs of rapid crystallization. This study is the first to document diverse quartz textures as consequences of rapid, disequilibrium crystal growth in a caldera-forming rhyolite. Studies that seek to extract time information from the crystal record have found that trace element diffusion in quartz indicates crystallization, or residence, timescales that are much shorter than previously thought for large silicic magma systems. This work provides at least some physical, textural evidence that dynamic processes must operate on short geologic timescales in these systems. The documentation of skeletal to dendritic quartz textures is expected to provide vital context to interpreting the chemical stratigraphy of quartz and may impact the application of quartz as a petrologic tool.

Constraints on non-isothermal diffusion modeling

This study by Rout et al., through a series of diffusion experiments, evaluates the non-isothermal diffusion model, which gives more realistic estimates of diffusion time in natural samples, assesses the errors, and provides a simpler and more accurate approach to better implement the model.

Machiite, a new oxide mineral from the Murchison carbonaceous chondrite: A new ultrarefractory phase from the solar nebula

Krot et al. report on a discovery of a new ultra-refractory mineral, machiite, found in the CM carbonaceous chondrite Murchison. Machiite is intergrown with corundum and most likely formed by gas-solid condensation from a gas of solar composition at the very beginning of the protoplanetary disk evolution. Machiite is in O-isotope disequilibrium with corundum. They suggest that both minerals condensed from an 16O-rich solar nebula gas. Subsequently, machiite experienced O-isotope exchange with an aqueous fluid on the CM chondrite parent body.

Partitioning of V and 19 other trace elements between rutile and silicate melt as a function of oxygen fugacity and melt composition: Implications for subduction zones

Holycross and Cottrell detail the results of 16 experiments to measure the partitioning of 20 trace elements between the mineral rutile (TiO2) and silicate melt as a function of redox state and melt composition. Special attention is given to the behavior of the multivalent trace element vanadium (V), which may speciate as V2+, V3+, V4+, or V5+ in the silicate Earth. The distribution of V between rutile and melt is a direct function of both oxidation state and melt composition. Vanadium is strongly compatible in rutile when V4+ is the dominant species, and when melt compositions are highly polymerized and contain low concentrations of TiO2. Their new data indicate that rutile may be a significant sink for V in the solid Earth. The flux of V will be coupled to Ti during mass transfer through the subduction system when rutile is present and will change as a function of the oxidation state of the subducting slab.

Cl-bearing Fluorcalciobritholite in high-Ti basalts from Apollo 11 and 17: Implications for volatile histories of late-stage lunar magmas

Greenwood et al. report the second fluorine-bearing mineral found from the Moon, a Cl-bearing fluorcalciobritholite. This mineral has only been found in mesostasis of high-Ti magmas with fluorapatite. This F- and Cl-bearing mineral crystallizes after fluorapatite, implying that high-Ti lunar magmas were not volatile-poor.

Amphibole-rich cumulate xenoliths in the Zhazhalong intrusive suite, Gangdese arc: Implications for the role of amphibole fractionation during magma evolution

Although the importance of precipitating amphibole has been inferred through distinctive geochemical fingerprints of derivative products, this study by Zhou et al. provides a direct snapshot of this differentiation process. With progressive accumulation, the amphibole-rich sponge will form in the arc crust, which is fundamental to interpreting many issues in petrology. Particularly, the amphibole-rich xenoliths have petrological similarities to appinites, and their results suggest that some appinites likely represent amphibole-rich cumulates.

Extraterrestrial, shock-formed, cage-like nanostructured carbonaceous materials

Németh and Garvie report the first natural carbon nano-onions resembling large multilayered fullerenes and bucky-diamonds from the impact-shocked Gujba (CBa) meteorite. They propose that the carbon nano-onions formed from the primitive carbonaceous materials, whereas bucky-diamonds could have formed through the high-pressure transformation of nano-onions. Both nano-onions and bucky-diamonds are fullerene-type structures, and as such could be a component of the astronomical 217.5 nm absorption feature.

Minerals Matter: Pyrite: fool's gold records starvation of bacteria

This paper by Gregory and Kohn gives an introduction to a high-school level audience on pyrite, and the sulfur isotopes contained within it, and how that can be used to understand past ocean conditions.

The issue ends with a list of 2019 reviewers for American Mineralogist ( and a book review by Tony Barber on Geological Belts, Plate Boundaries, and Mineral Deposits in Myanmar (2017) by Andrew Mitchell, Elsevier, eBook ISBN: 9780128033838; Paperback ISBN: 9780128033821, 524 p. (

January 2020

Highlights and Breakthroughs

Elliott’s Highlights and Breakthroughs paper details how Li and Zhou describe the role of halloysite in sorbing rare-earth elements (REE) in a regolith setting in SE China. This mineral-based model explains the occurrences of the ion-sorbed REE in this regolith setting, which is a leading source of the heavy, and technologically important, rare-earth elements (Gd-Lu, Y).

Review Paper

The founding of the MSA in 1919 followed the discovery of X-ray diffraction so closely that one might hypothesize a causal link between the events. This review by Peter Heaney explores the efforts of the MSA founders to communicate the importance of the crystallography revolution to their contemporaries, and it argues that the then-new conceptions of atomicity differentiated mineralogy from other styles of geological investigation, thereby warranting the establishment of a specialized professional society.

Deep Earth Carbon Reactions through Time and Space

Carbon reactions take place in the deep Earth and have played a critical role in our planet's development. McCammon et al. describe these reactions, including carbon capture during Earth's formation, freezing of liquid iron-carbon to form Earth's inner core, and oxidation-reduction reactions in the modern-day mantle. Although these reactions are "invisible" because they are not observed on Earth's surface, our planet would be vastly different if none of these reactions had taken place during its history.

Magmatic Carbon Outgassing and Uptake of CO2 by Alkaline Waters

The Earth in Five Reactions project was part of the Deep Carbon Observatory program, a decade-long effort to understand the quantities, movements, forms, and origin of carbon in Earth. In this paper, Edmonds et al. review the reactions involved when carbon exsolves from silicate melts in magmatic intrusions and volcanic eruptions and the reactions that occur when carbon is dissolved into aqueous solutions, with a particular reference to alkaline lakes. They discuss the implications of our understanding of these natural reactions for forecasting the longevity and effects of anthropogenic carbon release.

New Insights into the Evolution of MVT Hydrothermal System

The Upper Yangtze Pb-Zn metallogenic province (South China) covers an area of 170,000 km2 and contains approximately 400 carbonate-hosted epigenetic Pb-Zn deposits totaling ~26 Mt of metal reserves. Despite the Wusihe deposit resembling Mississippi Valley Type, MVT, deposits in North America, it shows many distinctive geological and geochemical features warranting further investigation. Luo et al. take the Wusihe Pb-Zn deposit as a case study, employing LA-ICPMS in situ quartz trace element analysis, bulk and NanoSIMS in situ S isotopes, and femtosecond LA-MC-ICPMS in situ Pb isotopes to reveal the evolution of this MVT hydrothermal system. The trend of trace element compositions in quartz indicates the role of an acid-producing process resulting from sulfide precipitation and acid consumption by carbonate buffering. New bulk δ34S values of sulfides imply that in addition to thermochemical sulfate reduction (TSR), bacterial sulfate reduction (BSR) may also play an important role in the formation of S2-. In situ δ34S values suggests a mixing of two isotopically distinct sources of S2- produced by TSR and BSR. New galena Pb isotopic ratios suggest that the metal sources in the Wusihe deposit are mainly Proterozoic basement rocks. Hence, a multi-process model (i.e., basin-mountain coupling, fluid mixing, local sulfate reduction, in situ acid-production, and involvement of black shales and carbonate sequences) was responsible for the formation of the Wusihe deposit, whilst S2- was produced by both TSR and BSR, providing new insights into the evolution of MVT hydrothermal systems.

Celestine Discovered in Hawaiian basalts

Celestine was discovered in basalt from a Hawaii. This represents the first report of celestine in an oceanic basalt. Garcia and Hellebrand document the mode of occurrence of celestine in Ka'ula Island basalts using backscattered electron imagery, present high precision electron microprobe analyses of celestine, examine the effects of alteration on the geochemistry of Ka'ula basalts, and discuss possible origins for the formation of celestine in these rocks. Future studies of hydrothermally altered basalts from active volcanoes on oceanic islands, especially for basalts with elevated Sr contents (e.g., 1000 ppm), should be aware of the possible presence of celestine in moderately altered lavas.

Microstructural Controls on the Chemical Heterogeneity of Cassiterite Revealed by Cathodoluminescence and Elemental X-ray Mapping

Cassiterite (SnO2) has gained attention recently as a geochronometer for Sn-bearing mineralized systems, but there is no clear framework for the interpretation of cathodoluminescence imaging frequently employed prior to analysis. In this paper, Bennett et al. use a combination of hyperspectral cathodoluminescence and quantitative X-ray element mapping to constrain the growth history of cassiterite crystals.

Hornblende as a Tool for Assessing Mineral-Melt Equilibrium and Recognition of Crystal Accumulation

Recent studies have proposed that hornblende major element compositions can be used to calculate equilibrium anhydrous melt compositions. In this study, Fe/Mg partitioning relationships and hornblende chemometry are used to evaluate the extent to which hornblende from several plutonic and volcanic rocks are in equilibrium with their associated bulk-rock and/or glass compositions. Werts et al. found that hornblendes from many plutonic and volcanic rocks are not in equilibrium with their bulk-rock compositions; instead they are in equilibrium with melts that are more silicic than the bulk-rock sample, indicating that some degree of crystal accumulation and/or melt loss has occurred and requiring removal and/or redistribution of silicic melts. The application of hornblende partitioning relationships and chemometry to bulk-rock and glass samples has wide-ranging implications on discerning volcano-plutonic connections, the ways in which magmatic processes are determined and quantified, and on the distribution of melt compositions in the crust.

The Role of Clay Minerals in Forming the Regolith-hosted Heavy Rare Earth Element Deposits

Heavy rare earth elements (HREEs) have become significantly important in our modern society. The dominant source of HREE is from regolith-hosted deposits, presumably occurring as adsorbed on the clay minerals in weathering crusts. However, the actual relationship between the HREEs and clay minerals has not been comprehensively examined before. In this study, for the first time, Li and Zhou link the properties of clay minerals to HREE accumulation through comprehensive mineralogical and geochemical studies exemplifying the world's largest regolith-hosted HREE deposit, Zudong, South China. Abundant poorly crystallized halloysite and kaolinite of high specific surface area, pore volumes, and low crystallinity in the lower part of soil profiles transform to well-crystallized vermicular kaolinite of low specific surface area, pore volumes, and high crystallinity in the upper part of the soil profile during progressive weathering, which is correlated with a significant decrease in the clay-adsorbed REE concentrations. Variation in physicochemical properties of the clay minerals during progressive weathering is one of the key factors for REE accumulation and enrichment in weathering crusts to form the regolith-hosted deposits.

The Tetrahedrite Group: Nomenclature and classification

Biagioni et al. show how the new IMA-CNMNC approved classification of the tetrahedrite group allows the full description of the chemical variability of these widespread sulfosalts and is able to convey important information not only to mineralogists but also to ore geologists and industry professionals.

Caseyite, a New Mineral Containing a Variant of the flat-Al13 Polyoxometalate Cation

The new mineral caseyite records the presence of technologically important polyoxometalate ions in geochemical fluids, both as cations and as anions. This report by Kampf et al. is the first of a naturally occurring nanocluster resembling the “flat-Al13” polyoxocation that is used to make thin films for electronics, optics, and optoelectronics applications, and it lends greater credence to research suggesting that the flat-Al13 cluster is an important aqueous ionic species in natural systems. The discovery of a new flat-Al13 variant containing a V5+ substituent suggests new avenues for developing technologically useful modifications of the flat-Al13 structure.

Incorporation of Mg in Phase Egg

Bindi et al. demonstrate that Phase Egg, considered one of the main players in the water cycle of the mantle, can incorporate large amounts of Mg in its structure and that there exists a solid solution with a new hypothetical MgSiH2O4 end-member. The new hypothetical MgSiH2O4 end-member would be a polymorph of Phase H, a leading candidate for delivering significant water to the deepest part of the lower mantle.

Imaging Trace Element Zoning in Pyroxenes Using Synchrotron XRF Mapping with the Maia Detector Array: Benefit of Low Incident Energy

Microbeam XRF mapping with synchrotron radiation lets Barnes et al. reveal subtle trace element zoning in igneous phases. Scanning with incident energy below the Fe K edge improves sensitivity and precision. An entire thin section can be imaged in a few hours. This is a powerful technique for imaging and understanding crystal cargoes in magmatic systems.

Memorial of Enver Murad 1941-2019

American Mineralogist Volume 104

December 2019

This issue of American Mineralogist starts with a Centennial Review on high-pressure minerals by Tschauner (page 1701). The review is dedicated to the occurrence, relevance, and structure of minerals whose formation involves high pressure. Over the past ten years more high-pressure minerals have been discovered than during the previous fifty years. The previously unexpected richness in distinct high-pressure mineral species allows for assessing differentiation processes in the deep Earth.

Talla and Wildner (page 1732) performed detailed crystal-chemical and spectroscopic studies on a continuous solid solution series between kieserite and szomolnokite, (Mg,Fe)SO4·H2O, under ambient and Martian temperature conditions. Their verification of the existence of this sold solution series suggests that respective intermediate compositions are expected to occur in Fe-rich environments, such as the surface of Mars. The presented data can generally assist in the characterization of kieserite, szomolnokite and their solid solutions based on FTIR-, Raman-spectroscopic and structural data.

Brenna et al. (page 1750) investigate the role of crystal mushes on intra-eruptive trachyte-phonolite transition from natural evidences and experimental constraints. The transition of magma composition from trachyte to phonolite within a single eruption is an uncommon occurrence and cannot be derived from closed-system fractionation processes. The authors performed experiments to test the conditions that generated such a transition. Both experiments and natural samples suggest the formation of phonolite from trachyte as a result of open-system interaction between trachyte magma and extensively crystallized framework of feldspar with interstitial phonolitic glass.

Yakymchuk and Acosta-Vigi (page 1765) compare the measured concentrations of P2O5 in nanogranites and melt inclusions with existing solubility expressions for apatite in anatectic systems. They first identify discrepancies between the measured values and the predicted values and then use this information to discuss the implications for the behavior for apatite in anatectic systems. They provide a detailed analysis of the factors that contribute to apatite behavior in high-temperature metamorphic rocks and finally highlight some broader implications for the Nd isotope values of granites and the mechanisms of anatexis in migmatites and granulites.

Breton et al. (page 1781) conducted static compression experiments of γ’-Fe4N up to 77 GPa to examine its stability and elastic properties. From the obtained data together with existing data, the authors established a pressure-temperature phase diagram for the system Fe-N. The diagram shows the high-pressure stability of γ’-Fe7N3 to 56 GPa at 300 K beyond which β-Fe7N3 phase is stable. If Earth's core contains nitrogen, β-Fe7N3 is the most likely stable iron nitride phase in the solid inner core.

Almeida and Jenkins (page 1788) investigate the stability field (P-T-X) of an intermediate composition scapolite and compare the stability field of this Cl-rich scapolite with that of the end-member marialite. The results demonstrate that a small change in the scapolite composition from marialite to intermediate scapolite (Ma85Me15) causes a shift in its stability, relative to the plagioclase plus salt assemblage, by 260 °C from 990 to 730 °C at 2.0 GPa. The combined effects of a broader thermal stability field and tolerance to lower salinities mean that intermediate scapolites will be more commonly encountered in shallow- to mid-crustal levels at temperatures of ~500 °C or higher.

Amulele et al. (page 1800) conducted high-pressure electrical conductivity measurements, via impedance spectroscopy, on the silica polymorphs quartz and coesite. The authors compare their data to previously published values for quartz and stishovite and apply these data to a magnetotelluric profile to provide geophysical context. This study holds significance as quartz and the associated polymorphs of coesite and stishovite are prevalent in the Earth's crust and upper mantle. The authors propose that these data and the interpretations therein will serve as a useful tool to the geophysical and mineral physics communities.

Owen et al. (page 1806) investigated REE-, Sr-Ca-aluminum-phosphate-sulfate (APS) minerals of the alunite supergroup and their role as hosts for radionuclides. Woodhouseite-svanbergite-dominant APS assemblages are interpreted as paragenetically late with respect to the Cu-(Fe)-sulfides and associated gangue which they consistently replace (they also replace earlier REE-bearing phosphates). This interpretation adds to the growing body of evidence for a low-temperature hydrothermal overprint at Olympic Dam and represents a valuable contribution to a holistic mineralogical-geochemical-geometallurgical model of REE behavior in the Olympic Dam IOCG-U-Ag deposit. The observed sorption properties of the Ca-Sr-dominant APS minerals and their ability to scavenge radionuclides from the 238U decay chain from 226Ra onwards highlight both the need to suppress natural APS phases in copper concentrates and prevent crystallization of new, anthropogenic APS phases during leaching or elsewhere during the processing-refining cycle. Alternatively, the capacity of APS minerals to scavenge radionuclides could be used in the removal of the products of U decay from processing streams, provided that these phases can be re-dissolved or otherwise removed from the final product.

Vigliaturo et al. (page 1820) determined the Fe-valence state on amosite-asbestos boundaries using acSTEM dual-electron energy-loss spectroscopy (EELS). The authors determined the Fe-valence state from the core-loss region of the electron energy-loss spectra and compared the results with those obtained independently from X-ray photoelectron spectroscopy and a simple geometric model of the fiber boundaries. Dual-EELS allows collection of simultaneously low-loss and high-loss spectra, making it possible in the near future to apply this approach to the "soft" organic materials, which can be intimately associated with asbestos fiber surfaces after the interaction with human cells. This study will help in promoting the use of acSTEM dual-EELS in both medical mineralogy and general mineralogy.

Oberti et al. (page 1829) investigated the thermoelastic behavior, cation exchange and deprotonation processes of a Fe-rich holmquistite using high-temperature single-crystal X-ray diffraction up to 1023 K. The results contribute to paving the way towards a general crystal-chemical model that can explain how orthorhombic amphiboles responds to changes in physiochemical conditions, especially high temperature, and allows the release of water in diverse geological settings.

Lorenz et al. (page 1840) conducted a detailed textural and geochemical study on a relatively unknown but important rare-earth element bearing mineral group called britholite. The studied britholite occurrence at the Rodeo de los Molles deposit, central Argentina, is exceptional in its high concentration of this unusual mineral within vein mineralizations. The authors discovered intergrowth textures of apatite and britholite, which makes it very likely that a miscibility gap between these two phases exists. The paper also discusses the strong alteration of the mineral, which could be of particular interest for the field of nuclear waste containment, as phases with the britholite structure have been used to store liquid nuclear waste.

Kampf et al. (page 1851) describe a new mineral, bicapite, KNa2Mg2(H2PV5+14O42)·25H2O, from the Pickett Corral mine, Montrose County, Colorado. The structure of this mineral contains the [H2PV5+12O40(V5+O)2]7- protonated bicapped α-isomer of the Keggin heteropolyanion. The mines of the Colorado Plateau are a rich source of minerals with complex heteropolyions, which have formed in low-temperature, post-mining mineral assemblages. The discovery of these minerals advances our knowledge and understanding of mineral complexity on Earth, provides new insights into complex ions that can exist in near-surface environments, and informs researchers in other fields of new avenues to explore in developing phases with potential technological uses.

Sokol et al. (page 1857) investigated carbon solubility in a Fe melt and the iron-loving behavior of C and N in a carbon-saturated Fe-C-N-S system at 5.5 and 7.8 GPa and 1450-1800 °C. Carbon becomes twice less soluble as nitrogen increases in a graphite-saturated melt but the solubilities of C and N are commensurate in the presence of 1-1.7 wt.% S. Nitrogen partitioning between diamond and an N- and S-bearing Fe melt, DNDm/Met, is in the range 0.012-0.025. This study is another step forward to the understanding of nitrogen "deficit" and carbon "excess" in the mantle.

Belakovskiy et al. (page 1866) introduce 11 new minerals, including cesiodymite, cryptochalcite, feodosiyite, fluoro-tremolite, itelmenite, ozerovaite, ramazzoite, redcanyonite, selivanovite, vanderheydenite, and wrightite.

This issue ends with an Editors’ Note (page 1872) on mineral localities in Kaminsky et al. (2019, American Mineralogist, 104, 140-149).

November 2019

Fu et al. (p. 1533). Granite is one of the most abundant rock types of the continental crust. Heat transfer and temperature distribution in the crust could be strongly influenced by the thermal properties of granite. In this study, the thermal diffusivity and thermal conductivity of four natural granitoids are simultaneously measured under high pressure and high temperature. The calculated geotherms suggest that the presence of partial melting induced by muscovite or biotite dehydration likely occurs in the upper-middle crust of southern Tibet. This study provides new insights into the origin of low-velocity and high-conductivity anomaly zones revealed by geophysical observations in this region.

Trudu et al. (p. 1546). A long-standing issue is the presence of linear Si-O-Si bridges in silicates. We investigate the structure of the all silica zeolite ferrierite and explain the origin of the instability of its linear linkages. The relationship between linear Si-O-Si angles and metastability of a zeolite framework is a key to better understanding of the possible phase-transition mechanisms of open-framework (alumino-) silicate minerals at elevated pressure and temperature conditions of geological relevance.

Díaz et al. (p. 1556). Composite materials based on natural stilbite zeolites from Faroe Islands have been prepared where nanoparticles of hydroxyapatite are grown in a particular orientation on the zeolite external surface. Such nanoparticles are very efficient for the removal of fluoride from water through anion-exchange with hydroxides, allowing reducing the fluoride concentration below the admissible level.

Flahaut et al. (p. 1578). The characteristics and mineralogic diversity of the Italian Solfatara fumarolic field were surveyed as an analog for some of Mars' hydrothermal environments. Several techniques of mineral identification (VNIR spectroscopy, Raman spectroscopy, XRD) were used and reveal the presence of varying alteration patterns as a distance from the crater center and vents. The authors discuss the possibility of identifying such environments -which represent prime astrobiological targets- on Mars.

Xie et al. (p. 1565). The results of our research have two major implications. Firstly, it can be used for continuous monitoring of burial sites and other geothermal engineering projects. The acoustic emission (AE) energy distribution exponent can easily be measured using highly sensitive thermostability AE sensors attached to the host rock or normal sensors located around the burial site. The data analysis method used in this study also can be easily implemented through online evaluation of the AE signals. In addition, the relationship between mineral structure and avalanche dynamics is the focus of many studies of crackling noise. Here we show that avalanches are heavily influenced by temperature and that local 'crumbling' is a key mechanism for softening of geomaterials (rather than elastic softening). This result will now attract more theoretical and simulation studies in this field.

Williams et al. (p. 1585). The Adirondack Highlands have been held as an example of the middle crust during continental collision. Gneissic rocks interpreted to have undergone partial melting are common, but the timing and setting of melting are uncertain. This contribution uses in-situ monazite petrochronology to suggest that melting occurred at two main times, ca. 1.15 Ga and 1.05 Ga. Further, the amount of melting in the second event depends on the degree of melt segregation in the first event. High-resolution microprobe mapping and analysis provide a powerful tool for linking geochronology to deformation fabrics and metamorphic reactions.

Oka et al. (p. 1603). We performed melting experiments on Fe-O alloys to 204 GPa and determined the change in liquidus phase diagram in the Fe-FeO system, in particular the eutectic liquid composition, with increasing pressure on the basis of ex-situ textural and chemical characterizations of recovered samples. Our main conclusions are the following: (1) The oxygen content in the Fe-FeO eutectic liquid increases remarkably upon reduction in the immiscible two-liquids region to ~40 GPa. (2) Such observation is different from that in the recent study by Morard et al. (2017) that was mainly based on in-situ XRD measurements. The difference is likely attributed to the contamination by carbon that occurred in their experiments and some of ours. (3) Our data to 204 GPa allows extrapolation to 330 GPa, indicating the eutectic liquid includes ~15 wt% O at the ICB. We estimated the range of possible liquid core compositions in Fe-O-Si-S-C, which crystallize solid Fe that forms a dense inner core and account for the density and sound velocity observed in the outer core.

Cuadros et al. (p. 1608). The controls on Fe(III) distribution between tetrahedral and octahedral sites in phyllosilicates have been a matter of great interest in order to understand the interplay between formation environment and crystal-chemical factors during crystallization of Fe-phyllosilicates. Here, for the first time, we present a model describing the controls of Fe(III) distribution between octahedral and tetrahedral sites in all 2:1 phyllosilicates. The results are from a survey of 70 analyses covering a wide range of dioctahedral and trioctahedral phyllosilicates, including endmember and interstratified minerals. Dioctahedral phyllosilicates have a steric control whereby tetrahedral Fe(III) is only allowed if at least five out of six octahedral atoms are larger than Al (typically Fe[III], Fe[II], Mg). In this expanded structure tetrahedral sites can accommodate Fe(III). After this threshold, ~73 % of further Fe(III) atoms occupy tetrahedral sites. In trioctahedral 2:1 phyllosilicates there is no steric hindrance; on average, Fe(III) enters tetrahedral and octahedral sites in similar proportion, and the only control on Fe(III) abundance is Fe(III) availability during crystallization.

Zhang et al. (p. 1620). In this study, Fe-bearing phase E (one of the dense hydrous magnesium silicates) coexisting with ringwoodite and wadsleyite has been synthesized at 18 and 19 GPa, and 1400 °C. The long heating duration time (27 hours) of syntheses implies that phase E may be a stable component of the mantle transition zone at near-geotherm temperatures, if transition-zone regions adjacent to subducting slabs are hydrated by fluids generated at the top of the lower mantle.

Maner IV et al. (p. 1625). The enrichment of Mn to the point of spessartine saturation in granitic liquids is explained using experimentally derived mineral-melt partition coefficients for garnet, tourmaline, and cordierite. The partition coefficients are used in a Rayleigh fractional crystallization model to show the amount of crystallization required to concentrate enough Mn to saturate melt with respect to spessartine. The model indicates that approximately 95% of an S-type granitic melt must crystallize to achieve spessartine saturation.

Cherniak and Watson (p. 1538). This paper reports on experimental measurements of Al and Si diffusion in rutile, investigating effects of oxygen fugacity and orientation on diffusion. Little diffusional anisotropy is observed, with only minor effects of oxgyen fugacity over the range of conditions studied. Si and Al are among the slowest-diffusing species in rutile. These slow diffusivities indicate that the recently developed Al-in-rutile crystallization geothermobarometer (Hoff and Watson, 2018) will be more resistant to diffusional alteration than the Zr-in-rutile crystallization thermometer.

Wei et al. (p. 1650). We have determined the compressional-wave velocity (VP) of Ne at simultaneously high P-T condition up to 53 GPa and 1100 K. The obtained results provide crucial constraints on the combined effect of pressure and temperature on VP. In particular, VP of super-critical Ne fluid exhibit a weak dependence on pressure above 800 K and 20 GPa. The velocity contrast in VP between the super-critical Ne fluid and the solid phase increases with increasing temperature.

O'Bannon, III and Williams (p. 1656). Topaz is an important subduction zone mineral that transports water and fluorine into the deep Earth. Previous studies have characterized the bulk crystallographic response of topaz to high-pressure. Here we used Cr3+ luminescence which is a sensitive site-specific probe of the bonding environment of the Al-sites in topaz. We have characterized for the first time the high-pressure behavior up to 60 GPa of all three Al environments simultaneously. Our results reveal that topaz is one of the most metastable tetrahedrally coordinated silicates that is known.

Pittarello et al. (p. 1663). Mesosiderite meteorites consist of a mixture of crustal basaltic or gabbroic material and core metal, and have experienced an extremely slow cooling rate from ca. 550 °C, as recorded in the metal (0.25-0.5 °C/Ma). Here we present a detailed investigation of two generations of exsolution features in a pyroxene from the Antarctic mesosiderite Asuka 09545, in an attempt to constrain the evolution of pyroxene from 1150 to 570 °C.

Epp et al. (p. 1688). Pyromorphite-group minerals are the most common Pb-secondary minerals worldwide and their halogen incorporation is not only governed by crystallographic effects, but more predominantly by the fluid-composition. This enables further fluid monitoring applications such as deciphering single fluid flow episodes and the determination of surface processes. Furthermore, we developed a methodology to simultaneously determine Cl, F, Br and I. Since halogen data, especially concerning Br and I, is generally scarce, this is of interest to the broad geoscience audience.

Feisel et al. (p. 1689). We applied the diffusion couple technique on a homogeneous rhyodacitic melt over a temperature range relevant for natural systems. We analyzed the samples for diffusion induced concentration profiles using an electron microprobe. Our analytical conditions allow for very low F and Cl detection limits, which in turn results in very clear and smooth F- and Cl-concentration gradients across the experimental couples. Our results show that both F and Cl exhibit the same temperature dependence on diffusivity, but the diffusion of F is significantly faster than that of Cl. This means that the potential for diffusive fractionation exists and may occur especially under conditions of slow magma ascent and bubble growth. Compared to other melt compositions for which F and Cl diffusion data is available, our diffusivities are generally slower and correlate well with the atomic radius of the diffusing ion. With this investigation we have conducted the first study that quantifies and compares the diffusion rates of F and Cl elements in natural high-silica melts.

October 2019

Kellogg et al. (page 1365) discuss two alternative hypotheses on the origin of carbon in Earth's continental crust. In the first hypothesis, the atmosphere is extracted directly from the early atmosphere through the Urey reaction. In the second hypothesis, the atmosphere is extracted from the mantle by volcanism. The authors argue that the current equilibrium content of carbon in the atmosphere is a balance between injection by volcanism and loss by the Urey reaction.

Stewart et al. (page 1369) discuss carbonation and decarbonation reactions, which are the primary mechanism for transferring carbon between the solid Earth and the ocean-atmosphere system, and their implications for Earth's climate and planetary habitability. These processes can be broadly represented by the reaction: CaSiO3 (wollastonite) + CO2 (gas) = CaCO3 (calcite) + SiO2 (quartz). The authors summarize their significance as part of the Deep Carbon Observatory’s “Earth in Five Reactions” project.

Sugiura et al. (page 1381) investigated the effects of PO4 on CaCO3 formation from calcium sulfate anhydrate (CaSO4) at high-pH conditions, under which calcite is likely to form. PO4 regulates calcite formation and growth by adsorbing onto the surface of calcite. Therefore, under high PO4 concentrations, calcite crystals become small and porous. In addition, vaterite is likely to form to counteract the thermal instability of the solution. The results also indicate that PO4, which is an essential component of carbonate apatite, can control the physico-chemical properties of CaCO3, which is a precursor of carbonate apatite, a bone-replacement material.

Vennari and Williams (page 1389) conducted high-pressure Raman and Nd3+ luminescence spectroscopy of bastnäsite, a rare earth element (REE)-bearing carbonate that has geologic and economic importance due to its high REE content. The high-P measurements on bastnäsite allowed investigation of bonding changes in carbonate ions juxtaposed with halogens and REE within a carbonate matrix. This study provides evidence for how REE ions are coordinated with carbonate ions at depth with implications for both REE and carbon retention at depth.

Hill et al. (page 1402) observed nanocrystals of -cristobalite that are epitaxially exsolved in cores of ultrahigh-pressure (UHP) clinopyroxene from the Bohemian Massif, Czech Republic, using transmission electron microscopy. This study revealed vacancies and OH- in high-pressure pyroxene that formed during subduction. When the host rock is exhumed, the incorporated OH- migrates out from the UHP pyroxene structure together with vacancies and excess silica in multiple forms of glass and cristobalite/amphibole precipitates. If the pyroxene-bearing slab subducts further into the mantle, the pyroxene may release the incorporated OH- into mantle during its phase transformation to garnet and/or perovskite.

Xu et al. (page 1416) investigated phase relations in the AlOOH–FeOOH system at 15–25 GPa and 700–1200 °C. δ-AlOOH and ɛ-FeOOH, which have a CaCl2-type structure, were found to form partial solid solutions over wide ranges of pressures and temperatures. This result implies that CaCl2-type hydroxides may carry a certain amount of hydrogen into deeper regions of Earth interior and even Fe-rich Martian core in cold subduction regions.

Tamer et al. (page 1421) investigated analyst and etching protocol effects on the reproducibility of apatite confined fission-track length measurement, and ambient-temperature annealing at decadal timescales. The authors show that fission-track etching is a continuous process, and they illustrate possible differences between two major apatite fission-track etching procedures, analyst biases on track recognition and measurements, and long-term ambient temperature fission-track annealing.

Wang et al. (page 1436) developed a new approach combining multiple scanning/transmission electron microscopic techniques (HRTEM, STEM with EDS) to identify mixed layers in a chlorite sample. Since interstratified mineral phases appear only as a few layers, they are difficult to identify by conventional methods. Via lattice spacings, HRTEM and HAADF images, and EDS mapping, the authors distinguished the parallel-growing monolayer mica and pyrophyllite from chlorite domains. This method may be extended to the studies of other minerals, especially those e-beam sensitive phases.

González-García et al. (page 1444) measured the diffusive exchange of six major elements in diffusion couple experiments, using natural shoshonitic and rhyolitic melts as endmembers. Experiments were run at atmospheric pressure and temperatures between 1230 and 1413 °C, and the concentrations were measured by electron microprobe. Arrhenius relationships were obtained. The results suggest that diffusive coupling is prevalent, i.e., all elements show similar diffusivities. These also expand a previous database with variably hydrous melts, allowing establishment of a linear relationship between diffusivity and the square root of water concentration. The results are of particular interest in the study of mass transfer phenomena in alkaline volcanic systems.

Fei et al. (page 1455) investigated the factors that control the crystal morphology and chemistry of garnets in the Cuihongshan polymetallic skarn deposit, China. Major findings include: 1) Strain from lattice mismatch at substitution and twin probably causes the birefringence in garnets; 2) REE concentrations are probably influenced by the relative proportion and temperature of the system; 3) LREE-HREE fractionation of garnets can be attributed to relative compositions of grossular-andradite system; and 4) W and Sn concentrations in garnets can be used as indicators for the exploration of W-Sn skarn deposits.

Vereshchagin et al. (page 1469) report a new mineral, gasparite-(La), La(AsO4), from Mn ores of the Ushkatyn-III deposit, Central Kazakhstan and from alpine fissure in metamorphic rocks of the Wanni glacier, Binn Valley, Switzerland. Gasparite-(La) has a monoclinic structure with the space group P21/n. The samples from the Ushkatyn-III deposit and Wanni glacier have different origins. La/Ce and As/P/V ratios in gasparite-(La) may serve as an indicator of formation conditions. Raman spectra of gasparite-(La) can be used to identify its origin.

Stachowicz et al. (page 1481) investigated the cation ordering, valence states, and symmetry breaking in a Nb-rich chevkinite-(Ce) from the Biraya rare-metal deposit, Russia, using electron probe microanalysis, X-ray diffraction, and photoelectron spectroscopy. Annealing the sample at 750 °C resulted in the structural transformation C2/mP21/α, which defines chevkinite stability relations. This transformation seems to be a rapid version of a naturally occurring process that possibly involves twinning of the crystals. Nb-rich chevkinite-(Ce) occurs naturally as two polymorphs, C2/m and P21/α, the latter of which is the stable form under ambient conditions. There are some distinct differences between the two structures, such as their mean M-O distances and site scattering values of particular sites, which can be associated with the redistribution of lighter cations, mainly Mg2+, within the structures.

Qiu et al. (page 1487) present geochronological data showing that Zr and REE mineralization events of the Baerzhe rare-metal deposit in Northeastern China spanned roughly two million years. The Zr and REE mineralization events are temporally distinct but both are genetically related to the same mineral system. Three distinct types of magmatic through deuteric zircon along with hydrothermal monazite are considered the most significant ore minerals.

Marger et al. (page 1503) present one of the first in-situ oxygen isotope analyses from zoned tourmaline crystals using secondary ion mass spectrometry. The authors document that tourmaline is refractory for oxygen isotope compositions, and can be used to track fluid-rock interactions. They developed a set of tourmaline reference materials spanning the composition range of the most abundant metamorphic and igneous tourmalines. By combining a detailed petrologic study with the oxygen isotope compositions of quartz and tourmaline, this study demonstrates that oxygen isotopes measured in-situ can be used to resolve the debated question about whiteschist formation in the western Alps. It also shows that the protolith of the whiteschists were Hercynian granites, and the metasomatism predated Alpine metamorphism.

Ma et al. (page 1521) report a new mineral, chenmingite, FeCr2O4, with a Pnma CaFe2O4-type structure. This is the third high-pressure mineral the authors have discovered (in addition to ahrensite and tissintite) during their nanomineralogy investigation of the Tissint shergottite. Further, chenmingite is one of thirteen newly-approved high-pressure minerals discovered in shocked meteorites since 2013. Their meteoritic occurrences can also inform the study of deep earth systems at high pressures and temperatures.

Satta et al. (page 1526) determined the elastic properties of phase E single crystals using Brillouin spectroscopy. The results show that phase E has the lowest isotropic compressional and shear velocities among the Dense Hydrous Magnesium Silicate (DHMS) phases and other relevant upper mantle and transition zone minerals. This study suggests that even small amounts of phase E can significantly reduce acoustic wave velocities in isotropic aggregates, opening the possibility of linking the presence of water-rich phase E assemblages with velocity anomalies detected by seismology in subduction scenarios.

Chen (page 1530) reviews a book entitled “Understanding Minerals and Crystals” by Terence McCarthy and Bruce Cairncross (2015), Struik Nature, 312 pp. The focus of this book is mineral resources education, and it distinguishes itself from other mineralogy books by containing rich conceptual and visual content as an introductory guide for mineralogy.

Johnson (page 1531) reviews a book entitled “A Practical Guide to Rock Microstructure (2nd edition)” by Ron H. Vernon (2019), Cambridge University Press, 440 pp. This book contains detailed descriptions of multiple hypotheses on the generation of each texture and provides extensive references for exploring the origin of textures in more depth. It can serve as a reference for graduate students, professionals, or undergraduate research projects.

September 2019

Steele-MacInnis this issue (page 1217) highlights the recent publications by Gavrilenko et al. (July issue, p. 936; that investigated the contradiction between experimental studies that predict that primitive arc melts may contain up to and greater than 15 wt% H2O, but that, curiously, the breadth of analytical data on melt inclusions consistently show to have lower values, mostly less than 6 wt%. Gavrilenko et al. showed experimentally that this apparent contradiction is likely rooted in a “quench control,” whereby wetter melts are incapable of being quenched to glass. This study neatly reconciles experimental and observational data and provides a key insight into how best to analyze and interpret the H2O contents of melt inclusions from subduction settings.

Takahashi (page 1219) discusses the breakthrough by Sato and Ozawa’s study of the lithosphere under the Japanese subduction zone (page 1285; The LABZ (lithosphere-asthenosphere boundary Zone) under oceanic plates, young continents and subduction zones has remained largely in question, due to the lack of proper geobarometry for spinel lherzolite facies. Y. Sato and K. Ozawa, for the first time, describe the petrologic features of the LABZ beneath a subduction zone. A reconstructed LABZ beneath Ichinomegata is similar to those reported from the bottom of the subcratonic lithospheric mantle in various aspects, but the boundary layer beneath Ichinomegata is much shallower (40-60 km) and colder (~1050 °C).

Hikosaka et al. (page 1356) present the results of their study of the stability of Fe5O6 and Fe4O5 at high pressures. While several new iron oxides were discovered above 10 GPa in the last decade, their stabilities at high pressure and temperature (P-T ) are not understood yet. Here we examined phase relations in both Fe5O6 and Fe4O5 and found that both decompose into the FeO + Fe3O4 assemblage above ~40 GPa. It indicates that the intermediate compounds between FeO and Fe3O4 (i.e., Fe4O5, Fe5O6, Fe7O9, and Fe9O11) are formed only in the deep upper mantle to the shallow lower mantle.

Boujibar et al. (page 1221) investigate the origin of the elevated concentration of K relative to U and Th on the surface of Mercury found by the MESSENGER mission. These measurements suggested an enrichment of volatile elements (such as K) relative to refractory elements (such as U and Th), in comparison to Earth and Venus. They conducted experiments in an O-depleted environment typical of Mercury's formation, to assess whether K, U, and Th were distributed differently between the different layers inside Mercury (mantle, core, and possible sulfide layer) and found that if an iron sulfide layer exists between the core and mantle, more U and Th then K would be sequestered in that layer, leading to higher K/U and K/Th ratios in the crust and mantle, measurable on the surface by MESSENGER spacecraft. By taking into account this possible sulfide layer, Mercury appears to be as volatile depleted as the Earth and Venus and more depleted than Mars. These results confirm the depletion of volatile elements in the inner part of the Solar System, where Mercury, Venus, and Earth formed.

Lee et al. (page 1238) study a new magnetic mineral (valleyite) that was discovered in a basaltic scoria. Valleyite has sodalite-type structure with low density. The magnetization hysteresis loop indicates the magnetic exchange coupling between valleyite (soft magnet) and luogufengite (hard magnet), which will help us better understand magnetic properties and paleo-magnetism of basaltic rocks. The new mineral with the sodalite-type cage structure, and the magnetic property is potentially a functional material.

Geiger et al. (page 1246) analyzed the magnetic and and Néel temperature, TN, properties of four silicate solid solutions. The four systems are: fayalite-forsterite olivine, Fe2+2SiO4-Mg2SiO4, and the garnet series, grossular-andradite, Ca3(Alx,Fe3+1-x)2Si3O12, grossular-spessartine, (Cax,Mn2+1-x)3Al2Si3O12, and almandine-spessartine, (Fe2+x,Mn2+1-x)3Al2Si3O12.

Mukherjee et al. (page 1256) demonstrate that the trace-element concentrations (and their ratios) of sedimentary pyrite in three black shale formations of the McArthur Basin provide evidence for a gradual increase in atmospheric oxygenation from 1730 to 1360 Ma. They observe a marked change in pyrite sulfur isotopic compositions in the three black shale formations, i.e., a marked increase in mean δ34Spyrite values from the Wollogorang Formation to the Velkerri Formation. This change is possibly indicative of the expansion of oxygenated waters and decreasing areal extent of anoxia. Results from both techniques have major implications on the atmospheric redox evolution in “Boring Billion”, a period known to witness non-fluctuating redox conditions.

Ohira et al. (page 1273) investigate the δ-(Al,Fe3+)OOH solid solution, one of the hydrous phases stable under lower mantle conditions. This study shows that the δ-(Al,Fe)OOH could cause an anomalously high ρ/νΦ ratio at depths corresponding to the spin crossover region (~900 to ~1000 km depth), whereas outside the spin crossover region a low ρ/νΦ anomaly would be expected. The results suggest that the presence of δ-(Al,Fe)OOH could be detectable and provide new insight for understanding the heterogeneity in the lower mantle.

Sato and Ozawa (page 1285) develop a method for accurate estimation of derivation depths of shallow upper mantle materials (xenoliths) occurring as fragments in volcanic ejecta. Its application to xenoliths from the Ichinomegata maar in the back-arc side of Northeast Japan Arc was successful in revealing, for the first time, the structure of lithosphere-asthenosphere boundary zone in arc settings, which was found to consist of water-saturated lithospheric mantle and underlying partially molten asthenosphere.

Li et al. (page 1307) investigate ammonia-water mixtures that are an important component of the giant ice planets. Using Raman spectroscopy, they investigate the phase stability of ammonia hydrate (AHH) in different ratios at high pressures. The experimental results show that all the ammonia hydrate will dehydrate into ammonia hemihydrate and ice-VII. More importantly, ice-VII will gradually separate out from ammonia hydrate and could grow to be a single crystal. They also measure the sound velocity and elasticity of ammonia hydrate and single-crystal ice-VII using Brillion scattering up to 53 GPa. Measured elasticity of ice-VII shows anomalous variations at 10-20 GPa and 42-53 GPa, respectively, which is associated with the structural change of single-crystal ice-VII. Modeling the velocity of AHH and ice-VII mixture indicates that the mantle of ice giants may have strong anisotropies in velocity.

Diego Gatta et al. (page 1315) re-investigate the crystal structure and crystal chemistry of kurnakovite by a multi-methodological approach (i.e., single-crystal neutron diffraction at room and low temperature, titrimetric analysis for the determination of B and Mg content, inductively coupled plasma atomic emission spectroscopy for REE and other minor elements, ion selective electrode for F, high-T mass loss for H2O content). Kurnakovite does not act as a geochemical trap of industrially relevant elements (e.g., Li, Be, or REE). It is a potentially B-rich addition to concretes used for the production of radiation-shielding materials due to the elevated ability of 10B to absorb thermal neutrons, because it can mitigate the risk of releasing undesirable elements, for example sodium, which could promote deleterious reactions affecting the durability of cements.

Yuan et al. (page 1323) treated diatomaceous opals using Focused Ion Beam (FIB) to characterize separately the composition of the internal siliceous parts and the surface/near-surface locations by means of EDS on a TEM. The results demonstrate that minor elements, such as Al, Fe, Ca, and Mg, conclusively exist within the siliceous framework. In addition, foreign minerals (mostly aluminosilicates) largely contribute to the bulk and surface properties of diatomaceous opal. These findings renew the knowledge that diatomaceous opal is “pure” silica mineral or materials.

Anzolini et al. (page 1336) report the discovery and full description of a new mineral, nixonite, ideally Na2Ti6O13 (IMA 2018-133). Nixonite was found as a part of a complex reaction rim around a rutile grain within a heavily-metasomatized pyroxenite xenolith from the Darby kimberlite field, beneath the west-central Rae Craton, Canada. Nixonite is the first occurrence in nature of Na2Ti6O13, which was previously known only as a synthetic material used in batteries, and is the Na-rich analogue of jeppeite, K2Ti6O13, which is commonly observed as a groundmass mineral in lamproites. This finding represents not only the first natural occurrence of Na2Ti6O13, but also the unique coexisting assemblage of the minerals rutile, priderite, perovskite, freudenbergite, ilmenite, and nixonite. We suggest that this complex Na-K-Ti rich metasomatic mineral assemblage may have been produced by an unusual metasomatic melt that percolated through the lithospheric mantle beneath the Darby field.

Meyer et al. (page 1345) report the discovery of the new mineral Goldschmidtite. New minerals are not common after years of mineral research, and new perovskite-structured minerals are even more rare. This new mineral, coming from a diamond, highlights a case of extreme metasomatic conditions in the lithosphere. Remarkably, this mineral is a well-known synthetic in ceramic science. It is also, justifiably, named after one of, if not the, founders of geochemistry, whose scientific impact is well known and far reaching.

Chi Ma and Alan Rubin (page 1351) report the discovery of the new mineral edscottite that occurs with low-Ni iron (kamacite), taenite, nickelphosphide (Ni-dominant schreibersite), and minor cohenite in the Wedder-burn iron meteorite, a Ni-rich member of the group IAB complex. The end-member formula is Fe5C2. The new mineral is named in honor of Edward (Ed) R.D. Scott, a pioneering cosmochemist at the University of Hawai’i at Manoa, for his seminal contributions to research on meteorites. Since the Carbon Mineral Challenge was launched in 2015, edscottite is the first carbide mineral approved by the IMA-CNMNC.

Belakovskiy et al. report on the new mineral names (page 1360). The paper contains entries for nine new minerals, including argentotetrahedrite-(Fe), bytízite, calamaite, fluorlamprophyllite, honzaite, katerinopoulosite, meitnerite, melcherite, and rozhdestvenskayaite-(Zn).

August 2019

This issue of American Mineralogist starts with the MSA Presidential Address given by Brown (Past-President) and Johnson (page 1065): Metamorphism and the evolution of subduction on Earth. Whether Earth always had plate tectonics or, if not, when and how a globally-linked network of narrow plate boundaries emerged are matters of debate. Here the authors use a dataset of the pressure (P), temperature (T), and age of metamorphic rocks to evaluate secular change associated with subduction and collisional orogenesis at convergent plate boundaries. The widespread appearance of two types of metamorphism with different thermobaric ratios (T/P) at the beginning of the Neoarchean is interpreted to be evidence of the stabilization of subduction during the emergence of plate tectonics.

Cerantola et al. (page 1083) studied the stability of siderite (FeCO3) in the Earth's lower mantle. The experimental results using Fe K-edge X-ray absorption near-edge structure (XANES) spectroscopy are supported by first-principles calculations and match well with recently reported observations on FeCO3 at extreme conditions. At conditions of the mid-lower mantle, ~50 GPa and ~2200 K, FeCO3 melts and partially decomposes to high-pressure Fe3O4. Diamond and oxygen are also inferred products of the reaction. Moreover, the incongruent melting of FeCO3 could be a key mechanism that partially preserves FeCO3 from decomposition, potentially supporting carbon influx into the deep Earth via carbonate subduction.

Pan et al. (page 1092) conducted a comparative study of fluid inclusions in coexisting wolframite and quartz crystals from a giant vein-type tungsten deposit, South China. A combined in situ analytical procedure, including cathodoluminescence (CL) imaging, infrared microthermometry, Raman microspectroscopy, and fluid inclusion LA-ICP-MS analysis, was used to reconstruct the detailed fluid evolution history. Based on elaborate petrography on the crystal scale, fluid inclusions in wolframite and coexisting quartz are found to exhibit distinct chemical signatures, despite sharing similar overall ranges of salinity and homogenization temperature. The revealed fluid evolution history provides insight into the fluid source and the wolframite deposition mechanism in vein-type tungsten deposits.

Rusiecka and Baker (page 1117) obtained new data on monazite and xenotime solubility in boron-bearing rhyolitic melts at 1000-1400 °C and 800 MPa in a piston cylinder apparatus, as well as the diffusivity of the components of these two minerals (LREE, P, and Y). This study provides first-ever data on the solubility of xenotime-(Y) in rhyolitic melt, as well as new data on monazite solubility in the boron-bearing rhyolitic melt. The authors also discussed implications of the results on the understanding of natural, silicic (granitic/rhyolitic), and magmatic systems.

Moy et al. (page 1131) report a new low-accelerating voltage electron probe microanalysis (EPMA) method to quantify Fe using the combined Fe L X-ray lines. This method takes advantage of the reduction of the electron beam offered by Schottky field emission source in conjunction with the reduction of the electron interaction volume permitted by decreasing the accelerating voltage from traditional 15-20 kV to 7 kV, to accurately quantify submicrometer-sized features. The method has been successfully applied to olivines as a test bench. Compared with large errors using the traditional EPMA quantification method using the Fe L X-ray line, the new method gives much more accurate results, with average relative deviations of 3.6% from actual compositions.

Locmelis et al. (page 1143) present the results of a comprehensive study on the concentrations of first-row transition elements, Ga and Ge in olivine from komatiites measured via laser ablation ICP-MS. The data show that (1) elevated Ga/Sc ratios in olivine reflect garnet retention in the komatiite source, (2) high Ge contents in olivine may be indicative of melting under hydrous conditions, (3) V/Sc and Mn/Fe ratios in olivine can potentially be used to constrain local oxygen fugacity in the komatiite magma, and (4) Cu-abundances reflect the sulfide saturation state of a komatiite magma during olivine crystallization.

Tao et al. (page 1156) report a mechanism of hydrotalcite (Ht) conversion into saponite after formed by the hydrothermal alteration of metal oxides. The conversion was through a hydration-dissolution-precipitation pathway. It was favored under the conditions of relatively low Mg/Al ratios with high Al and Si contents, and greatly accelerated by the Al3+—Si4+ substitution in silicate oligomers to generate the extra negative charge in tetrahedral sheets. During the process, CO2 was initially incorporated into the interlayer of Ht as CO32-, but was expelled by the formation of saponite, due to the layer charge reversal from positive in Ht to negative in saponite. These findings provide an explanation for the rare occurrence of hydrotalcite deposits on the Earth's surface.

Křížová et al. (page 1165) report the occurrence of shenzhuangite close to its ideal end-member composition (NiFeS2), Ni1.007Fe0.998Cu0.016Co0.058S1.922, in Australasian Muong Nong-type tektites from Laos. This was the first discovery of shenzhuangite in terrestrial materials; originally it was found in meteorite Suizhou. This shenzhuangite was identified by electron probe microanalysis and electron back scatter diffraction. The authors also presented a Raman spectrum with a tentative assignment of spectral bands based on the analogy with synthetic chalcopyrite-structured phases.

Wang et al. (page 1173) conducted acoustic velocity measurements of a natural orthopyroxene, (Mg1.77Fe0.22Ca0.01)Si2O6, up to 13.5 GPa and 873 K. It is known that the end-member orthoenstatite, Mg2Si2O6, undergoes velocity softening at high pressure and room temperature, due to its transition to the metastable, high-pressure clinoenstatite. This study reports a similar, high-pressure velocity softening in Fe-bearing orthopyroxene at temperatures up to at least 673K, providing an upper bound on the P-T conditions where such softening may occur in subduction zones. The direct measurement of velocity jump across the orthopyroxene to high-pressure clinopyroxene transition further proves that it could be a possible contributor to the seismic X-discontinuity.

Yin et al. (page 1180) present a potential mechanism for the uptake of trace element at a fluid-mineral interface in hydrothermal systems. More specifically, they determine the role of mineral nanoparticles at a fluid-magnetite interface using high-resolution transmission electron microscopy. The results show that the Al concentration in magnetite measured on a micron-scale is caused by three different effects: Al solid solution, Al-rich nanometer-sized lamellae and zinc spinel nanoparticles in the host magnetite, and the authors propose a genetic relationship among the three phases. The fluid-mineral interface in this mechanism has been repeatedly utilized during crystal growth, providing an efficient way for the uptake of trace element from a related undersaturated bulk fluid.

Deng and Lee (page 1189) investigated the effects of the electronic spin transition of iron on the melting temperature of Earth's two most abundant minerals, ferropericlase, and bridgmanite, using both Lindemann's Law and thermodynamic analyses. Lindemann's Law predicts a negligible melting temperature depression for bridgmanite but a substantial depression for ferropericlase across the spin transition of iron, consistent with extant experimental results. Thermodynamically, the melting depression likely derives from a more negative Margules parameter for a liquid mixture of high- and low-spin end-members as compared to that of a solid mixture. This melting depression across the spin transition of iron may be the process responsible for the formation of a deep molten layer during the crystallization of a magma ocean in the past, and a reduced viscosity layer at present.

Chen et al. (page 1197) investigated epidote spherulites and radial euhedral epidote aggregates in a greenschist facies metavolcanic breccia hosting an ultrahigh-pressure eclogite in Dabieshan, China, and discussed the implications for dynamic metamorphism. Because these non-equilibrium textures would recrystallize into equilibrium ones if the P-T -H2O conditions were maintained sufficiently long, they likely formed in response to P-T and fluid pulses, possibly related to seismicity.

Liu et al. (page 1213) synthesized a new (Mg0.5Fe3+0.5)(Si0.5Al3+0.5)O3 LiNbO3-type phase (space group R3c) at lower-mantle conditions. Fe3+ and Al3+ cations substitute into A (Mg2+) and B (Si4+) sites, respectively, through a charge-coupled substitution mechanism. This phase is probably recovered from bridgmanite at lower-mantle conditions by a diffusionless transition because of the displacement of A cations and distortion of BO6 octahedra on releasing pressure. Bridgmanite can thus contain the FeAlO3 component (50 mol%) beyond the previously reported solubility limit (37 mol%). The present study shows that the Earth's most abundant elements form a new FeAlO3-dominated LiNbO3-type compound from bridgmanite at lower mantle conditions. It provides new insight into the complicated crystal chemistry of LiNbO3-type phase/bridgmanite and constrains the P-T conditions for shocked meteorites.

July 2019

Swanner et al. (page 917) in their article “Fate of cobalt and nickel in mackinawite during diagenetic pyrite formation” report their investigations of Ni and Co incorporation into freshly-precipitated mackinawite, and after experimental diagenesis to pyrite at 65 °C. Fe and S K-edge micro-X-ray absorption near edge spectroscopy identified the oxidation state and mineralogy within experimentally synthesized and diagenetically transformed minerals. Results indicate that Co and Ni may inhibit the transformation of mackinawite to pyrite or slow it down. Cobalt concentrations in the solid diminished by 30% during pyrite transformation, indicating that pyrite Co may be a conservative tracer of seawater or porewater Co concentrations. Ni concentrations increased several-fold after pyrite formation, suggesting that pyrite may have scavenged Ni from the dissolution of primary mackinawite grains. Nickel in pyrites thus may not be a reliable proxy for seawater or pore water metal concentrations.

Liu et al.’s research on page 929 in the “Influence of aluminum on the elasticity of majorite-pyrope garnets” used an ultrahigh pressure convection technique to fabricate a series of gem-quality majorite-pyrope garnets and measured the velocity of these garnets by means of ultrasonic interferometry measurements. They found that both velocity and elastic moduli increase linearly with increasing Al along the majorite-pyrope system. The Al component plays a dominant role for the variation of elasticity (velocity and modulus) for majorite-pyrope garnets, while the phase transition due to cation ordering or disordering cannot significant affect these elastic properties. Therefore, seismic velocity modeling of a garnet-bearing mantle transition zone is more associated with garnet’s composition rather than the phase transitions due to cation order or disorder in garnet.

Gavrilenko et al.’s article on page 936, “The quench control of water estimates in convergent margin magmas,” explores the limits of melt inclusions (MIs) as hydrous magma recorders in an experimental study, showing that there is a limit of dissolved H2O that mafic glassy MIs can retain. These results show there is potential bias in the glassy MI data set; they can only faithfully record pre-eruptive H2O contents in the upper-most part of the Earth’s crust where H2O-solubility is low. The current MI database cannot be used to robustly estimate the full range of arc magmas and therefore assess volatile budgets in primitive or evolved compositions. Such magmas may contain much larger amounts of H2O than currently recognized, and the diversity of magma evolutionary pathways in subduction zones is likely being significantly underappreciated.

Yin et al. (page 949) studied the “Textural and chemical variations of micas as indicators for tungsten mineralization: Evidence from highly evolved granites in the Dahutang tungsten deposit, South China.” They found that micas are effective indicators not only for the magmatic-hydrothermal evolution of granite but also for the tungsten mineralization. The texture of zoned micas and geochemical variations of micas are important for reconstructing tungsten ore-forming processes, including the enrichment and transportation of tungsten during the magmatic-hydrothermal evolution. Tungsten is unlikely to be deposited directly in the granite, and reducing fluids and fluid-rock interaction play an import role in forming large ore deposits.

Hirth et al. (page 966) in “A topological model for defects and interfaces in complex crystal structures” introduce a tractable method for applying the topological model to characterize defects in these complex materials. They illustrate how structural groups, each with a motif containing multiple atoms, provide lattices and structures that are useful in describing dislocations and disconnections in interfaces. They illustrate the model for twinning in albite owing to its potential application for constraining the rheological properties of the crust at conditions near the brittle-plastic transition, where plagioclase is a major constituent of common rock types. The concept of structural groups makes an analysis of the twinning process easier in complex minerals and explicitly predicts the interface structure of the deformation twins.

Kampf et al. (page 973) describe the new mineral “Phoxite, (NH4)2Mg2(C2O4)(PO3OH)2(H2O)4, the first phosphate-oxalate mineral.” Phoxite is a new mineral species found in an unusual bat-guano-related, post-mining assemblage of phases in the Rowley mine, Maricopa County, Arizona, U.S.A. It is the first mineral known to contain both phosphate and oxalate groups and it possesses a novel layer structure that can be considered a “soft framework” due to strong hydrogen bonding between layers. The phase may have potential uses in agricultural applications for soil conditioning, fertilizing, and as a natural pesticide.

Keller and Ague (page 980) investigated “Corundum, and apatite lamellae from garnet.” Multiple lines of evidence support the precipitation of rutile, ilmenite, apatite, and corundum lamellae from garnet. Crystallographic orientation relationships (COR) between inclusion and host are consistent from independent occurrences worldwide, and minerals of the same crystal system share preferred relationships. Rutile forms a COR angularly equivalent to the Widmanstätten pattern in meteorites. These COR are valuable for comparing precipitation behavior across materials and suggest lamellae are precipitates indicative of precursor garnet chemistry.

Brugman and Till’s (page 996) “A low-aluminum clinopyroxene-liquid geothermometer for high-silica magmatic systems” presents a new clinopyroxene-liquid geothermometer calibrated for use with high-Fe, low-Al clinopyroxene from high-silica systems. It lowers calculated temperatures by 85 °C on average relative to a popular geothermometer (Putirka 2008, Eq. 33) and reduces the uncertainty by a factor of two (standard error of estimate ± 20 °C). When applied to natural systems, this clinopyroxene-liquid geothermometer reconciles many inconsistencies between experimental phase equilibria and preexisting geothermometry results for silicic volcanism, including those from the Bishop Tuff and Yellowstone caldera-forming and post-caldera rhyolites. Clinopyroxene is found not restricted to near-liquidus temperatures in rhyolitic systems and can be stable over a broad temperature range, often down to the solidus.

Berryman et al. (page 1005) in “Compressibility of synthetic Mg-Al tourmalines to 60 GPa” established the pressure-volume equation of state of tourmaline and its room-temperature metastability to 60 GPa in a series of high-pressure single-crystal X-ray diffraction experiments. The use of synthetic tourmalines representing five distinct end-member species revealed the primary role of the octahedral sites, particularly the Z site, on tourmaline’s compressibility and its remarkable metastability. This study highlights the utility of synthetic crystals in exploring compositional and structural controls on mineral properties at high pressure.

Hao et al. (page 1016) measured “The single-crystal elastic properties of the jadeite-diopside solid solution and their implications for the composition-dependent seismic properties of eclogite.” The 13 single-crystal adiabatic elastic moduli (Cij) of a C2/c jadeite sample close to the ideal composition (NaAlSi2O6) and a natural P2/n diopside-rich omphacite sample were measured at ambient conditions by Brillouin spectroscopy. Voigt-Reuss-Hill averaging of the Cij values yields an aggregate bulk modulus, KS, = 138(3) GPa and shear modulus, G, = 84(2) GPa for jadeite. The vpvs of omphacite decrease with diopside content, though the velocity changes are small as diopside component exceeds 70%. They also found that both the isotropic vpvs, as well as the seismic anisotropy of eclogite, changed strongly with the bulk-chemical composition. The relationship between the anisotropic velocities of eclogite and the chemical composition can be a useful tool to trace the origin of the eclogitic materials in the Earth’s mantle.

Fan et al.’s (page 1022) “Elasticity of single-crystal low water content hydrous pyrope at high-pressure and high-temperature conditions” reports measurements of the acoustic wave velocities and density of a single-crystal, hydrous pyrope with ~900 ppmw H2O by Brillouin light scattering combined with in situ synchrotron X-ray diffraction in the diamond anvil cell up to 18.6 GPa at room temperature and up to 700 K at ambient pressure. The modeling results indicate that hydrous pyrope remains almost elastically isotropic at relevant high P-T conditions and may have no significant contribution to seismic anisotropy in the upper mantle. Furthermore, hydrogen has no significant effect on the seismic velocities and the Vp/Vs ratio of pyrope at the upper mantle P-T conditions, especially for the limited hydration level (<100 ppmw H2O) of mantle-derived garnets.

Le Losq et al.’s study (page 1032) “Determination of the oxidation state of iron in Mid-Ocean Ridge basalt glasses by Raman spectroscopy” used conventional, as well as machine learning, data reduction methods to measure the iron oxidation state of Mid-Ocean Ridge Basalt (MORB) glasses, a key parameter for understanding upper mantle conditions and oceanic seafloor production, from their Raman spectra. The approaches allow evaluation of the average iron oxidation state in MORB glasses as 0.09 and to predict MORB glass chemical composition. Because Raman spectroscopy is fast, non-destructive, has microscale resolution and has the potential to be portable (e.g., the SHERLOC system that equips the Mars 2020 rover), its combination with machine learning approaches have a strong potential for analysis of materials in environments inaccessible by other conventional techniques, like Mid-Ocean ridges.

Pieczka et al.’s paper (page 1043) “Lepageite, Mn2+3(Fe3+7Fe2+4)O3[Sb3+5As3+8O34]], a new arsenite-antimonite mineral from the Szklary pegmatite, Lower Silesia, Poland” presents data on a new mineral, lepageite, that is a representative of a rare mineral group of arsenite-antimonites, discovered in the Szklary LCT pegmatite in Poland. We describe its chemical composition and crystal structure and finally explain by genetic implications why arsenite-antimonite minerals are rare species in a pegmatitic environment.

Igami et al. (page 1051) investigated the “High-temperature structural change and microtexture formation of sillimanite and its phase relation with mullite.” Synchrotron X-ray diffraction experiments and transmission electron microscopy of heated sillimanite at various pressures were conducted to clarify the detailed phase relations between sillimanite and mullite. As a result, they propose a new P-T diagram for the Al2SiO5 system with the mullitization boundary and the Al/Si order parameter of sillimanite. Investigations of sillimanite/mullite based on the present results can yield new information about thermal histories in high-temperature regions that is easy to be lost in general.

Hawthorne et al. (page 1062) in their “Memorial of Paul Brian Moore 1940–2019” remember one of the most prolific mineralogists of the 20th century and highlight his contributions to our science.

June 2019

This issue of American Mineralogist starts with an interesting and thoughtful Editorial by the former Editor Keith Putirka (page 785): Why scientists should study chess.

In a “highlights and breakthroughs” article, Ferrand (page 788) comments on the paper “Kinetics of antigorite dehydration: Rapid dehydration as a trigger for lower-plane seismicity in subduction zones” by Liu et al. (2019, Vol. 104, no. 2, pages 282-290). The study by Liu et al. confirms that antigorite dehydration is fast enough to trigger brittle failure under subduction conditions. The dehydration was found to involve two dehydroxylation mechanisms, allowing better understanding of the two-step antigorite destabilization observed in high-pressure experiments.

Dunn et al. (page 791) performed calcite-graphite carbon isotope thermometry on 150 marble samples from the western Central Metasedimentary Belt (CMB) of the Ontario segment of the Grenville Province, which represents the deeply eroded and exposed core of a major mountain chain that existed one billion years ago. The obtained data show a gradual increase in the peak metamorphic temperature from <500 °C in the Tudor Township area in the east to >700 °C along the western margin of the CMB. These results refine our understanding of orogenic buildup and collapse in the Grenville, including the styles of deformation of crystalline rocks in continental crust. The authors also found unusually high carbon isotope ratios (13C/12C) in the sample which provides a useful chemical marker to correlate rocks that formed in similar marine settings at around the same time roughly 1.3 billion years ago, well before the large mountain-building event.

In his Roebling Medal Paper, Hazen (page 810) describes a classification of planetary materials based on natural kind clustering. Minerals reveal the nature of the co-evolving geosphere and biosphere through billions of years of Earth history. Mineral classification systems have the potential to elucidate this rich evolutionary story; however, the present mineral taxonomy, based as it is on idealized major element chemistry and crystal structure, lacks a temporal aspect and thus cannot reflect planetary evolution. A complementary evolutionary system of mineralogy based on the quantitative recognition of "natural kind clustering" for a wide range of condensed planetary materials with different paragenetic origins (best revealed through the data-driven methods of cluster analysis) has the potential to amplify, though not supersede, the present classification system.

Putirka and Rarick (page 817) survey the compositions and mineralogy of >4000 nearby stars from the Hypatia Catalog (the most compositionally broad of such collections) to determine whether such exoplanets might be geologically similar to Earth. They find that most exoplanets will have rocky compositions that are similar to Earth and so most exoplanets might exhibit plate tectonics, and so might also be Earth-like in other respects, for example, by harboring life.

Stangarone et al. (page 830) used density functional theory (DFT) simulations of the structures of zircon and reidite (polymorphs of ZrSiO4) to show that above 20 GPa zircon undergoes a displacive phase transition to a new polymorph (space group I-42d) which may trigger the reconstructive transformation to reidite. Thus, this study provides new insights into the zircon-reidite transition, which may be induced by shock in meteorite impacts. The results clarify the discrepancies between previous observations on natural and experimental samples.

Papike et al. (page 838) investigated the effects of contrasting Ti and Al activities on Mn/Fe systematics in pyroxene from lunar mare basalts. In terms of using Mn/Fe ratios for determining planetary parentage and the reasons for dispersion in this trend for each body, variation in oxygen fugacity appears to be the most important factor for Martian basalts. For lunar mare basalts, however, high Ti activity in the melting zone and the melts, and crystallization sequence differences among high-Ti, low-Ti, and very low-Ti basalts account for almost all of the observed dispersion in the Mn/Fe ratios. This study gives important insight into the challenges of establishing planetary parentage by use of Mn/Fe ratios in lunar pyroxene, and explores the effects of crystallization sequence (order of appearance on liquidus) on this parameter.

Wood et al. (page 844) recalculated the temperatures at which the so-called "moderately volatile elements" such as Zn, In, Tl, Ga, Ag, Sb, Pb, and Cl, would condense from a gas of solar composition at 10-4 bar during formation of the solar system. The calculations highlighted three areas where currently available estimates of condensation temperature could be improved. The newly calculated 50% condensation temperatures are generally similar to or, because of the improvements, lower than those of Lodders (2003). Thus, this work provides a more accurate measure of the relative volatilities of the elements at the earliest stages of planetary formation.

Heller et al. (page 857) explore the possibilities of estimating radiation damage (-dose) in titanites using Raman spectroscopy. Raman spectra of randomly oriented titanite fragments with known thermal history were related to their -dose, calculated from the concentration of -emitting elements. The intensity-weighted mean width of all Raman bands of a spectrum is independent from orientation and proves to be the most robust measure of -dose. This approach provides a pre-selection method to optimize the range of -doses of titanite crystals to be dated by (U-Th)/He thermochronology.

Yoder et al. (page 869) have explored the IR spectra of carbonated calcium and strontium apatites that contain carbonate in the channel (A-type substitution) as well as in both the channel and in place of phosphate (B-type substitution). The results show that a correlation of the band position of the high frequency A-type carbonate band with weight percent carbonate exists for the calcium apatites, whereas a correlation of the band positions of both the low and high frequency B-type carbonate bands with carbonate weight percent occurs for the strontium apatites. On the other hand, correlations of band frequencies with sodium content are weaker than those for carbonate, even though carbonate and sodium are correlated with each other in the calcium apatites. This study also confirms previous conclusions about the distribution of A and B-type carbonate for most synthetic calcium apatites formed under a wide range of temperature and pressure conditions.

Yang et al. (page 878) carried out in situ high-temperature and high-pressure IR spectroscopic investigations on hydrogen storage sites in the natural olivine and synthetic Fe-free forsterite. The results show that hydrogen does not transfer between storage sites with increasing temperature, but displays disordering at temperatures over 600 °C. In contrast, pressure can induce re-configuration of hydrogen storage sites corresponding to the 3610 and 3579 cm–1 bands. Hydrogen storage sites also exhibit disordering at high pressure. In addition, the dehydrogenation experiments of the natural olivine indicate interactions of hydrogen storage sites. Protons released from titanium-clinohumite defects move to pure Si vacancies, and also to Mg vacancies coupling with trivalent cations. This study is of importance to understand water distribution and its impact on the upper mantle.

Glazner (page 890) conducted a thermodynamic analysis to reevaluate the ascent of water-rich magma and decompression heating. It has long been assumed that water-saturated magmas move into the subsolidus field and freeze upon ascent. However, this assumption ignores the considerable thermal energy released by crystallization. The new analysis shows that if magma ascent is treated as an adiabatic, reversible (isentropic) process then water-saturated magma can ascend without freezing, following the solidus to shallow depth and higher temperature as it undergoes modest crystallization and vapor exsolution. Decompression heating is an alternative to magma recharge for explaining pre-eruptive reheating seen in many volcanic systems and accounts for paradoxical growth of quartz during heating events. The viscosity increase that accompanies vapor exsolution as magma rises to shallow depth explains why silicic magmas tend to stop in the upper crust rather than erupting, producing the observed compositional dichotomy between plutonic and volcanic rocks.

Lazarz et al. (page 897) conducted in situ single-crystal X-ray diffraction experiments to investigate high-pressure phase transitions of clinoenstatite (Mg2Si2O6): 1) from a low-pressure form (LPCEN, space group P21/c) to a high-pressure form (HPCEN, space group C2/c) at ~6 GPa; and 2) from HPCEN to a P21/c-structured polymorph (HPCEN2) at ~45 GPa (discovered in this work). High-pressure structure refinements of HPCEN were carried out to determine its P-V equation of state and structural evolution over an expanded pressure range relevant to pyroxene metatstability. The newly discovered HPCEN2 phase is related to the P21/c structure previously observed in diopside at 50 GPa and in clinoferrosilite at ~30-36 GPa.

Matrosova et al. (page 905) performed in situ high-pressure single-crystal X-ray diffraction experiments to determine compressibility of two synthetic Na-rich clinopyroxenes, a Na-Ti-pyroxene with formula (Na0.86Mg0.14)(Mg0.57Ti0.43)Si2O6 and a Na-pyroxene with composition (Na0.886Mg0.085Fe0.029)(Si0.442Mg0.390Fe0.168)Si2O6, up to 40 GPa. These phases were found to be monoclinic with the space group C2/c and exhibit KTo of 106.8(2) and 121.8(4) GPa, respectively. Na-Ti-pyroxene is more compressible than Fe-bearing Na-Mg-Si-pyroxene, likely because the FeO6 octahedron is significantly more rigid than MgO6 at high pressure. The formation of Na-rich pyroxenes in the deep mantle is related to crystallization of low-degree alkaline carbonate-silicate melts formed when the crust and mantle interact during the slab descent and its stagnation in the transition zone.

This issue ends with a review by Hummer (page 914) on a textbook entitled “An Introduction to X-ray Physics, Optics, and Applications” by Carolyn MacDonald (2017), Cambridge University Press, 368 pp. The book presents a comprehensive, rigorous, but understandable explanation of X-ray physics and the many contexts in which this physics is useful in modern technologies. Hummer highly recommends this book to any student or researcher with an adequate background in physics who is seeking advanced knowledge of any system that utilizes X-rays.

May 2019

Hudson-Edwards (p. 633) discusses research on the importance of alunite, jarosite, and beudantite group minerals as sinks for arsenic and antimony. These minerals can immobilize these elements and restrict their bioavailability in acidic, oxidizing environments. This ability to store arsenic and antimony can protect humans and other biota from their toxic effects. Aerobic and abiotic As release from alunite and natroalunite is limited, especially between pH 5 and 8. Release of As is also very limited in As-bearing jarosite, natrojarosite, and ammoniumjarosite at pH 8 due to the formation of secondary maghemite, goethite, hematite, and Fe arsenates that resorb the liberated As. Abiotic reductive dissolution of As-bearing jarosite at pH 4, 5.5, and 7 is likewise restricted by the formation of secondary green rust sulfate, goethite, and lepidocrocite that take up the As. Similar processes have been observed for the aerobic dissolution of Pb-As-jarosite (beudantite analog), with secondary Fe oxyhydroxides resorbing the released As at pH 8. Higher amounts of As are released, however, during microbial-driven jarosite dissolution. Natural jarosite has been found to contain up to 5.9 wt% Sb5+ substituting for Fe3+ in the B-site of the mineral structure. Sb(V) is not released from jarosite at pH 4 during abiotic reductive dissolution, but at pH 5.5 and 7, up to 75% of the mobilized Sb can be structurally incorporated into secondary green rust sulfate, lepidocrocite, or goethite. Further research is needed on the co-incorporation of As, Sb, and other ions in, and the uptake and release of Sb from, alunite, jarosite, and beudantite group minerals, the influence of microbes on these processes, and the long-term (>1 yr) stability of these minerals.

Barton (p. 641) examines the social, cultural, scientific, and technological factors that affected the rate and types of the 4,046 mineral discovery reports (roughly 3/4 of all known minerals) from 1917 to the present. The number of new minerals discovered per year was steady over time from 1917 to the early 1950s, when it began a rapid increase punctuated by spikes in 1962-1969, 1978-1982, and 2008-2016, the last of which is probably still ongoing. A detailed breakdown of the technological, geographic, institutional, and other characteristics of mineral discovery demonstrates that the availability of instrumentation for a particular analytical technique has a far larger impact on the rate of its uptake in mineral discovery than the technique's invention or computer-automation. Around 2/3 of all new mineral discoveries are found in samples associated with resource exploration and exploitation, with peralkaline intrusions and volcanic fumaroles as the next most productive source of new mineral discoveries. New mineral discovery is highly concentrated in specific laboratories or work groups Interestingly, although the number of analytical techniques continues to grow, the average number of methods used to characterize new minerals has not changed significantly since 1960, and about half of new mineral descriptions are made using roughly the minimum of analyses required for a new mineral to be recognized. Although some minerals have been discredited or redefined, most of those were due to changes in nomenclature and classification, and only five cases of fraudulent mineral discovery are known. This article presents the data underlying these analyses and discusses some possible reasons for the observed trends in the rate of new mineral discovery, as well as the implications for the history (and future) of mineralogy.

Gu et al. (p. 652) investigated enigmatic milky diamonds and revealed that dislocations, nano-inclusions, and polycrystalline textures are the possible origins of their appearance. Through application of cathodoluminescence (CL), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM), Gu et al. suggest dislocation loops, nano-sized inclusions (negative crystals) and/or characteristic grain boundaries of the radiating fibrous crystals are the origins for the milky appearance of the type IaB diamonds studied.

Spürgin et al. (p. 659) present an unconventional zeolite deposit type and discuss the geological framework as an important factor for the generation of zeolite deposits hosted in subvolcanic alkaline rocks. This work is focused on the Miocene Kaiserstuhl Volcanic Complex, Germany, which shows that economic grades of zeolitization occur in phonolites intruded into water-rich, shallow marine sediments; whereas low degrees of zeolitization are found in phonolite intruded in dry, subaerial pyroclastic strata of the volcanic edifice. Pseudomorphic replacement textures indicate that zeolites formed from magmatic feldspathoid minerals. The common sequence from Ca-Na-rich zeolites (thomsonite, mesolite, gonnardite) towards pure Na-zeolites (natrolite, analcime) is the result of closed-system alteration. The late formation of Ca-K-rich chabazite in the latter phonolite is interpreted as open-system behavior with an influx of water in equilibrium with the leucite-bearing country rock, after the main alteration phase. This work concludes that zeolite deposits may not only be found in sheet-like, often very young tuffs but also in older rocks in a different geological context.

Dong et al. (p. 671) measured the melting point of barium carbonate (BaCO3) at pressures up to 11 GPa using the ionic conductivity and platinum (Pt) sphere methods in a multi-anvil press. The melting point decreases with pressure from 2149 ± 50 K at 3 GPa to a minimum of 1849 K at 5.5 GPa, and then it increases with pressure to 2453 ± 50 K at 11 GPa. The negative slope of the BaCO3 melting curve between 3 and 5.5 GPa indicates that the liquid is denser than the solid within this pressure range. Synchrotron X-ray diffraction measurements in a laser-heated diamond-anvil cell showed that crystalline BaCO3 transformed from the aragonite structure (Pmcn) to the post-aragonite structure (Pmmn) at 6.3 GPa and 1026 K as well as 8 GPa and 1100 K, and the post-aragonite structure remained metastable upon quenching and only reverted back to the witherite structure upon pressure release. The local minimum near 5 GPa is attributed to the triple point where the melting curve of BaCO3 meets a phase transition to the denser post-aragonite structure (Pmmn). Local minima in the melting curves of alkaline earth carbonates would lead to incipient melting of carbonated rocks in Earth's mantle.

Williams et al. (p. 679) demonstrate that packing changes in glasses have little influence on how glasses compact under pressure through their study of the elasticity of a sequence of SiO2-TiO2 glasses at high pressures and temperatures. The effect of changing Ti content on the bulk moduli of these glasses is monotonic, and no systematic effect of possible coordination changes is observed. In contrast, there is an apparent decrease in the pressure derivative of the bulk modulus above ~3 wt% TiO2. This change occurs at a similar composition to that at which a transition from predominantly 5-fold to 4-fold of Ti has been proposed to occur in these glasses. This shift in the pressure derivative of the bulk modulus is attributed to a stiffening of the equation of state for these glasses generated by the substitution of 5-fold Ti species relative to TiO4 units. These results provide rationales for the onset of coordination changes producing a minimal change in the equation of state of silicate melts/glasses, and for bulk moduli determined at ambient pressure producing relatively accurate silicate melt volumes even within liquids that have begun to undergo coordination changes. Thus, this study supports the general validity of the single equation of state formulations that describe the densities of silicate melts through the transition zone and shallow upper mantle.

Wu et al. (p. 686) explored the thermodynamics of interaction between nano oxides and small gas molecules by gas adsorption calorimetry to investigate the energetics of ethanol and carbon dioxide adsorbed on surfaces of nanoscale anatase, rutile, and γ-alumina particles. The measured zero-coverage adsorption enthalpies per mole of gas adsorbed are -97.7, -107.3, and -84.4 kJ/mol for C2H5OH on anatase, rutile, and γ-Al2O3, respectively. The corresponding values for CO2 adsorption are -61.6, -47.4 and -47.1 kJ/mol. The results indicate the ethanol adsorption is generally more exothermic than carbon dioxide and water adsorption. The isotherm and differential enthalpies show type II isotherms and step-wise patterns for ethanol adsorption in all three oxides. However, CO2 adsorption shows simple continuous isotherms and energetics that suggest dominantly physical adsorption occurred. The repeated adsorption cycle shows that ethanol adsorption on these nanoparticles is partially reversible at room temperature. This thermodynamic evidence indicates that ethanol and similar organics may protect mineral oxide surfaces from reaction with aqueous solutions, which may affect crystal growth, dissolution, and biomineralization.

Knafeic et al. (p. 694) investigated the effect of olivine oxidation on its magnetic properties using a time series of 1 bar oxidation experiments at 600 °C and 900 °C. They found rapid olivine oxidation and alteration at both 600 and 900 °C, forming magnetite and hematite associated with a change from paramagnetic to ferromagnetic behavior after oxidation. Magnetite and hematite nucleated along dislocations and impurities in the crystal structure, along with surface coatings and within cracks in the crystals. Fresh, unaltered mantle xenoliths containing magnetite have been interpreted as having formed in cold tectonic regimes in the mantle, rather than through oxidation during or after the ascent. Mantle xenoliths rapidly ascend through the mantle with estimates of ascent of up to 90 km/hour (3 GPa/hour) based on the diffusion profile of water in mantle olivine. The rates correspond to xenoliths ascending through the mantle over hours and not days or weeks. Our results show that olivine oxidation and alteration can occur in days to weeks at 600°C and within minutes at 900 °C. Therefore, if the xenolithic material is transported to the surface in a cold magma (at temperatures less than or equal to 600 °C), then the time scale of ascent is likely not long enough for oxidation to cause magnetite formation or a ferromagnetic signature to occur. However, if the material is transported in a hot oxidized basaltic magma (with temperatures greater than or equal to 900 °C), then oxidation can cause magnetite formation and a ferromagnetic signature.

Mansor et al. (p. 703) investigated metal sulfide nanoparticles (NPs) because they are present in the environment and are important in controlling the availability of bio-essential and toxic metals in environmental remediation and in resource recovery. Characterizing the basic attributes of these NPs is the first step in understanding their behaviors in various processes. Experiments were performed in the presence and absence of the sulfate-reducer Desulfovibrio vulgaris to elucidate biological controls on NP formation. First, the single-metal end-member NPs were determined by precipitation in a solution containing either aqueous Fe(II) or Cu(II). Limited differences are observed between biogenic and abiogenic precipitates aged for up to one month; the Fe-only experiments resulted in 4-10 nm mackinawite (FeS) NPs that aggregate to form nanosheets up to ~1,000 nm in size, while the Cu-only experiments resulted in mixtures of covellite (CuS) NPs comprised of < 10 nm fine nanocrystals, 20-40 x 6-9 nm nanorods and ~ 30 nm nanoplates. The crystal sizes of biogenic mackinawite and covellite are respectively larger and smaller than their abiogenic counterparts, indicating a mineral-specific response to biological presence. Structural defects are observable in the fine nanocrystals and nanorods of covellite in both biogenic and abiogenic experiments, indicative of intrinsic NP instability and a formation mechanism via particle attachment. In contrast, covellite nanoplates are defect-free, indicating high stability and potentially rapid recrystallization following particle attachment. Mixed-metal sulfide NPs were precipitated at variable initial aqueous Fe-to-Cu ratios (2:1, 1:1 and 1:5). With increasing ratios of Fe-to-Cu, Fe-rich covellite, nukundamite (Cu5.5FeS6.5), chalcopyrite (CuFeS2), and Cu-rich mackinawite are formed. The Fe-rich covellite NPs are larger (100-200 nm) than covellite precipitated in the absence of Fe, indicating a role for Fe in promoting crystal growth. Chalcopyrite and nukundamite are formed through incorporation of Fe into precursor covellite NPs while retaining the original crystal morphology, as confirmed by doping a covellite suspension with aqueous Fe(II), resulting in the formation of chalcopyrite and nukundamite within days. Additionally, in the biological systems we observe the recrystallization of mackinawite to greigite (Fe3S4) after six months of incubation in the absence of Cu, and the selective formation of chalcopyrite and nukundamite at lower initial Fe-to-Cu ratios compared to abiotic systems. These observations are consistent with NP precipitation that ise influenced by the distinct (sub)micro-environments around bacterial cells. Comparative TEM analyses indicate that the synthetic NPs are morphologically similar to NPs identified in natural environments, opening ways to studying behaviors of natural NPs using experimental approaches.

Tateno et al. (p. 718) determined the pressure-volume-temperature (P-V-T) relationship of the B2 (CsCl-type) phase of KCl at 9-61 GPa at 1500-2600 K and up to 229 GPa at room temperature using synchrotron X-ray diffraction measurements in a laser-heated diamond-anvil cell (DAC). The nonhydrostatic stress conditions inside the sample chamber were critically evaluated based on the platinum pressure marker. With thermal annealing by laser after each pressure increment, the deviatoric stress was reduced to less than 1% of the sample pressure even at the multi-Mbar pressure range. The obtained P-V-T data were fitted to the Vinet equation of state with the Mie-Grüneisen-Debye model for thermal pressure. The thermal pressure of KCl was found to be as small as ~10 GPa even at 3000 K at any given volume, which is only half of that of common pressure markers (i.e. Pt, Au, or MgO). Such a low thermal pressure validates the use of a KCl pressure medium as a pressure marker at high temperatures.

Wang et al. (p. 724) studied the petrology, mineralogy, and geochemistry of lawsonite blueschists from the Tavşanli zone in NW Turkey - one of the best-preserved blueschist terranes in the world. The blueschist samples contain lawsonite + sodic amphibole + phengite + chlorite + titanite + apatite ± aragonite ± quartz ± relict igneous pyroxene ± Mn-rich garnet and opaque phases. Lawsonite is a significant repository for Sr, Pb, Th, U, and REE, whereas phengite carries the most LILE; titanite hosts the highest Nb and Ta as well as considerable amounts of HFSE, and apatite strongly controls Sr. Two groups of blueschists with different origins were identified: one from enriched continent-derived terrigenous sediments and another from MORB-like submarine basalts. Lawsonite in blueschists with the enriched origin exhibits strong Th/La fractionation, raising the possibility of the involvement of blueschist facies mélange to explain the origin of Mediterranean potassium-rich magmatism, because similarly high Th/La ratios are also observed in the Mediterranean potassium-rich lavas. We propose that subduction-induced tectonic imbrication took place entirely at shallow depths (less than 80 km), giving rise to newly formed lithosphere where oceanic and continental crustal materials, sediments, strongly depleted peridotite blocks and metamorphic rocks are all imbricated together, and in which many of the compositional characteristics of the lawsonite blueschist are sequestered. Subsequent melting of the fertile and enriched components in this new lithosphere would result in the generation of potassium-rich post-collisional mafic magmas with diagnostic geochemical affinities.

Dey et al. (p. 744) studied a calc-silicate rock from part of the Chotanagpur Granite Gneiss Complex (CGGC), East India, that contains veins and patches of vesuvianite (F: 2.3-3.9 apfu, Fe3+: 1.7-2.1 apfu) and garnet (Gr71-80Alm12-17Adr1-9) proximal to amphibole-bearing quartzo-feldspathic pegmatitic veins. The vesuvianite-garnet veins are both parallel to, and cross-cutting, the gneissic banding of the host calc-silicate rock. Two contrasting mineralogical domains that are rich in garnet and vesuvianite respectively develop within the vesuvianite-garnet veins. Textural studies support the view that the garnet- and vesuvianite-rich domains preferentially develop in the clinopyroxene- and plagioclase-rich layers of the host calc-silicate rocks respectively. Some of the vesuvianite-rich domains of the veins develop the assemblage vesuvianite + quartz + calcite +anorthite (as a result of the reaction diopside + quartz + calcite + anorthite = vesuvianite) which was deemed metastable in the commonly used qualitative isobaric T-XCO2 topology in the system CaO-MgO-Al2O3-SiO2-H2O-CO2 (CMASV). Using an internally consistent thermodynamic database, quantitative petrogenetic grids in the P-T and isobaric T-XCO2 spaces were computed in the CMASV system. The influence of the non-CMASV components (e.g., Na, Fe3+, F) on the CMASV topologies are discussed using the published a-X relations of the minerals. This study shows topological inversion in the isobaric T-XCO2 space, which is primarily dependent upon the composition of the vesuvianite. The quantitative CMASV topologies presented in this study successfully explain the stabilities of the natural vesuvianite-bearing assemblages, including the paradoxical assemblage vesuvianite + quartz + calcite + anorthite. Application of the activity-corrected CMASV topology suggests that infiltration of F-bearing oxidizing aqueous fluids into the calc-silicate rocks developed the vesuvianite-garnet veins in the studied area. A genetic link between quartzo-feldspathic pegmatites and the vesuvianite-garnet veins seems plausible. This study demonstrates controls of topological inversion in the complex natural system, owing to which, certain mineral assemblages that are deemed metastable in one set of reaction geometries can develop in nature.

Rezvukhin et al. (p. 761) recognized LILE-enriched chromium titanates of the magnetoplumbite (AM12O19) and crichtonite (ABC18T2O38) groups as abundant inclusions in orthopyroxene grains in a mantle-derived xenolith from the Udachnaya-East kimberlite pipe, Daldyn field, Siberian craton. The studied xenolith consists of three parts: an orthopyroxenite, a garnet clinopyroxenite, and a garnet-orthopyroxene intermediate domain between the two. Within the host enstatite (Mg# 92.6) in the orthopyroxenitic part of the sample, titanate inclusions are associated with Cr-spinel, diopside, rutile, Mg-Cr-ilmenite, and pentlandite. Crichtonite-group minerals also occur as lamellae inclusions in pyrope grains of the intermediate domain adjacent to the orthopyroxenite, as well as in interstitial-to-enstatite oxide intergrowths together with Cr-spinel, rutile, and ilmenite. Yimengite-hawthorneite inclusions in enstatite contain (wt%) 3.72-8.04 BaO, 2.05-3.43 K2O, and 0.06-0.48 CaO. Their composition is transitional between yimengite and hawthorneite end-members with most grains exhibiting K-dominant chemistry. A distinct feature of the studied yimengite-hawthorneite minerals is a high content of Al2O3 (5.74-7.69 wt%). Crichtonite-group minerals vary in composition depending on the occurrence in the xenolith: inclusions in enstatite are moderate-to-high in TiO2 (62.9-67.1 wt%), moderately Cr-rich (12.6-14.0 wt% Cr2O3), Ba- or K-specific in the A site, and contain low ZrO2 (0.05-1.72 wt%), whereas inclusions in pyrope are moderate in TiO2 (61.7-63.3 wt% TiO2), relatively low in Cr (8.98-9.62 wt% Cr2O3), K-dominant in the A site, and are Zr-enriched (4.64-4.71 wt% ZrO2). Crichtonite-group minerals in polymineralic oxide intergrowths show highly diverse compositions even within individual aggregates, where they are chemically dominated by Ba, Ca, and Sr. P-T estimates indicate the orthopyroxenite equilibrated at ~800 °C and 35 kbar. Preferentially oriented lamellae of enstatite-hosted Cr-spinel and diopside, as well as pyrope, diopside, and Cr-spinel grains developed around enstatite crystals, are interpreted to have been exsolved from the high-T, Ca-Al-Cr-enriched orthopyroxene precursor. The observed textural relationships between inclusions in enstatite and exotic titanate compositions imply that the studied orthopyroxenite has undergone metasomatic processing by a mobile percolating agent; this highly evolved melt/fluid was enriched in Ba, K, HFSE, and other incompatible elements. The prominent textural and chemical inhomogeneity of the interstitial oxide intergrowths is either a consequence of the metasomatic oxide crystallization shortly prior the kimberlite magma eruption, or arose from the intensive modification of pre-existing oxide clusters by the kimberlite melt during the Udachnaya emplacement. Our new data provide implications for the metasomatic treatment of orthopyroxenites in the subcontinental lithospheric mantle from the view of exotic titanate occurrences.

Bindi et al. (p. 775) report the first natural occurrence and single-crystal X-ray diffraction study of the Fe-analogue of wadsleyite [a = 5.7485(4), b = 11.5761(9), c = 8.3630(7) Å;, V = 556.52(7) Å3; space group Imma], spinelloid-structured Fe2SiO4, a missing phase among the predicted high-pressure polymorphs of ferroan olivine, with the composition (Fe2+1.10Mg0.80Cr3+0.04Mn2+0.02Ca0.02Al0.02Na0.01)Σ2.01(Si0.97Al0.03)Σ1.00O4. The new mineral was approved by the International Mineralogical Association (No. 2018-102) and named asimowite in honor of Paul D. Asimow, the Eleanor and John R. McMillan Professor of Geology and Geochemistry at the California Institute of Technology. It was discovered in rare shock-melted silicate droplets embedded in Fe,Ni-metal in both the Suizhou L6 chondrite and the Quebrada Chimborazo (QC) 001 CB3.0 chondrite. Asimowite is rare, but the shock-melted silicate droplets are very frequent in both meteorites and most of them contain Fe-rich wadsleyite (Fa30-45). Although the existence of such Fe-rich wadsleyite in shock veins may be due to the kinetic reasons, new theoretical and experimental studies of the stability of (Fe,Mg)2SiO4 at high temperature (> 1800 K) and pressure are clearly needed. This may also have a significant impact on the temperature and chemical estimates of the mantle's transition zone in Earth.

Belakovskiy et al. (p. 779) present names for 11 new minerals, including ammoniovoltaite, belousovite, chlorellestadite, clino-suenoite, marcobaldiite, markeyite, martinandresite, parisite-(La), plumbopharmacosiderite, somersetite, and ziminaite.

April 2019

This issue of American Mineralogist starts with an article to introduce the special collection “Earth in Five Reactions: A Deep Carbon Perspective” by Li et al. (page 465). This special collection features review papers on the role of carbon-related reactions in Earth’s dynamics and evolution, and includes original studies on carbon-bearing phases and the impact of chemical and polymorphic reactions on Earth’s deep carbon cycle.

Hazen (page 468) then describes the five reactions that influence the Earth’s history, which were identified at the Earth in Five Reactions Workshop held at the Carnegie Institution for Science, Washington, D.C., March 22–23, 2018. The workshop posed two challenges: (1) the formulation of a conceptual definition of “reaction” and (2) the identification and ranking of the “most important reactions” in the context of planetary evolution.

Knipping et al. (page 471) determined grain-to-grain and intra-grain Fe isotope variations in magnetite grains from the Los Colorados Kiruna-type iron oxide-apatite (IOA) deposit, Chile, using in situ femtosecond laser ablation MC-ICP-MS. The results reveal an igneous and magmatic-hydrothermal growth mechanism of magnetite, which is consistent with the formation model of this IOA deposit proposed based on trace element zonation in its magnetite grains. This is a contribution to the special collection “From Magmas to Ore Deposits”.

Wu et al. (page 485) report the discovery of ophiolite-hosted diamonds in the podiform chromitites of the Skenderbeu massif of the Mirdita ophiolite in the western part of Neo-Tethys. Carbon and nitrogen isotopes and mineral inclusions in diamonds demonstrate recycling of oceanic crust into the mantle. This discovery not only provides new evidence of diamonds in these settings but also sheds light on deep cycling of subducted oceanic crust and mantle composition.

Pe-Piper et al. (page 501) investigated the geochronology and trace element mobility in rutile from a Carboniferous syenite pegmatite using SEM, Raman spectroscopy, LA-ICP-MS and in situ U-Pb analysis. In particular, they determined the role of halogens in the mobility of Ti and associated Zr, U and REE, which led to hydrothermal alteration of magmatic rutile in syenite. The complexities of rutile chemistry in this hydrothermal setting could be reproduced in deeper subduction settings as a result of variations in halogen content of fluids released by prograde metamorphism.

Jenkins (page 514) synthesized calcium amphiboles from ferro-pargasite and hastingsite bulk compositions at 600–950 °C, 0.1–0.45 GPa and logfH2 of 1.4 to 2.4 for durations of 111–672 h, and determined how variations in formation conditions (temperature, pressure, hydrogen fugacity), bulk composition (Na and K ratio), and choice of starting material salts affect the Cl contents of synthesized calcium amphiboles. The results imply that the crystal-chemical controls for Cl incorporation in calcium amphiboles are dominated by substitution of Fe2+ for Mg, TAl for Si, and K for Na into the crystallographic A site with a linear dependence at the rate of 0.45 Cl per FeAlK index above a minimum value of about 0.34.

Zhang et al. (page 525) performed three series of amphibole crystallization experiments from hydrous basaltic melt at 0.6–2.6 GPa and 860–970 °C and measured the amphibole crystal size distribution (CSD). The results show that the amphibole growth rate increases with increasing temperature in the isobaric series and with increasing pressure at constant temperature. By contrast, the growth rate is negatively correlated with crystallization time at constant temperature and pressure. The authors developed a functional form for evaluating growth rate at known pressure and temperature from an observed amphibole CSD and applied to a diorite from the eastern Tianshan Mountains, NW China.

Yuguchi et al. (page 536) investigated the role of micropores, mass transfer, and reaction rate in the hydrothermal alteration process of plagioclase from the Toki granitic pluton in central Japan. Important observations include: 1) Micropores form during the incipient stage of plagioclase alteration by dissolution of the anorthite component, and then contribute to the infiltration of hydrothermal fluid into the plagioclase. 2) The mass transfer of the components released from biotite by chloritization involves the inflow of H4SiO4, Al3+, Fe2+, Mn2+, Mg2+, K+, CO2, and F-, and the outflow of H2O, H+, and Ca2+. 3) The infiltration rate of the hydrothermal fluid and the potassium transfer rate through the micropores into the plagioclase represent the mass transfer rate of the alteration.

Beyer et al. (page 557) measured the diffusivity of Pb in CaTiO3 perovskite (commonly used for dating kimberlites and carbonatites) using Rutherford backscattering and TOF-SIMS in the depth-profiling mode. Experiments were performed on oriented CaTiO3 single crystals with (Ca0.83Pb0.07)Ti1.05O3 thin film or (Ca0.9Pb0.1)TiO3 powder as the Pb-source, which were annealed at 736—1135 °C for 2—283 h. The measured Pb profiles show two regions — a steep gradient at the diffusion interface that transitions sharply to a low concentration tail that penetrates deeper into the crystal. Moreover, Pb is trapped in the planar defects formed due to the CaTiO3/Pb-bearing perovskite lattice mismatch, and the closure temperature for Pb in CaTiO3 is found to be between 300 and 400 °C for a range of different cooling scenarios if diffusive resetting of Pb in CaTiO3 occurs. At typical cooling rates of hours to days for ascending kimberlite, the age of crystal growth is preserved, with closure temperatures similar to the magma temperature.

Jonckheere et al. (page 569) compare the traditional bulk etch rate (vB) and an alternative radial etch rate (vR) model for fission-track etching in apatite. A skeletal vR-model, based on the inferred orientations of the vR minima and maxima, accounts for the main geometrical features of etched fission tracks, unifies the diverse appearances of etched tracks, and embeds fission-track etching in the mainstream theories of crystal growth and dissolution. The authors suggest that the anisotropic-vB-model may be replaced with an anisotropic-vR-model based on the radial etch rate.

Zhang et al. (page 580) investigated the compressional behavior of a synthetic liebenbergite, Ni2SiO4, that has the olivine structure, up to 42.6 GPa using single-crystal synchrotron X-ray diffraction. Over this pressure range, liebenbergite retains the orthorhombic Pbnm structure. Fitting the pressure-dependent variation in its unit-cell volume to a third-order Birch-Murnaghan equation of state yielded a bulk modulus of 163(3) GPa — the most incompressible olivine-structured silicate.

Tsujino et al. (page 588) determined the wadsleyite-ringwoodite phase transition loop in the Mg2SiO4-Fe2SiO4 system under dry conditions from 1473–1873 K using in situ high P-T synchrotron X-ray diffraction. Assuming an equilibrium composition of wadsleyite and ringwoodite coexisting with garnet in a pyrolite model and an adiabatic temperature gradient with a potential temperature of 1550–1650 K, the phase transition depth and effective width of the seismic discontinuity were found to be 500–514 and 20–22 km, respectively. Considering wet and oxidized conditions, the depth of the wadsleyite-ringwoodite phase boundary could be >520 km. Variation in the depth of seismic anomaly may be attributed to water content or oxygen fugacity of the transition zone.

Stachowicz et al. (page 595) investigated the cation ordering, valence states, and symmetry breaking in chevkinite-(Ce) from the Biraya rare-metal deposit, Russia, using single-crystal X-ray diffraction and X-ray photoelectron spectroscopy (XPS). Nb-rich chevkinite-(Ce) typically possesses a space group of C2/m, though a specimen of lower, P21/α, symmetry has also been recognized. While XPS shows that both C2/m and P21/α structures contain Ti4+ and Ti3+, it also indicates that Ti2+ may occur in the P21/α phases. In addition to the substitution CFe3+ + DTi4+CFe2+ + DNb5+, the authors propose that another substitution, 2DTi4+DNb5+ + DTi3+, can occur, leading to substantial Nb-enrichment.

Kampf et al. (page 603) describe the crystal structure, chemical composition and physical properties of meyrowitzite, Ca(UO2)(CO3)2·5H2O, a new mineral from the Markey mine, Red Canyon, San Juan County, Utah, U.S.A. The structure is monoclinic, P21/n, and contains a novel uranyl-carbonate sheet. Meyrowitzite is a secondary phase found on calcite-veined asphaltum in association with gypsum, markeyite and rozenite.

Bindi et al. (page 611) describe the discovery of the first natural metal hydride, gamma-VH2. .It was discovered in xenoliths within volcanic rock on Mount Carmel, Israel. The hydride coexists with metallic V, which requires oxygen fugacities of ΔIW -9 or lower. The presence of VH2 is interpreted as a signature of deep-seated basaltic magmas with mantle-derived CH4+H2 at high fluid/melt ratios.

Lindsley et al. (page 615) describe the synthesis of pigeonite samples and offer them for non-destructive study. As pigeonite does not survive as a discrete phase in coarse plutonic rocks, natural samples suitable for study are difficult to come by. Eight samples of differing composition, the results from more than 125 trials, in sizes 5-50 micrometers are available for bulk analysis.

This issue contains four 2018 MSA award presentations: (1) page 619–620: “Presentation of the 2018 Roebling Medal of the Mineralogical Society of America to E. Bruce Watson” by Frank Richter; (2) page 621–622: “Acceptance of the 2018 Roebling Medal of the Mineralogical Society of America” by E. Bruce Watson; (3) page 623: “Presentation of the Dana Medal of the Mineralogical Society of America for 2018 to Jörg Hermann” by Bradley R. Hacker; and (4) page 624: “Acceptance of the Dana Medal of the Mineralogical Society of America for 2018” by Jörg Hermann.

Belakovskiy and Cámara (page 625) introduce eight new minerals, including fengchengite, ferriperbøeite-(Ce), genplesite, heyerdahlite, millsite, saranchinaite, siudaite and vymazalováite, and summarize new data on lavinskyite-1M.

Gysi (page 630 ) reviews the book: “Thermodynamics of Natural Systems: Theory and Applications in Geochemistry and Environmental Science”, 3rd ed. (2017) by Greg Anderson, Cambridge University Press, 428 p. Compared to the two earlier editions, this edition is shorter and more concise and is suitable as an introduction to thermodynamics book.

March 2019

Rodeghero et al. (p. 317) study the ability of zeolite ZSM-12 to remove the groundwater contaminants chloroacetanilides and their degradation products. They measured the removal of 2-ethyl-6-methyl-aniline [C2H5C6H3(CH3)NH2, labeled EMA] from water by combining chromatographic, thermogravimetric, and synchrotron X-ray powder diffractometric techniques and demonstrate that ZSM-12 can rapidly incorporate about 4 EMA molecules per unit cell. The authors document the strong interaction with framework O atoms in ZSM-12 that confers stability to the pollutants in the zeolite cages. The rapid kinetics combined with the good adsorption capacity makes ZSM-12 a promising material to control and minimize water pollution from acetanilide compounds as well as other agro-chemical contaminants.

Zhu et al. (p. 325) report chemical and structural analyses of synthetic Fe7C3, a potential host of reduced carbon in Earth’s mantle and a candidate component of the inner core. They synthesized Fe7C3 utilizing a diffusive reaction between iron and graphite that contained 31 to 35 at% carbon. They found that more carbon-rich Fe7C3 has smaller unit-cell volumes, suggesting that excess carbon atoms substituted for iron atoms instead of entering the interstitial sites of the closed-packed iron lattice as in FeCx steel. This substitution leads to a larger reduction in the unit-cell mass than the volume so that the carbon-rich end-member may be as much as 5% less dense than stoichiometric Fe7C3. If Fe7C3 solidifies from Earth’s iron-rich liquid core, it is expected to have a nearly stoichiometric composition with a compositional expansion coefficient of ~1.0. However, laboratory experiments using carbon-rich synthetic Fe7C3 to model the inner core may overestimate the amount of carbon that is needed to account for the core density deficit.

Hunt and Lamb (p. 333) applied mineral equilibria to estimate values of aH2O in rocks that originated below the Moho. The chemical compositions of olivine + orthopyroxene + clinopyroxene + amphibole + spinel ± garnet were used to estimate values of temperature (T), pressure (P), aH2O, hydrogen fugacity (fH2), and oxygen fugacity (fO2) in 11 amphibole-bearing mantle xenoliths from the southwestern U.S.A. The activity of water was first calculated by amphibole dehydration equilibria, and then oxygen fugacity calculated from coexisting olivine, spinel, and orthopyroxene was combined with hydrogen fugacities calculated from amphibole dehydrogenation equilibria to construct a separate estimate of the water activity. The two separate estimates of aH2Ogenerally agree to within 0.05. This agreement indicates that the amphibole in these samples has experienced little or no retrograde H-loss and that amphibole equilibria yields robust estimates of aH2Othat, in these xenoliths, are generally <0.3, and are often 0.1 or less.

Macdonald et al. (p. 348) review chevkinite-group minerals (CGM), minerals that are dominantly monoclinic REE-Ti-Fe sorosilicates [(REE, Ca)4Fe2+(Fe2+,Fe3+,Ti)2Ti2(Si2O7)2O8)], with REE2O3 contents up to ~50 wt%, although some members with predominant Mg, Al, Mn, Cr, Sr, or Zr in one of the cation sites are also known. They show that these minerals can be found in igneous and metamorphic rocks on Earth as well as on the Moon and Mars; these minerals may form over the pressure range 50 to <10 kbar, and over a wide temperature range. In common with other REE-bearing accessory minerals, CGM are prone to alteration by hydrothermal fluids. The nature and extent of the alteration are primarily determined by the composition of the fluids. Fluids poor in ligands tend to generate a Ti-enriched phase whose nature is unknown but is probably amorphous. With increasing F + CO2 levels, complex replacement assemblages are formed. This review also discusses some of our ignorance. The stability of CGM vis-à-vis other REE-Ti-bearing accessories is poorly understood. They are often the major carriers of REE and actinides, and they have a high potential for fractionating the light lanthanides and Th from U, but very little systematic work has been done in determining CGM-melt partition coefficients, yet such data are critical in, inter alia, geochemical modeling. And although observational evidence of the effects of alteration and element mobility is accumulating and chemical equations can be constructed to approximate the reactions, there is still no firm geochemical basis for understanding element redistribution during these processes.

Treiman et al. (p. 370) investigate the origin of magnesium aluminate spinel, (Mg,Fe)Al2O4, in lunar anorthositic rocks Although uncommon, recent near-infrared spectra of the Moon have delineated regions where spinel is the only ferromagnesian mineral, and the rock is inferred to be spinel anorthosite. The authors consider multiple alternative hypotheses for the origin of spinel anorthosites: formation at high pressure, low-pressure assimilation of anorthosite by picritic magmas, and crystallization of superliquidus anorthite-rich melts created by impacts. The authors conclude that near the lunar surface, the most likely process of spinel formation is rapid crystallization of impact melts of anorthosite + picrite or peridotite compositions. The presence of spinel anorthosite on the walls and central peaks of impact craters results from rapid cooling and partial crystallization of superliquidus melts produced in the impacts, and not from the uplift of deep material to the Moon's surface.

Kuroda et al. (p. 385) investigated the diffusion of deuterated water into silica glass at 900–750 °C and a water vapor pressure of 50 bar and found it to be an order of magnitude greater than previously measured. Their analysis indicates that the species responsible for the fast diffusion is not molecular hydrogen, but molecular water, and hypothesize that water diffuses through the free volume of the glass in a manner similar to noble gases. The abundance of free volume in the silica glass structure estimated previously is higher than that of 2H observed in the fast diffusion of this study, suggesting that the free volume was not fully occupied by 2H2O under the present experimental conditions. This implies that the contribution of the fast water diffusion to the total water transport in volcanic glass becomes larger at higher water vapor pressure conditions.

Cao et al. (p. 391) characterized textural and compositional microscale (10–100 μm) and nanoscale (10–100 nm) zoning in a plagioclase phenocryst from a fresh, syn-mineralization diorite porphyry (Black Mountain porphyry Cu-Au deposit, Philippines) by electron microprobe, laser ablation-inductively coupled plasma-mass spectrometry, and atom probe tomography. The complex plagioclase crystal (3.0 × 5.4 mm) has a patchy andesine core (An41–48 mol%), eroded bytownite mantle (An71-80 mol%), and oscillatory andesine rim (An39–51 mol%). Microscale variations with a periodic width of 50 to 200 μm were noted for most major and trace elements (Si, Ca, Al, Na, K, Fe, Mg, Ti, Sr, Ba, Pb, La, Ce, and Pr) with a ΔAn amplitude of 4–12 mol% in both the core and rim. The mantle has a distinct elemental composition, indicating the addition of hotter mafic magma to the andesitic magma. Atom probe tomography shows an absence of nanoscale variations in the andesine rim but alternating nanoscale (25–30 nm) Al-rich, Ca-rich, and Si-rich, Na-rich zones with a Ca/(Ca+Na)at% amplitude of ~10 in the bytownite mantle. The estimated physiochemical parameters for crystallization suggest that microscale oscillatory zoning was likely controlled by internal crystal growth mechanisms, not by periodic variations in physiochemical conditions. The micro-scale zoning in plagioclase indicates a minimum cooling rate of 0.0005 °C/yr during crystallization, but the retention of nanoscale zoning (~28 nm) requires a minimum cooling rate of 0.26 °C/yr. Given that this is significantly faster cooling than would occur in a magma chamber, this texture likely records the post-crystallization emplacement history.

Masci et al. (p. 403) investigated Fe3+ in chlorite using the electron microprobe and XANES to assess the importance of oxychlorite and how ferric iron influenced cation site distributions and thermobarymetric calculations. Their analyses show iron oxidation states varying from ferrous to ferric; iron is in octahedral coordination in all ferromagnesian chlorites but to ~25% tetrahedral in the lithian chlorite cookeite (1.0 wt% Fe2O3(total)). Absolute amounts of ferric iron cover an unprecedented range (0 to ~30 wt% Fe2O3). For highly magnesian, ferric chlorite, Fe concentrations are low and can be accounted for by Al = Fe3+ substitution. In Fe-rich samples, Fe3+ may exceed 2 atoms per formula unit (pfu, 18 oxygen basis). When structural formulas are normalized to 28 charges corresponding to the standard O10(OH)8 anionic basis, these measurements define the exchange vector of a di-trioctahedral-type substitution: 3 VI(Mg, Fe2+) = VI☐ + 2 VIFe3+, as described in earlier studies. However, structural formulas calculated on the basis of the oxygen contents actually measured by EPMA show that this trend is an artifact, due to the neglect of variations in the number of protons in the structure. Our measurements indicate increasing hydrogen deficiency with increasing Fe3+ content, up to ~2 H+ pfu in the Fe3+-rich chlorite samples, corresponding to a net exchange vector of the type R2+ + H+ = Fe3+. These results highlight the need to consider substitution toward an “oxychlorite” (i.e., H-deficient) ferric component, close to tri-trioctahedral, with an O12(OH)6 anionic basis, even in green, pristine-looking chlorite. The effects of iron oxidation and H deficiency on chlorite geothermometers were explored and it was found that, given the sensitivity of most thermometers to octahedral vacancy, the assumption FeTotal = Fe2+ is still safer than using high measured Fe3+ contents and the standard 28 charge basis, which artificially increases vacancies. With the help of constraints from thermodynamic models, charge balance, crystal symmetry, and proton loss, a new cation site distribution is proposed for di-tri- to tri-trioctahedral chlorites in the Fe2+-Fe3+-Mg-Al-Si-O-H system, allowing a more realistic thermodynamic handling of their solid solutions.

Zhao et al. (p. 418) used Brillouin scattering spectroscopy to study variations in sound velocity across calcite phase transitions at pressures to 10.3 GPa. Dramatic decreases in the velocities of the compressional wave (Vp) and shear wave (Vs) and abrupt increases in the Vp anisotropy (Ap) and maximum Vs anisotropy (Asmax) were detected across the phase transition from CaCO3-I to CaCO3-II. Dramatic increases in the Vp and Vs and an abrupt decrease in Ap were observed across the phase transition from CaCO3-II to CaCO3-III. The phase transition from CaCO3-I to CaCO3-II may potentially explain the Gutenberg discontinuity at 51 km in the Izu-Bonin region. The Vp and Vs values of calcite were low. The new results combined with literature data suggest that the low velocities of CaCO3 could potentially explain the low-velocity zone occurring in northeastern (NE) Japan.

Wei et al. (p. 425) studied the distribution of trace elements in sulfosalts (bournonite, jamesonite, tetrahedrite, boulangerite, semseyite, heteromorphite, robinsonite and (Cu)-Pb-Bi-Sb sulfosalts) and coexisting base-metal sulfides in auriferous veins from the Gutaishan Au-Sb deposit, China, by electron probe microanalysis and by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace-element analysis. Two generations of native gold are documented; the first is coarse-grained, Ag- and Bi-bearing, and is associated with the main (Cu)-Pb-Sb sulfosalts (bournonite, jamesonite, tetrahedrite, and boulangerite). The second generation is fine-grained and has the highest fineness. Increase in the complexity of sulfosalt assemblages, re-distribution of Ag within coarse native gold and dissolution-reprecipitation reactions among the sulfosalt-gold association increase the gold fineness. All (Cu)-Pb-Sb sulfosalts analyzed were found to be remarkably poor hosts for gold. Trace, yet measurable, concentrations of Au are, however, noted in the (Cu)-Pb-Bi-Sb sulfosalts, in agreement with published data indicating that (Cu)-Pb-Bi-Sb sulfosalts may be minor Au-hosts in some ore systems. Silver is preferentially partitioned into tetrahedrite at the expense of other sulfosalt phases, and tetrahedrite is thus the major host for Ag in the Gutaishan deposit. Cd and Co are typically enriched in sphalerite relative to any sulfosalt, and when present, pyrite is always enriched in Au and Co relative to all other phases. The present study shows that linking petrographic aspects at the micrometer-scale with minor/trace element distributions in complex sulfidesulfosalt assemblages can track a complex history of Au deposition and enrichment.

Bollmeyer et al. (p. 438) report their investigation of carbonate substitution into apatite. The substitution of carbonate is particularly important because of the presence of carbonate in bone mineral and the recent suggestion that most of the substituted carbonate resides in the apatite channels (A-type substitution) rather than in place of phosphate (B-type substitution). To better understand the A-type substitution, a Sr homolog of Ca hydroxylapatite was studied because of its larger channel volume and a greater unit-cell a-axial length than its unsubstituted parent. Strontium hydroxyl-, chlor-, and fluorapatites, containing incorporated 13C-carbonate up to 7 wt%, were synthesized by aqueous precipitation reactions in the presence of Na, K, and ammonium counter cations. These samples were studied by infrared spectroscopy (IR) and 13C MAS NMR. These IR and NMR spectra were interpreted as representing three channel environments (A-type substitution: A, A′, A″) and one B-type substitution. Heating samples to 600 °C resulted in the loss of carbonate and conversion to A-type carbonate demonstrating the stability of A-type carbonate at higher temperatures. Analysis of the populations of A-, A′-, and A″-, and B-sites for the hydroxyl-, chlor-, and fluorapatites prepared under both low Na and high Na conditions revealed that high Na/carbonate ratios produce a larger amount of channel substitution, contrary to observations for Ca homologs. It is speculated that multiple A-environments also exist for Ca hydroxylapatite prepared by aqueous precipitation, which is consistent with previous analysis of apatite prepared at high temperature and high pressure.

Ma and Liu (p. 447) report the first discovery of a Zn-rich mineral on the pristine surface of orange pyroclastic beads from Apollo sample 74220. This Zn-rich mineral is widely occurring, trigonal or hexagonal in shape, with a normalized composition of ~59 wt% Zn, ~26 wt% O (calculated), ~6 wt% S, ~5 wt% Na, and ~4 wt% Cl. The crystal morphology, homogeneity, and chemistry of individual grains are most consistent with gordaite, a zinc chlorohydroxosulfate mineral, showing an empirical formula of Na1.02Zn3.98[(SO4)0.84(OH)0.30](OH)6[Cl0.50(OH)0.50]·nH2O, albeit the exact amounts of OH and H2O are uncertain. The authors concluded that this zinc-rich mineral likely formed through rapid alteration (oxidation and hydration) by terrestrial air of the original vapor-deposited Zn, Cl, S, and Na-bearing solids. The composition of the zinc-rich mineral indicates that the vapor condensates consist of metallic Zn and metallic Na with either ZnS or native S, and either ZnCl2 or NaCl. This is the first direct evidence that metallic Zn and Na are key components in the vapor condensates of lunar volcanic gas, which implies lunar volcanic gas may be under higher pressure than previously thought, and the gas composition may be different than previously inferred. Additionally, the formation of this mineral indicates that detailed protocols for the handling of extra-terrestrial samples must be constructed to minimize sample modifications (e.g., destruction of previous minerals or growth of new minerals) during collection, handling, curation, and sample preparation.

McCanta et al. (p. 453) investigated ferric iron variations in lunar glasses by X-ray absorption spectroscopy mapping. Multivariate analysis (MVA) allows selection of specific channels in a spectrum to inform predictions of spectral characteristics. Here, the sparse model of the least absolute shrinkage and selection operator (Lasso) is used to select key channels in XAS channels that can be used to predict accurate in-situ Fe3+ analyses of silicate glasses. By tuning the model to use only six channels, analytical time is decreased enough to allow mapping of Fe3+ variations in samples by making gridded point analyses at the scale of the XAS beam (1–2 μm). Maps of Fe3+ concentration can then be constructed using freely available, open source software ( This result shows the enormous potential of using MVA to select indicative spectral regions for predicting variables of interest across a wide variety of spectroscopic applications. Redox gradients in lunar picritic glass beads first observed with point analyses are confirmed through this XAS mapping and suggest degassing processes during ascent and eruption are responsible for the range of Fe3+ values measured in these samples.

The Letter by Li et al. (p. 459) reports their success in using high-angle annular dark-field scanning transmission electron microscopy to depict the structural motifs in Pb-(Bi-Sb)-sulfosalts. Using two homologs from the kobellite homologous series, a group of “chessboard derivative structures,” represented by Bi-, and Sb-rich pairs of natural phases (the kobellite-tintinaite isotypic series and giessenite-izoklakeite homeotypic series), we visualize the slabs underpinning crystal structural modularity for the N = 2 homolog kobellite and the N = 4 homolog, in this case, a Bi-rich izoklakeite [Sb/(Sb+Bi) = 0.35]. The homolog number, N, can be readily calculated as N = n1/6 – 1 and N = n2/4, where n1 and n2 are the numbers of atoms in the PbS- and SnS-motifs, respectively. Atom-scale imaging of thinned foils extracted in situ from samples for which compositional data are available also reveals syntactic unit-cell scale intergrowths on [001] zone axis with akobellite || bizoklakeite. These are as small as half-unit cells of bizoklakeite and one-unit cell akobellite. Replacement relationships are also observed as irregular slabs of kobellite “intruding” into izoklakeite. Both banded and irregular intergrowths account for the compositional fields measured at the micrometer scale.

Johnson (p. 463) Reviews the new book entitled Mineralogy of Uranium and Thorium. The author concludes that Mineralogy of Uranium and Thorium is an accessible and engaging book for anyone with an interest in the mineralogy and crystallography of U- and Th-bearing minerals and the ore deposits from which they are mined.

February 2019

This issue of American Mineralogist starts with an extensive review by Luguet and Pearson (pages 165-189) on Re-Os isotopic dating of mantle peridotites using the main Re-Os host minerals—base metal sulfides (BMS) and platinum group minerals (PGM)—versus whole-rock peridotites. Comparison of the results obtained at the two scales indicates that (1) BMS may provide a record of much older partial melting event, pushing back in time the age of the lithospheric mantle stabilization (BMS±PGM are considered as the mantle equivalents of crustal zircons), (2) if only whole-rock peridotite Re-Os analyses are possible, the best targets for constraining the timing of lithospheric stabilization are BMS-free/poor ultra-refractory spinel-bearing peridotites with very minimal metasomatic overprint, (3) while lherzolites are “fertile” in terms of their geochemical composition, they do not have a “primitive,” unmodified composition, and (4) the combined Re-Os isotopic investigations of BMS and whole-rock in BMS-rich mantle peridotites would provide a complementary view on the timing and nature of the petrological events responsible for the chemical and isotopic evolution and destruction of the lithospheric mantle. In addition, the 187Os/188Os composition of the BMS±PGM within any single peridotite may define several age clusters—in contrast to the single whole-rock value—and thus provide more accurate information on the formation and evolution of the lithospheric mantle.

D’Errico et al. (pages 190-206) conducted in situ measurements of Pb and other trace elements in 150 abyssal peridotite sulfide grains (predominantly pentlandite) from the Gakkel and Southwest Indian ridges using the Sensitive High-Resolution Ion Microprobe with Reverse Geometry (SHRIMP-RG). The goal was to provide constraints on the storage of Pb and associated elements in the mantle. The authors developed a model for sulfide petrogenesis to explore the implication of assuming that all mantle Pb is hosted in mantle sulfides prior to melting. The results indicate that the measured average Pb concentration of 4 ppm (varying from 0.1 to 36 ppm) can be reproduced by >90% fractional crystallization from a sulfide melt. The remaining sulfide melt, which is modeled to contain 800 ppm Pb, will dissolve into silicate melt, as it rises through the mantle due to the increasing solubility of sulfur in silicate melt as pressure decreases. However, the amount of sulfide melt that remains after fractional crystallization is too low to contribute a significant amount of Pb to mid-ocean ridge basalts. Therefore, sulfides are not the main host for mantle Pb, even prior to the onset of any melting, and that the majority of mantle Pb is stored in silicate phases.

Griffin et al. (pages 207-219) reported a unique occurrence of magmatic hibonite-grossite-spinel assemblages, crystallized from highly reduced Ca-Al-rich silicate melts trapped within aggregates of hopper/skeletal corundum, found as ejecta from Cretaceous pyroclastic deposits on Mt Carmel, N. Israel. The crystallization conditions were comparable to those of their meteoritic counterparts. Coarse-grained aggregates of hibonite + grossite + spinel +fluorite ± krotite ± perovskite ± Ca4Al6O12F2 represent a further evolution of the silicate melts. Native vanadium occurs as rounded inclusions in the hibonite, grossite, and spinel of the coarse aggregates. The presence of V0 requires fO2 ≤ ΔIW-9, suggesting a decline in fO2 by ~3 log units during the crystallization of this assemblage. The late crystallization of Ca4Al6O12F2 together with fluorite in the hibonite-grossite-spinel aggregates suggests that crystallization of the aggregates began at T >1400 °C, cooled to the pseudo-eutectic grossite + fluorite + Ca4Al6O12F2 + liquid at ca. 1375 °C, and remained at T >1150 °C until crystallization was terminated by the volcanic eruption. This study reported the first terrestrial example of the crystallization of hibonite and grossite from high-T silicate melts, the first terrestrial occurrence of krotite, and the first occurrence of native vanadium melts.

Bollinger et al. (pages 220-231) performed deformation experiments of polycrystalline forsterite at pressures of 3.5–5.0 GPa, temperatures of 1000–1200 °C, and a strain rate of ˜2 × 10–5s–1 at various applied strains in a 6-axis Mavo press to determine the contribution of individual mechanism (dislocation creep, diffusion creep, grain boundary sliding) to olivine deformation. They developed a methodology that allows the usage of the split-cylinder technique to extract information of the deformation mechanisms from an internal surface (with engraved strain markers) in the sample before and after the deformation experiments. The results suggest the dominance of intragranular deformation, in agreement with the fact that the samples have been deformed in the dislocation creep regime. Moreover, from strain markers and out-of-plane displacements of grains, the authors obtained the first microstructural evidence for a contribution of grain boundary sliding to plastic deformation at upper mantle pressure. Electron backscatter diffraction data indicate that grain boundary processes become increasingly relevant at temperatures above 1100 °C and ensure homogenous plastic strain distribution in the aggregate. Since olivine is the major component of the upper mantle, this study sheds important light on the plasticity and rheological behavior of the mantle.

Aja (pages 232-243) reported the thermodynamic properties of two natural chlorites—a magnesian chamosite and a ferroan clinochlore )—derived from calorimetric and low-temperature hydrothermal measurements. Moreover, the author developed a molecular modeling approach to calculate the excess thermodynamic properties of chlorite solid solutions. The obtained excess entropy of mixing in the ternary Al-rich and Si-rich system exhibits a curvilinear dependence on composition, and at 25 °C, the excess Gibbs energy of mixing varies from about –72 to 413 kJ/mol, implying a significant deviation from ideality. The molecular solid solution model further reveals significant deficiencies in the available database of standard state thermodynamic properties of chlorites. Application of the results to examine the neoformation of authigenic iron chlorites in green rusts suggests that green rusts will readily transform to berthierine and Fe-chlorites except under oxidizing conditions atypical of aquatic environments and ferrugineous sediments.

Gao et al. (pages 244-261) investigated the pyroxene lamellar exsolutions and associated Fe–Ti oxides and spinels in clinopyroxne of an olivine gabbro sample from the Panzhihua intrusion, Southwest China, using high-angle annular dark-field scanning transmission electron microscopy, electron diffraction, and energy dispersive spectroscopy. The results indicate a sequence of nanoscale processes: from higher-T (~1030–1100 °C): (1) (clino)enstatite exsolutions in low-Ca diopside, followed by (2) slightly Ca-richer diopside overgrowths and high-T titanomagnetite exsolution in diopside; to lower-T (<450 °C) (3) titanomagnetite exsolutions into ulvöspinel + magnetite; followed by (4) sub-solidus re-equilibration in clinopyroxenes and among Fe–Ti oxides + hercynite. Using the exact phase boundary theory, the authors estimated the pressures of lamellar exsolution within the host diopside to be ~2 GPa. This study demonstrates that a nanoscale approach can help constrain the petrogenetic evolution during formation of layered intrusions.

Fan et al. (pages 262-275) measured the acoustic wave velocities and density of a periclase single-crystal by Brillouin light scattering combined with in situ synchrotron X ray diffraction up to ~30 GPa and 900 K in an externally heated diamond-anvil cell. Based on a comparison of the obtained elastic moduli of periclase with those of ferropericlase reported in the literature, the authors developed a comprehensive thermoelastic model for ferropericlase with up to ˜20 mol% FeO to evaluate the effect of Fe-Mg substitution on the elasticity and seismic parameters of ferropericlase at the lower mantle P-T conditions. The modeling results indicate that both the increase of the Fe content in ferropericlase and the increasing depth could change the compres¬sional wave anisotropy and shear wave splitting anisotropy of ferropericlase in the upper parts of the lower mantle. Furthermore, the authors conclude that Fe-induced lateral heterogeneities can significantly contribute to the observed seismic lateral heterogeneities in the lower mantle.

Burnley and Kaboli (pages 276-281) conducted a suite of low strain deformation experiments on polycrystalline San Carlos olivine using a deformation DIA apparatus combined with in situ synchrotron X-ray diffraction at temperatures of 440–1106 °C and pressures of 3.8–4.6 GPa. The obtained data were fitted using elastic plastic self-consistent (EPSC) models, which incorporate an isotropic deformation mechanism that permits a small amount of non-elastic defor¬mation during the initial elastic portion of the experiment. This deformation mechanism mimics the observed reduction in the elastic modulus as a function of temperature and allows for better modeling of the remainder of the stress-strain curve. The critical resolved shear stresses (CRSS) for slip obtained from these models are in good agreement with those measured in single-crystal deformation experiments. Hence, polycrystalline deformation experiments analyzed with an EPSC model may be a viable approach to measure CRSS under conditions where single-crystal deformation experiments are more challenging.

Liu et al. (pages 282-290) studied the dehydration kinetics of antigorite by thermogravimetric analysis using different heating rates of 10, 15, 20, and 25 K/min at temperatures up to 1260 K. The data were best fitted with the double-Gaussian distribution activation energy model (2-DAEM), in which a compensation effect exists between the pre-exponential factor and the average activation energy. The determined activation energy of the first step of antigorite dehydration stretches over a wide interval, whereas the second step has a significantly higher activation energy, distributed over a narrower interval. The release rate of water is 8.0×10–5 and 2.1×10–3 m3fluidm3rocks–1 at 893 and 973 K, respectively, which are near the onset temperature for the isothermal dehydration reaction. The results indicate that antigorite dehydration is fast enough to induce mechanical instabilities that may trigger seismicity in the lower plane of the double seismic zone.

Deng et al. (pages 291-299) measured the sound wave velocities and density (ρ) of the Fe5Si (9 wt% Si) alloy that possesses a body-centered cubic (bcc) structure using ultrasonic technique and synchrotron X ray radiography combined with a Paris-Edinburgh press at pressures of 2.6–7.5 GPa and temperatures of 300–1173 K. The results show that at room temperature, the addition of Si to bcc-Fe increases the compressional wave velocity (vP) but decreases the shear wave velocity (vS). At high temperatures, a pronounced effect of pressure on the vS-T relations is observed. In the studied P-T range, the vP-ρ relationship follows the Birch’s law, whereas the vS-ρ relation exhibits complex behavior. Combined with planetary/spacecraft observations, these results have important implications in constraining the compositions of the lunar and Mercurian cores.

Merkulova et al. (pages 300-306) reported the first observation of the incorporation of As3+ in goldfieldite [Cu12(As,Sb,Bi)2Te2S13] and As5+ in colusite [Cu26V2(As,Sb)4Sn2S32] inclusions in pyrite (which contains up to 50 ppm As1-) from high-sulfidation deposits in Peru, using electron probe microanalysis, synchrotron X-ray fluorescence and absorption spectroscopy. The two Cu sulfide inclusions range from several to one hundred micrometers in size, and the As3+/As5+ concentration varies from a few ppm to 17.33 wt%. The results indicate that oxidizing hydrothermal conditions prevailed during the late stage of the mineralization process in the ore deposits, and provide new insights into the substitutional mechanisms of As3+ and As5+ in copper sulfosalts. From an environmental perspective, high concentrations of potentially toxic As contained in pyrite may pose a heretofore unrecognized threat to ecosystems in acid mine drainage settings.

Brounce et al. (pages 307-312) performed measurements of the oxidation state of S in lunar apatites and associated mesostasis glass using synchrotron X‑ray absorption near edge structure spectroscopy. The results show that lunar apatites and glass contain dominantly S2–, whereas Earth apatites are only known to contain S6+. It is likely that many terrestrial and martian igneous rocks contain apatites with mixed sulfur oxidation states. The S6+/S2– ratios of such apatites could be used to quantify the fO2 values at which they crystallized, given information on the portioning of S6+ and S2– between apatite and melt and on the S6+/S2– ratios of melts as functions of fO2 and melt composition. Such a S-in-apatite oxybarometer could be developed and applied to igneous rocks from various planetary bodies in our solar system.

The issue ends with a list of the 2018 reviewers for American Mineralogist, who are thanked for their invaluable services to the journal.

January 2019

The January issue of American Mineralogist starts with a “Highlights and Breakthroughs” by Su and Liu (page 1). In their short contribution, they discuss the importance of the study by Lai et al. (published in last October's issue of American Mineralogist) on the thermoelastic properties of Fe7C3, a candidate component for the Earth's inner core.

On page 2, Cambell et al. demonstrate that the combination of zeolitized proxy-glass signatures in alkaline-mafic pyroclastic deposits and Rhyolite-MELTS can provide new insights into the magmatic evolution of mafic alkaline systems. The predictive capability of the novel procedure is demonstrated in the case of a major caldera-forming eruption, the 355 ka Villa Senni event of the quiescent Colli Albani volcano, Rome, Italy, and its pervasively zeolitized Tufo Lionato deposit (>50 km3). The key finding is that a more-evolved residual melt fraction has been revealed, based on a reconstructed SiO2/Al2O3 ratio of 2.05 relative to that of the parent magma at 2.68, with implications for a reappraisal of pre-eruptive conditions and eruption mechanisms, and potentially for similar patterns across the volcanic stratigraphy and for other alkaline volcanoes.

Chapman et al. (page 17) used large-scale large-scale electron backscatter diffraction (EBSD) and microbeam analysis to investigate crystallographic orientation and mineral chemistry data and quantify the proportion of relict igneous and neoblastic minerals forming a variably deformed, Cretaceous orthogneiss from Fiordland, New Zealand. Distinct metamorphic stages can be identified by texture and chemistry and were at least partially controlled by strain magnitude. At the grain-scale, the coupling of metamorphism and crystal plastic deformation appears to have permitted efficient transformation of an originally igneous assemblage. The effective distinction between igneous and metamorphic paragenesis and their links to deformation history enables greater clarity in interpretations of the makeup of the crust and their causal influence on lithospheric scale processes.

Mosefelder et al. (page 31) investigate nitrogen incorporation in Earth materials by a combination of chemical (SIMS, EPMA, and laser-extraction mass spectrometry) and spectroscopic (FTIR) observations to study nitrogen contents and speciation mechanisms in silicate glasses, metal alloys, and an N-bearing silicate mineral (hyalophane). They demonstrate the general veracity of EPMA analysis of N in these samples and using SIMS show that the N content determined by EPMA (or laser extraction) are best fit with exponential functions rather than the linear regressions that are most commonly applied to SIMS data. They infer that under reducing conditions at high pressure and temperature N is dissolved in basaltic melts chiefly as NH−2 and NH2–, with N2 and/or nitride (X-N3–) complexes becoming increasingly important at low fO2, increasing N content, and decreasing H content. Our results have implications for future studies seeking to accurately measure N by SIMS and for studies of N partitioning at high pressure relevant to planetary accretion and differentiation.

Wang et al. (page 47) measured deformation mechanisms in anhydrous and hydrated (4-60 ppm H2O) olivine. The hydrated and dehydrated olivines were sheared in the [100] direction on the (001) plane at pressures of 2 to 5 GPa and temperatures of 1473 or 1573 K then observed by transmission electron microscopy on the (001) plane to determine whether the [100](001) slip system was activated or not. Only c-elongated [100] dislocations were observed for the anhydrous samples, while [100](001) dislocations dominated in the hydrous samples. These results support the idea that E-type fabrics can exist under hydrous conditions and that a transition to this fabric may be the cause of seismic anisotropy decrease with depth in the asthenosphere.

Liu et al. (page 53) collected in-situ high-temperature Raman and Fourier transform infrared (FTIR) spectra for both a synthetic [Mg9Si4O16(OH)2] and a natural, F-bearing, [Mg7.84Fe0.58Mn0.01Ti0.25(SiO4)4O0.5(OH)1.30F0.20], clinohumite sample up to 1243 K. Three OH bands above 3450 cm–1 are detected for both the natural and synthetic samples with negative temperature dependence, due to neighboring H-H repulsion in the crystal structure. Additional OH peaks are detected for the natural sample below 3450 cm–1 with positive temperature dependence, indicating that F- substitution significantly changes the high-temperature behavior of hydrogen bonds in the humite-group minerals. The mode Grüneisen parameters (γiP, γiT), as well as the intrinsic anharmonic parameters (ai) for clinohumite, chondrodite, and phase A, the dense hydrous magnesium silicate (DHMS) phases along the brucite–forsterite join were also evaluated. The averaged anharmonic parameters for the DHMS phases are systematically smaller (no more than 2% at 2000 K) than those of olivine and suggest that quasi-harmonic approximations are valid for clinohumite at subduction zone temperatures. Hence, the classic Debye model can reasonably simulate the thermodynamic properties (e.g., heat capacity) of these DHMS phases in subduction zones.

Liu et al. (page 64) experimentally investigated the stability of hydrous phases in mafic oceanic crust under deep subduction conditions by high-pressure and high-temperature experiments at 17–26 GPa and 800–1200 °C. In contrast to previous studies, three hydrous phases, including Fe-Ti oxyhydroxide, Al-rich phase D and Al-rich phase H, were present at the investigated P-T conditions. These results, in combination with published data on the stability of hydrous phases at lower pressures, suggest that a continuous chain of hydrous phases may exist in subducting, cold, oceanic crust (≤1000 °C): lawsonite (0–8 GPa), Fe-Ti oxyhydroxide (8–17 GPa), Al-rich phase D (18–23 GPa), and Al-rich phase H (>23 GPa). Therefore, in cold subduction zones, mafic oceanic crust, in addition to peridotite, may also carry a substantial amount of water into the mantle transition zone and the lower mantle.

Diego Gatta et al. (page 73) investigated ettringite, (Ca6Al2(SO4)3(OH)12·26H2O), a secondary-alteration mineral with more than 40 wt.% H2O and an important crystalline constituent of Portland cements. The crystal structure and crystal chemistry of ettringite were investigated by electron microprobe analysis, infrared spectroscopy, and single-crystal neutron diffraction at 20 K. Anisotropic neutron structure refinement allowed the location of (22+2) independent H sites, the description of their anisotropic vibrational regime and the complex hydrogen-bonding schemes. Analysis of the difference-Fourier maps of the nuclear density showed a disordered distribution of the inter-column (“free”) H2O molecules of the ettringite structure. Because disorder is still preserved down to 20 K, the authors are inclined to consider that as a “static disorder.” The structure of ettringite is largely held together by hydrogen bonding: the building units (i.e., SO4 tetrahedra, Al(OH)6 octahedra, and Ca(OH)4(H2O)4 polyhedra) are interconnected through an extensive network of hydrogen bonds. The effect of the low-temperature stability of ettringite and thaumasite on the pronounced “Sulfate Attack” of Portland cements, observed in cold regions, is discussed.

Yang et al. (page 79) Investigated the behavior of hydrogen defects in 10 natural clinopyroxene crystals at temperatures up to 1000 °C using in situ and quenched experiments. The in situ high-T Fourier transform infrared (FTIR) spectra indicate no proton transfer between point defects, but the local environments of hydrogen defects vary. Dehydration rates at 1000 °C of the six samples are not only slightly site-specific but also increase with Fe and tetrahedrally coordinated Al contents. Near-FTIR spectra suggest that the dehydration of the studied samples involves oxidation of Fe2+. For two diopsides with a mantle affinity, the diffusivity is about 10–12 m2/s at 1000 °C. The results imply that the different local environments of hydrogen defects between high T and low T may be responsible for the different mechanism of water impact on electrical conductivity between high and low T experiments; and because hydrogen diffusivities are positively related to Fe and IVAl contents, more care is required for interpretation of measured water concentrations in clinopyroxenes with high Fe and IVAl contents. Based upon the hydrogen diffusivities of olivine, orthopyroxene, and clinopyroxene in mantle peridotite, clinopyroxene should be the most reliable recorder of water from a given depth.

Komabayashi et al. (page 94) examined the phase transition between a face-centered cubic (fcc) and hexagonal close-packed (hcp) structures in Fe-4wt% Si and Fe-6.5wt% Si alloys to 71 GPa and 2000 K by in situ synchrotron X-ray diffraction. The fcc-hcp phase boundaries in the Fe-Si alloys are located at higher temperatures than that in pure Fe, indicating that the addition of Si expands the hcp stability field. The dP/dT slope of the boundary of the fcc phase in Fe-4wt% Si is similar to that of pure Fe, but the two-phase region is observed over a temperature range that widens with increasing pressure, from 50 K at 15 GPa to 150 K at 40 GPa. The triple point, where the fcc, hcp, and liquid phases coexist in Fe-4wt% Si, is placed at 90–105 GPa and 3300–3600 K with the assumption that the melting curve is same as Fe. These results support the hypothesis that the hcp phase is stable at Earth's inner core conditions. The core of Mercury (well below the triple point), containing an Fe-Si alloy with a Si content up to 6.5 wt% would likely crystallize an inner core with an fcc structure. Both cores from Venus and Mars are currently believed to be totally molten. Upon secular cooling, Venus is expected to crystallize an inner core with an hcp structure, as the pressures are similar to those of the Earth's core (far higher than the triple point), whereas the Martian inner core will take an hcp or fcc structure depending on the actual Si content and temperature.

Hong et al. (page 100) studied distinctive quartz-rich unidirectional solidification textures (USTs) in apical carapaces of the Sn-mineralized Heemskirk Granite in western Tasmania (SE Australia). Individual UST layers consist dominantly of hexagonal quartz (>95%) with minor K-feldspar, plagioclase, biotite, muscovite, and magnetite. Multiple UST-quartz layers are intercalated with aplitic layers, and can locally extend for hundreds of meters. The Ti-in-quartz geothermometer yields temperatures of 545 ± 40 and 580 ± 20 °C (at 130 MPa) for the UST and aplitic quartz, respectively. The UST-quartz have higher Al/Ti values and Ge/Ti values than quartz phenocrysts in aplite layers, which is consistent with crystallization from a highly evolved fluid. LA-ICP-MS analyses show that UST-quartz has lower Ti, Li, and Sn than aplitic quartz, but higher Al, Li, Na, K, Mn, Fe, Ge, Rb, and Cs concentrations. The O-isotopic compositions (+5.1 to +10.2‰) of UST and aplitic quartz are consistent with magmatic source circulated by minor meteoric and/or formation waters. Scanning electron microscope-cathodoluminescence (SEM-CL) reveals that aplitic quartz is homogeneous and CL-bright with minor CL-dark patches. The bases of the UST quartz crystals are homogeneous and CL-bright with minor thin CL-dark fractures, whereas the trigonal apexes of the UST-quartz display CL-oscillatory growth zones. The results show that the UST layers in the Heemskirk Granite precipitated from magmatic-hydrothermal aqueous fluid exsolved from granitic melt during emplacement into the shallow crust (6–10 km). Such UST layers are characteristics of mineralized intrusions, and therefore provide significant indications for mineral exploration.

Cheng et al. (page 118) evaluate controls on cassiterite crystallization under hydrothermal conditions based upon the texture and geochemistry of cassiterite from a traverse from close to the host granitic pluton out into the mineralized country rock (Gejiu tin district, southwest China). The cassiterite samples feature diverse internal textures, as revealed by cathodoluminescence (CL) imaging, and contain a range of trivalent (Ga, Sc, Fe, Sb), quadrivalent (W, U, Ti, Zr, Hf), and pentavalent (Nb, Ta, V) trace elements, with Fe, Ti, and W being the most abundant trace elements. Cassiterite Ti/Zr ratios tend to decrease with distance away from the granite intrusion, and potentially can be used as a tool for vectoring toward a mineralized intrusive system. Elemental mapping of cassiterite grains reveals that trace-element concentration variations correspond closely to CL zoning patterns. The exceptions are distinct irregular domains that sharply cut across the primary oscillatory zoning, as defined by the concentrations of W, U, Sb, and Fe. Zones with low W and U (and Sb) and high Fe are interpreted to have formed during interaction with relatively oxidized fluids in which W and U are stripped from cassiterite due to cation exchange with Fe3+. Systematics of W, U, Sb, and Fe partitioning into cassiterite can, therefore, be used as a monitor of the relative oxidation state of the hydrothermal fluid from which cassiterite precipitates. Cassiterite U-Pb ages determined on zones of dissolution-reprecipitation are similar to ages for primary cassiterite growth and demonstrate a short (<3 m.y.) timespan of hydrothermal activity, indicating the potential of U-Pb dating of cassiterite for constraining the timing of Sn deposition.

Mookherjee et al. (page 130) performed high-pressure, high-temperature experiments on lithological compositions resembling hydrated sedimentary layers in subducting slabs and found that the phase egg, AlSiO3(OH), is stable to pressures of 20–30 GPa or depths equivalent to the transition zone to lower mantle. Thus, phase egg is a potential candidate for transporting water into the Earth's mantle transition zone. First-principles simulations based on density functional theory explored the pressure dependence of crystal structure and its influence on energetics and elasticity. The phase egg exhibits anomalous behavior of the pressure dependence of the elasticity at mantle transition zone depths (~15 GPa). The anomalous behavior is related to changes in the hydrogen bonding O-H···O configurations, which were delineated as a transition from a low-pressure to a high-pressure structure of phase egg. Full elastic constant tensors indicate that phase egg is anisotropic, resulting in a maximum anisotropy of compressional wave velocity, AvP ≈ 30% and of shear wave velocity, AvS ≈ 17% at zero pressure. Results indicate that the phase egg has one of the fastest bulk sound velocities (vP and vS) compared to other hydrous aluminous phases in the Al2O3-SiO2-H2O ternary, which include topaz-OH, phase Pi, and δ-AlOOH. At depths corresponding to the base of the mantle transition zone, phase egg decomposes to a mixture of δ-AlOOH and stishovite. The changes in compressional ΔvP and shear ΔvS velocity associated with the decomposition is ~0.42% and –1.23%, respectively. Although phase egg may be limited to subducted sediments, it could hold several weight percentages of water along a normal mantle geotherm.

Kaminsky et al. (page 140) found polycrystalline diamond grains within the Valizhgen Peninsula in Koryakia, northern Kamchatka, Russia. One grain from the Aynyn River area studied by TEM contained diamond crystallites, 2–40 μm in size, that are twinned and have a high dislocation density. The crystallites are cemented by tilleyite Ca5(Si2O7)(CO3)2, SiC, Fe-Ni-Mn-Cr silicides, native silicon, graphite, calcite, and amorphous material. Three polymorphs of SiC were discriminated: hexagonal 4H and 6H and cubic C3 (β-SiC). Silicides have variable stoichiometry with (Fe,Ni,Mn,Cr)/Si = 0.505–1.925. Native silicon is an open-framework allotrope of silicon S24, which appears to be a new natural mineral phase. Three types of amorphous material were distinguished: a Ca-Si-C-O material, similar in composition to tilleyite; amorphous carbon and amorphous SiO2. Diamond crystallites and moissanite are intensively twinned, which is characteristic when these minerals formed by gas phase condensation or chemical vapor deposition (CVD) processes. The synthetic analogs of all other cementing compounds (β-SiC, silicides, and native silicon) are typical products of CVD processes. This confirms the earlier suggested CVD mechanism for the formation of Avacha diamond aggregates. Both Avacha and Aynyn diamond aggregates are not related to “classic” diamond locations within stable cratons, but to areas of active and Holocene volcanic belts. The studied diamond aggregates from Aynyn and Avacha, by their mineralogical features and by their origin during the course of volcanic eruptions via a gas phase condensation or CVD mechanism, may be considered a new variety of polycrystalline diamond and may be called “kamchatite.” Kamchatite extends the number of unusual diamond localities. It increases the potential sources of diamond and indicates the polygenetic character of diamond.

Zhang et al. (page 150) describe spherical (Mg,Fe)-oxides with a protrusion surface in a shock-induced melt pocket from the Martian meteorite Northwest Africa 7755. Transmission electron microscopic observations demonstrate that the (Mg,Fe)-oxides are structure-coherent intergrowth of ferropericlase and magnesioferrite. The magnesioferrite is mainly present adjacent to the interface between (Mg,Fe)-oxides spherules and surrounding silicate glass, but not in direct contact with the silicate glass. Thermodynamic and kinetic considerations suggest that development of the spherical (Mg,Fe)-oxides can be best interpreted with crystallization by particle attachment and subsequent Ostwald ripening. This indicates that crystallization by particle attachment (previously hypothesized to occur in low-temperature aqueous natural and synthetic systems) can take place in high-temperature melts and has potential implications for understanding the nucleation and growth of early-stage crystals in high-temperature melts, such as chondrules in the solar nebula, erupted volcanic melts, and probably even intrusive magmas.

Etschmann et al. (page 158) provide an experimental confirmation of the suggestion, based on thermodynamic simulations and extrapolations (Zhong et al. 2015), that Zn is transported in the form of chloride complexes in most acidic, shallow hydrothermal systems; while bisulfide complexes become increasingly important in deep, pH neutral to basic hydrothermal systems. We used in situ X-ray absorption spectroscopy (XAS) diamond-anvil cell experiments to determine Zn(II) speciation in a 1 m NaHS + 0.2 m HCl solution in contact with sphalerite. XANES data indicate that Zn coordinates to oxy/hydroxyl/chloride ligands from room temperature up to and including 200 °C, and then at higher temperatures (≥300 °C) and pressures (>200 MPa) it changes to complexing with sulfur. Our data confirm that bisulfide complexes become increasingly important in neutral-alkaline solutions at high pressure and temperature, due to an increase in sulfur solubility and to favorable entropy contributions for bisulfide vs. chloride complexes.

Elimi (page 162) reviews the book: Infrared and Raman Spectroscopies of Clay Minerals, Volume 8, Developments in Clay Science, 1st Edition, by Will Gates, J. Theo Kloprogge, Jana Madejova, and Faïza Bergaya. (2017) Elsevier, pp. 620.