All our papers are special, so each month, the American Mineralogist editors will share highlights on each. We hope this information is enjoyable and useful.
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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.
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.
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.
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.
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/a, symmetry has also been recognized. While XPS shows that both C2/m and P21/a structures contain Ti4+ and Ti3+, it also indicates that Ti2+ may occur in the P21/a 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.
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 (http://cars.uchicago.edu/xraylarch/). 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  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.
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.
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  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 (001) slip system was activated or not. Only c-elongated  dislocations were observed for the anhydrous samples, while (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.