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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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 aH2O generally 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 aH2O that, 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.