Volume 107, Issue 4

Smectites are common clay minerals in surface and near surface terrestrial environments and have recently been shown to be ubiquitous on Mars. Because these minerals are products of water-rock interaction, the thermodynamics of their interaction with fluids constitutes a vital part of resolving the utility of smectite mineralizations as petrogenetic and paleoenvironmental indicators on both Mars and Earth’s near surface environments. Smectites, and other clay minerals of comparable compositional complexity, have been purported to be disequilibrium solids whose complexity derives from steep chemical gradients in their environments of formation rather than from crystal-chemical constraints. Solubility investigations of several natural smectites wherein none exhibited the predicted inverse correlation between pH-1⁄3pAl and pSi(OH) 4 were adduced by May et al. (1986) as empirical proof of the disequilibrium solid concept, and hence they asserted unequivocally that “it is obviously impossible to obtain valid ion activity quotients for smectite solubilities in these systems.” However, the unattainability of equilibrium smectite solubility in those experimental systems was probably an artifact of the extremely high fluid-to-solid ratios employed therein. In subsequent experimental studies using significantly lower fluid-to-solid ratios, smectite-fluid interactions (Kittrick and Peryea 1989; Gaboreau et al. 2020), and chlorite-fluid interactions (Aja and Dyar 2002) yielded solubility data amenable to laws of chemical thermodynamics and thus invalidated the disequilibrium solid model. Therefore, the notion of smectite metastability and/or instability anchored on the disequilibrium solid model is false and warrants a more constrained definition of smectite metastability.

In this study, we investigated an unusual natural Mn oxide hollandite-group mineral from the Kohare Mine, Iwate Prefecture, Japan, that has predominantly water molecules in the tunnels, with K, Na, Ca, and Ba. The specimens are labeled as type manjiroite, but our analyses show that Na is not the dominant tunnel species, nor is it even the primary tunnel cation, suggesting either an error in the original analyses or significant compositional variation within samples from the type locality. Chemical analyses, X‑ray photoelectron spectroscopy, and thermal gravimetric analysis measurements combined with Rietveld refinement results using synchrotron X‑ray powder diffraction data suggest the chemical formula: (K0.19Na0.17Ca0.03Ba0.01H2O1.60)(Mn5.024+Mn2.823+Al0.14Fe0.02)O13.47(OH)2.53.$\left( {{\text{K}}_{0.19}}\text{N}{{\text{a}}_{0.17}}\text{C}{{\text{a}}_{0.03}}\text{B}{{\text{a}}_{0.01}}{{\text{H}}_{2}}{{\text{O}}_{1.60}} \right)\left( \text{Mn}_{5.02}^{4+}\text{Mn}_{2.82}^{3+}\text{A}{{\text{l}}_{0.14}}\text{F}{{\text{e}}_{0.02}} \right){{\text{O}}_{13.47}}{{(\text{OH})}_{2.53}}.$Our analyses indicate that water is the primary tunnel species, and although water has been reported as a component in natural hollandites, this is the first detailed study of the crystal structure and dehydration behavior of a natural hydrous hollandite with water as the predominant tunnel species. This work underscores the rarity of natural Na-rich hollandite phases and focuses new attention on the role of hydrous components of hollandite-like phases in determining their capacities to exchange or accommodate various cations, such as Li + , Na + , Ba 2+ , Pb 2+ , and K + in natural systems.

Boninite samples from several intervals within Hole U1439C, recovered during IODP Expedition 352, show highly variable mineral chemistries that imply complex crystallization histories. Small pyroxene grains show oscillatory zoning with cores and zones ranging from pigeonite to augite. Late crystallizing augite has highly variable Al 2 O 3 contents (1.9–13.7 wt%) and Ca-Tschermak component contents (3–13 mol%), which reflect disequilibrium conditions. Large, euhedral, low-Ca pyroxene (i.e., enstatite/clinoenstatite) crystals exhibit complex sector and oscillatory zoning patterns. Cr-rich spinel is found as inclusions both in olivine and low-Ca pyroxene. Early crystallized olivine phenocrysts have embayed and reacted margins, and some early crystallized olivines exhibit zoning. A few olivines have multiple zones, with both normal and reverse zoning between Fo 86–92 . Olivine xenocrysts also have embayed and reacted margins; however, xenocrysts do not exhibit chemical zoning patterns and have consistent Fo 88 compositions. Disequilibrium crystallization of pyroxene rims reflects rapid cooling during an eruption. Sector zoning in low-Ca pyroxenes is the result of crystallization during periods of moderate undercooling between mixing events. Oscillatory, normal, and reverse zoning in olivine and pyroxene appears to have formed in response to multi-stage magma mingling or mixing processes, which introduced additional Ca, Fe, Ti, and Al into parental boninitic melts. The presence of olivine xenocrysts and orthopyroxene indicate equilibrium at 2–4 kbar (Whattam et al. 2020) indicates that boninite magma mixing events likely occurred within shallow magma chambers containing olivine-rich cumulate piles. Large mixing events probably destabilized the magma chamber, resulting in devolatilization and eruption. In contrast, small mixing events lacked the energy to destabilize the chamber, resulting in repeated compositional oscillations in minerals affected by multiple small mixing events.

It is well known that some garnet compositions can incorporate hydrogen and/or fluorine at levels up to several wt%. However, accurate measurement of these elements can be difficult at trace to minor concentration levels, so they are frequently ignored in routine chemical analysis. Furthermore, the mechanisms of H incorporation are still under debate, and only one mechanism for F substitution is commonly considered. We employed infrared spectroscopy (FTIR), elastic recoil detection analysis (ERDA), secondary ion mass spectrometry (SIMS), and electron probe microanalysis (EPMA) to measure H and F concentrations and constrain incorporation mechanisms in ten grossular garnets. We also present SIMS data for 11 spessartine and two andradite garnets. Three grossular garnets were measured with ERDA to obtain an infrared integral molar absorption coefficient (ε i ) for H 2 O of 13 470 L/(mol·cm 2 ). Grossular H 2 O and F concentrations range from 0.017 to 0.133 wt% and 0.012 to 0.248 wt%, respectively. Correlations between 16 OH and 19 F and interpretation of FTIR spectra prompt us to consider various coupled substitutions of H and F for Si, which can explain some high-frequency IR absorption bands that have been attributed previously to “hydrogrossular clusters” (variably sized clusters in which 4H substitute for Si) or to inclusions of hydrous minerals. A strong correlation between 16 OH and 19 F in spessartine and similar high-frequency IR bands implies a similar role for H-F substitution. Coupled H-F substitution is also probably relevant to some andradite-rich garnets, rare pyrope from the Dora Maira massif, and some synthetic garnets. Improvements in analytical methods for trace to minor H and F open up more possibilities for using these elements to calculate the activities of H 2 O and F-species in fluids that were in equilibrium with garnet-bearing phase assemblages, as well as constraining the recycling of these elements into the mantle via study of xenoliths.

Pyrite is one of the most important carriers for Au in hydrothermal deposits, but the incorporation mechanism of structurally bound Au has long been a matter of debate. This is due to a poor understanding of the chemical state and coordination environments of Au in the pyrite structure. Arsenian pyrite is the dominant Au-hosting mineral from the Shanggong Au deposit (130 t Au) in the southern margin of the North China Craton. Our EPMA and LA-ICP-MS results show that the arsenian pyrite has remarkably high Au (0.05–0.78 wt%) and As (0.39–4.60 wt%) contents. Au is negatively correlated with Fe but positively correlated with As. Z-contrast HAADF-STEM imaging reveals that Au atoms sit at the Fe atom sites in the arsenian pyrite structure. Our μ-XANES results and previously reported data suggest that Au and As in arsenian pyrite are predominant of chemically bound Au + and As – , respectively. We thus propose that Au + and As – are both structurally bound and sit at the Fe and S atom sites of pyrite, respectively. As may not be necessary but favorable for the incorporation of structurally bound Au in pyrite. These results have significant implications for understanding the enrichment mechanism of Au from ore-forming fluids with low-Au and low-As concentrations into arsenian pyrite and for revealing the role of As in Au mineralization of hydrothermal deposits.

Volatile-sensitive electrical soundings are becoming more widely adopted, with large nationwide arrays currently being acquired globally. This boom in new data is despite several key uncertainties relating to the electrical responses of a wide range of minerals that make up crustal regions. Complications include the influence of mineral chemistry, hydrous or nominally hydrous phases, and oxygen fugacity on charge-carrying ion activity within a mineral substrate. Feldspars are the most abundant mineral group in the Earth’s crust, comprising about 60% of its mineral assemblages and are particularly prevalent within subduction zones and lower crustal sequences. These areas are known locations where ore systems are commonly rooted, making them among the most widely studied regions in the Earth. To date, few studies exist that cover the electrical behavior of the feldspar mineral albite. To help address some of these issues and complications, we have undertaken electrical conductivity investigations on a single crystal of gem-quality albite (Ab 49 An 48 Or 3 ) from Nuevo Casas Grande, Chihuahua, Mexico. Electrical conductivity measurements using impedance spectroscopy were performed at a pressure of 1 GPa over a temperature range of 373–1273 K in a multi-anvil high-pressure apparatus. Experiments were carried out using different metal electrodes: molybdenum, nickel, and rhenium to vary the oxygen fugacity during the experiments. FTIR measurements of the starting and final materials confirm that the initial samples are completely dry but absorb an average of 67 ppm H 2 O by mass during the experiments from the surrounding pressure medium materials. We observe no correlation in the amount of water absorbed in the feldspar to the oxygen fugacity under water-undersaturated conditions. Our investigations show that the activation enthalpy increases from ~0.77 to ~1.0 eV from the nominally hydrous to the completely dry feldspar. The activation enthalpy decreases with increasing oxygen fugacity for comparable water contents. An oxygen fugacity exponent of –0.069 is calculated at the nominal water content measured in the experiments, indicating an electrical conductivity mechanism that also involves the mobility of hydrogen.

Abstraact Clinopyroxenes with excess Si have been described in run products from high-pressure experiments and inferred to have existed in nature from retrograde transformation phases. Here, we present the discovery of albitic jadeite, (Na,Ca,□ 1/4 )(Al,Si)Si 2 O 6 —the first natural, sodic clinopyroxene with excess Si occupying the octahedral cation site, M1, and a corresponding ¼ vacancy on the M2-site in the Ries impact structure and in a suite of L6 ordinary chondrites, EET 13014, EET 13052, NWA 1662, and TIL 08001. Garnet compositions in these samples indicate shock pressures of 18–22 GPa. Based on our survey, albitic jadeite is likely to be rather common in terrestrial and meteoritic shock-metamorphic environments. Shock-generated jadeite should be reexamined with respect to excess Si.

While the water storage capacities of the upper 700 km depths of the mantle have been constrained by high-pressure experiments and diamond inclusion studies, the storage capacity of the lower mantle remains controversial. A recent high-pressure experimental study on CaSiO 3 perovskite, which is the third most abundant mineral in the lower mantle, reported possible storage of H 2 O up to a few weight percent. However, the substitution mechanism for H in this phase remains unknown. We have conducted a series of density functional theory calculations under static-lattice conditions and high pressures to elucidate hydration mechanisms at the atomic scale. All of the possible dodecahedral (Ca 2+ → 2H + ) and octahedral (Si 4+ → 4H + ) substitution configurations for a tetragonal perovskite lattice have very small energy differences, suggesting the coexistence of multiples of H configurations in CaSiO 3 perovskite at mantle pressures and temperatures. The dodecahedral substitutions decrease the bulk modulus, resulting in a smaller unit-cell volume of hydrous CaSiO 3 perovskite under pressure, consistent with the experimental observations. Although the octahedral substitutions also decrease the bulk modulus, they increase the unit-cell volume at 1 bar. The H atoms substituted in the dodecahedral sites develop much less hydrogen bonding with O atoms, leading to a large distortion in the neighboring SiO 6 octahedra. Such distortion may be responsible for the non-cubic peak splittings observed in experiments on hydrous CaSiO 3 perovskite. Our calculated infrared spectra suggest that the observed broad OH modes in CaSiO 3 perovskite can result from the existence of multiples of H configurations in the phase. Combined with the recent experimental results, our study suggests that CaSiO 3 can be an important mineral phase to consider for the H 2 O storage in the lower mantle.

Abstact The discovery of chromite nanoparticles in silicates of ultramafic rocks may change our approach on the environmental risk assessment of mine waste associated with chromitite mining. This experimental study shows for the first time that the alteration of Cr-rich silicates results in the release of chromite nanoparticles and that their interaction with oxidizing Mn-oxide nanoparticles causes the dissolution of chromite and Mn-oxide nanoparticles and the precipitation of Fe 3+ - and Cr 3+ -hydroxides. Transmission electron microscopy (TEM) in combination with ultra-microtomy, centrifugation, chromatography, ICP-MS, and UV-VIS is used to characterize release and alteration of chromite nanoparticles and the concentrations and speciation of Cr aqueous species. Alteration of clinochlore grains containing chromite nanoparticles results in the release of the nanoparticles and their attachment to Si-bearing Al-hydroxide colloids. Chromite nanoparticles with the end-member composition FeCr 2 O 4 are synthesized, and their interaction with Mn-oxide nanoparticles (hausmannite, Mn 3 O 4 ), Fe 2+ -silicates (chamosite, [(Fe 3.9 Mg 0.62 Al 0.48 )Al(Si 3 Al)O 10 (OH) 8 ] and organic matter is studied over a period of six to nine months in suspensions of pH = 5. The interaction of chromite and hausmannite nanoparticles is facilitated by the aggregation of the nanoparticles and dissolution-precipitation processes. Processes on the surfaces of the hausmannite nanoparticles include the reductive dissolution of the substrate (reduction of Mn 3+ to Mn 2+ by Fe 2+ species) and its replacement by amorphous or nanocrystalline Cr 3+ -bearing Fe 3+ -hydroxides. Processes on the surfaces of the chromite nanoparticles involve the attachment of hausmannite nanoparticles, the oxidative dissolution of the substrate (oxidation of Fe 2+ and perhaps Cr 3+ by Mn 3+ species), its replacement by an amorphous or nanocrystalline Mn-bearing Cr 3+ -hydroxide matrix and the formation of the Cr 3+ -hydroxides bracewellite and grimaldite within the latter matrix. Analyses of the suspensions indicate only minor amounts of Cr 6+ in the suspensions (<5 μg kg −1 ) suggesting that formation or occurrence of Cr 6+ species can be limited in complex geochemical and mineralogical systems even in the presence of hausmannite nanoparticles.

Anisotropic absorption in crystals is routinely observed in many spectroscopic methods and is recognized in visible light optics as pleochroism in crystalline materials. As with other spectroscopies, anisotropy in Fe K -edge X‑ray absorption spectroscopy (XAS) can serve both as an indicator of the general structural arrangement and as a conundrum in quantifying the proportions of absorbers in crystals. In materials containing multiple absorbers, observed anisotropies can typically be represented by a linear relationship between measured spectroscopic peak intensities and relative absorber concentrations. In this study, oriented XAS analysis of pyroxenes demonstrates that the macroscopic theory that describes visible light absorption anisotropy of triaxially anisotropic materials can also be applied to X‑ray absorption in pyroxenes, as long as the orientation and magnitude of the characteristic absorption vectors are known for each energy. Oriented single-crystal XAS analysis of pyroxenes also shows that the measured magnitude of characteristic absorption axes at a given orientation is energy-dependent and cannot be reproduced by linear combination of intermediate spectra. Although the macroscopic model describes a majority of the anisotropy, there is distinct discordance between the observed and interpolated spectra in the pre-edge between 7109 and 7115 eV, which is marked by spikes in RMSE/mean intensity ratio. Absorption indicatrices for samples analyzed in the visible and at X‑ray wavelengths are modeled with a three-dimensional (3D) pedal surface, which functions as an empirical way of interpolating between the observed absorption data. This surface only requires a maximum of three coefficients, and results from the summation of 3D lemniscates. An absorption indicatrix model can be used to characterize anisotropic absorption in crystals and provides a way of comparing XAS spectra from randomly oriented crystals, such as those from polished sections, to a database of characterized crystals.

Hydroxyapatite (HAp) has been widely used to remove cadmium (Cd) in contaminated water and soils via Cd-Ca substitution. The Cd incorporation into HAp afects its structure; however, the detailed mechanism remains unclear. In this study, a series of Cd-substituted hydroxyapatites were synthesized and characterized with various techniques. Cd incorporation causes a decrease in a- and c- lattice parameters due to the radius of Cd 2+ being slightly smaller than that of Ca 2+ . As the Cd content increases, the particle sizes of the synthesized samples decrease and their specific surface areas increase. Raman bands shift linearly and the ν 1 (PO 4 ) peak at 961 cm −1 becomes broadened with increasing Cd content. Change in X‑ray absorption near edge structure (XANES) spectra of the P K- edge indicates distortion of phosphate with Cd incorporation. Total electron yield (TEY) spectra of the Ca L 2,3 -edge show a decrease in the octahedral symmetry, suggesting preferential occupancy of Cd over the Ca2 site. Extended X‑ray absorption fine structure (EXAFS) analysis of the Ca K- edge reveals no obvious change in the local environment of Ca induced by Cd incorporation. However, EXAFS analysis of the Cd K- edge indicates that the substituted Cd occupies one Ca2 site in hexagonal Ca2 positions at low-Cd contents [<10 mol% of Cd/(Cd+Ca)], while both Ca1 and Ca2 sites are occupied at higher Cd contents. This study provides atomistic insight into the mechanism for Cd incorporation in HAp, which will help develop an approach for effective Cd removal using HAp for environmental remediation.

A Cu-rich mantle source may play a key role in generating giant magmatic Ni-Cu deposits worldwide, but evidence for the source’s Cu enrichment and its mechanism is still rare. Copper isotopes can provide novel and direct insights into this issue since metasomatism that causes Cu enrichment in the mantle is commonly associated with a huge Cu isotope fractionation. Here we present the first Cu isotopic study on the world-class Jinchuan magmatic Ni–Cu deposit in China, including disseminated, net-textured, and massive sulfides. The disseminated and net-textured sulfides have variable δ 65 Cu values (+0.36 ± 0.38‰, n = 42), which are higher than those of massive sulfides (−0.44 ± 0.28‰, n = 11). The country rocks have a narrow δ 65 Cu range of 0.21 to 0.23‰, which is unlikely to have caused the large δ 65 Cu variations. The absence of a relationship between δ 65 Cu and whole-rock Cu contents rules out the possibility of surface weathering and diffusion-driven processes. Furthermore, the lack of correlation between δ 65 Cu and whole-rock Cu/Ni and Pd/Ir ratios excludes large Cu isotopic variations as a result of the progressive evolution of parental magma or sulfide melt. Numerical modeling indicates that the initially segregated sulfide melt has a mean δ 65 Cu of 0.44 ± 0.22‰ (2SD). Sulfide-liquid fractionation could have contributed to the enrichment of 65 Cu in the Cu-rich net-textured sulfides and depletion of 65 Cu in massive sulfides, respectively. The fractionated sulfide melts were fragmented and assimilated by new magma pluses, and consequently, the new segregated sulfide melts acquired lighter and more variable δ 65 Cu values in comparison with the initially accumulated sulfide melts. The estimated Cu isotopic composition of parental magmas for the Jinchuan Ni-Cu deposit is 0.54 ± 0.22‰ (2SD), which is up to ~0.5‰ higher than the mantle value. Copper transportation from oxidized subducted slabs to mantle peridotites and/or dissolution of Cu-bearing sulfides in the mantle caused oxidative breakdown and reprecipitation of sulfides and shifted the hybridized mantle source toward heavy δ 65 Cu as previously observed in mantle xenoliths. Our study, therefore, suggests that the source’s pre-enrichment is a key step in the generation of giant magmatic Ni-Cu deposits.

Quartz is one of the most common rock-forming minerals and crystallizes over a wide range of temperature and pressure conditions. This diversity of quartz crystallization environments is reflected by trace-element compositional variations, which can be used to distinguish between different source sediments. Trace elements that are incorporated into the quartz lattice form corresponding paramagnetic centers (impurity centers, such as Al and Ti centers), which can be detected using the electron spin resonance (ESR) method. However, whether the quartz impurity center ESR signal intensity (quartz ESR-SI) can be used for quartz sediment provenance tracing remains uncertain. In the present study, five present-day (modern) fluvial sediments from the Songhua, Yellow, Yangtze, Huai, and Pearl rivers in China and eight ancient fluvial sand lenses from the Yichang Gravel Layer (YGL) located in the middle Yangtze River were sampled for major- and trace-element determinations by ICP-OES and ICP-MS for the purpose of provenance discrimination. A total of 1404 ESR spectra were also collected to evaluate the effect of γ-ray dose (varying from 50 to 50 000 Gy) on quartz ESR-SI to establish the relationship between quartz element contents and quartz ESR-SIs and thereby to assess the potential utility of quartz ESR-SI for sediment provenance analysis. Results indicate that: (1) quartz collected from the different studied locations can be distinguished by element contents; (2) the quartz Al center ESR-SI increases with increasing γ-ray dose from 50 to 50 000 Gy; (3) the quartz Ti center ESR-SI increases within a γ-ray dose of 10 000 Gy and decreases beyond 10 000 Gy; (4) quartz Al and Ti center ESR-SIs are closely related to the contents of Al and Ti in quartz; and (5) a plot of quartz Ti center ESR-SI vs. Al center ESR-SI using data for a γ-ray dose range of 4000–7000 Gy is the best indicator of fluvial sediment provenance using the ESR method.

Oxygen fugacity ( f O 2 ) is typically controlled in high P-T experiments by using solid-state redox buffer assemblages. However, these are restricted to impose discrete f O 2 values, often with significant gaps between neighboring assemblages. Semi-permeable hydrogen membranes (Shaw 1963) are often used in internally heated pressure vessels for more flexible f O 2 control in hydrous experiments; however, their implementation in more widely available externally heated pressure vessels has not yet gained space. We propose a prototype molybdenum-hafnium carbide (MHC) pressure vessel apparatus that simultaneously allows rapid quenching and flexible, precise, and accurate redox control via a custom-designed hydrogen membrane. Test runs with two membranes at a time, one imposing and another one monitoring f H 2 , $f_{H_{2}},$demonstrated that 95% of the imposed hydrogen pressure was attained inside the pressure vessel within 2 h at 800–1000 °C, after which a steady state equilibrium was established. Furthermore, experiments comparing redox-dependent Cu solubility in silicate melts at f O 2 imposed by the fayalite-magnetite-quartz, Re-ReO 2 , and MnO-Mn 2 O 3 buffers and identical target f O 2 imposed by the hydrogen membrane confirmed consistency between the two methods within 0.25 log units f O 2 deviation at T = 900 °C and P = 2000 bar. This powerful yet cost-effective and low-maintenance apparatus may open up new pathways for studying redox reactions in hydrous magmas and magmatic fluids. As a proof of concept, we conducted near-liquidus phase-equilibrium experiments with H 2 O-saturated calc-alkaline basalt and shoshonite melt compositions at five different f O 2 values equally distributed between half log unit below the Ni-NiO buffer (NNO-0.5) and NNO+2.7. Most experiments crystallized olivine, clinopyroxene, and Ti-magnetite. The Mg# of the olivine increased with f O 2 , and the Fe 3+ /Fe total ratios in the silicate melt were determined based on Fe(II)-Mg exchange between olivine and melt. The Fe 3+ /Fe total ratios in the shoshonite melt were systematically higher by about 0.06 ± 0.01 than those in the calc alkaline basalt melt at identical f O 2 . The values determined for the basaltic melt were consistent within 1σ error (<0.033 deviation) from those predicted by the equation of Kress and Carmichael (1991). The Fe-Ti exchange coefficient between magnetite and silicate melt increases from 1.73 ± 0.19 (1σ) at NNO –0.5 to +7.12 ± 0.36 at NNO+2.7 for shoshonite and has a similar range for the calc-alkaline basalt.

As a major nominally anhydrous mineral (NAM) in the Earth’s upper mantle, orthopyroxene could host up to several hundred parts per million H 2 O in its crystal structure and transport the H 2 O to the deep Earth. To study the effect of structural H 2 O on the elasticity of orthopyroxene, we have measured the single-crystal elasticity of Mg 1.991 Al 0.065 Si 1.951 O 6 with 842–900 ppm H 2 O and 1.64 ± 0.20 wt% Al 2 O 3 at ambient conditions using Brillouin spectroscopy. The best-fit single-crystal elastic moduli ( C ij s), bulk ( K S 0), and shear ( G 0 ) modulus of the hydrous Al-bearing orthopyroxene were determined as: C 11 = 235(2) GPa, C 22 = 173(2) GPa, C 33 = 222(2) GPa, C 44 = 86(1) GPa, C 55 = 82(1) GPa, C 66 = 82(1) GPa, C 12 = 75(3) GPa, C 13 = 67(2) GPa, and C 23 = 49(2) GPa, K S0 = 111(2) GPa, and G 0 = 78(1) GPa. Systematic analysis based on the results presented in this and previous studies suggests that the incorporation of 842–900 ppm H 2 O would increase C 13 by 12.0(7)% and decrease C 23 by 8.6(8)%. The effects on C 11 , C 22 , C 33 , C 44 , C 66 , K S 0, and V P are subtle if not negligible when considering the uncertainties. The C 55 , C 12 , G 0 , and V S are not affected by the presence of structural H 2 O. Although laboratory experiments show that Fe,Al-bearing orthopyroxenes can host up to 0.8 wt% H 2 O in its structure, future high-pressure-temperature elasticity measurements on orthopyroxene with higher H 2 O content are needed to help better quantify this effect.

Lithium aluminum chloride layered double hydroxide ([Li-Al-Cl] LDH) sorbents selectively recover lithium from geothermal brines, paving the way for increased domestic production of lithium for rechargeable batteries. In this work, cryogenic heat capacity measurements ( CP ) were performed from 1.8 to 300 K on several undoped and Fe-doped [Li-Al-Cl] LDH samples with a generalized compositions Li 1– x Al2(OH) 6 Cl 1– x (undoped) and Li x Fe y Al 2–y (OH) 6 Cl x (Fe-doped). Thermodynamic functions were generated from these measurements, and values of S298.15∘ $S_{298.15}^{\circ }$are reported based on both the C P measurements and configurational entropy (Sconfig ∘) $\left( S_{\text{config }\!\!~\!\!\text{ }}^{\circ } \right)$arising from positional disorder in the layered structure. These results are combined with previous enthalpy of formation (ΔHf∘) $\left( \text{ }\!\!\Delta\!\!\text{ }H_{\text{f}}^{\circ } \right)$measurements to calculate the Gibbs energy of formation (ΔGf∘) $\left( \text{ }\!\!\Delta\!\!\text{ }G_{\text{f}}^{\circ } \right)$for the samples. In these samples, a higher water content results in a less negative ΔGf∘ $\text{ }\!\!\Delta\!\!\text{ }G_{\text{f}}^{\circ }$when doped and undoped samples are considered separately. °Limited iron substitution for aluminum results in the most negative ΔGf∘, $\text{ }\!\!\Delta\!\!\text{ }G_{\text{f}}^{\circ },$but a larger dopant °amount destabilizes the LDH structure. One of the samples had an anomaly in the heat capacity from 210 to 300 K, which is likely related to the movement of water in the structure due to the large H 2 O/Cl − ratio and the presence of vacancies in the interlayer where H 2 O resides. This indicates that the interactions between these species in the interlayer play an important role in stabilizing the LDH structure, and this effect should be further studied using different water/anion ratios.

Barium silicates may be found in contact aureoles and are used in several important technologies (e.g., LEDs). The BaO-SiO 2 system stabilizes 13 crystalline phases with different silicate tetrahedral and connectivity profiles. Aside from phases composed of a single structural unit (isolated or homo-connected tetrahedra), one encounters the relatively rare case of hetero-connected tetrahedra in which varying proportions of several Q n species are linked together. Here, we analyze the 29 Si MAS NMR and Raman spectroscopic manifestations of the connectivities in seven barium silicates: Ba 2 SiO 4 , high-BaSiO 3 , Ba 4 Si 6 O 16 , Ba 5 Si 8 O 21 , Ba 6 Si 10 O 26 , high-BaSi 2 O 5 , and sanbornite (low-BaSi 2 O 5 ). The structures and purity of these phases were confirmed by Rietveld refinement. From a Raman spectroscopic database of 144 predominantly homo-connected crystalline silicates, the mean Q n mode frequencies vQn(±1σ) ${{v}_{{{\text{Q}}^{\text{n}}}}}(\pm 1\sigma )$are found at 828 (±14) cm −1 for Q 0 , 905 (±22) cm −1 for Q 1 , 994 (±26) cm −1 for Q 2 , and 1068 (±18) cm −1 for Q 3 units. Experimentally, homo-connected barium silicates show good agreement with these values, whereas the hetero-connected phases show a wider range of ν Q2 than of ν Q3 frequencies. While the 29 Si NMR chemical shifts of the barium silicates are in agreement with known structural trends, those measured for the Q 2 resonances remain essentially constant, which may be caused by the lattice distortion around the large Ba 2+ cations. To complement and rationalize experimental measurements, first-principles calculations at the density functional theory level have reproduced measured frequencies within a mean absolute deviation of <7 cm −1 . Our work highlights how the results provided by 29 Si NMR and Raman spectroscopies and ab initio calculations can be combined to rationalize the structure of complex systems. The present findings also shed light on the vibrational modes that may be used to track bond lengths in situ at extreme conditions and the behavior of homo- vs. hetero-connectivity, revealing clear implications for evaluating silicate glasses and melts where hetero-connectivity is the rule rather than the exception.

Magmatic oxygen fugacity ( f O2 ) exerts a primary control on the discrete vanadium (V) valence states that will exist in quenched melts. Vanadium valence proxies for f O2 , measured using X‑ray absorption near-edge spectroscopy (XANES), can provide highly sensitive determinations of the redox conditions in basaltic melts. However, X‑ray beam-induced changes in V speciation will introduce uncertainty in the calculated average V valence (V*) that must be properly evaluated to make meaningful interpretations of the igneous evolution of the system. The study presented here showed that beam-induced modifications in V speciation are observed in silicate glasses that are dependent on the radiation dose rate used during analysis. Changes in V speciation are observed to be most pronounced at the highest flux density tested, 9.25 × 10 11 ph/s/μm 2 (photons per second per square micrometer), with rapid changes occurring in the first 200 s of analysis. The high-dose rate conditions result in changes in calculated V* ~0.3 valence unit for the most oxidized glass analyzed (V* = 4.94), which can correspond to ~0.5 log unit reduction in calculated f O2 . However, at flux densities ≤1.13 × 10 9 ph/s/μm 2 , measured changes in V* were found to be <0.03 for all standard glasses analyzed. The degree of reduction observed during analysis is also found to be progressively smaller as the initial V* of the glass decreases, such that magmatic glasses with V* values ≤3.7 show no statistically significant change in calculated valence during analysis at any flux density tested. For most terrestrial magmatic glasses, where V* is found to be <4, beam-induced changes in V* can be effectively minimized (<0.04), within analytical uncertainty of the XAFS analysis, by limiting flux densities to be ≤1 × 10 9 ph/s/μm 2 .

The Fe-S system was explored in a laser-heated diamond-anvil cell at 89(2) GPa and 2380(120) K to better understand the phase stability of Fe 2 S. Upon temperature quenching, crystallites of Fe 2 S were identified, and their structure was investigated using single-crystal X‑ray diffraction techniques. At these conditions, Fe 2 S adopts the C 23 structure (anti-PbCl 2 , Co 2 P) with space group Pnma ( Z = 4). This structure consists of columns of corner-sharing, FeS 4 tetrahedra, and columns of edge-sharing FeS 5 square pyramids linked along edges in the b direction. Sulfur is in ninefold coordination with Fe. This study marks the first high-pressure structural solution and refinement of Fe 2 S synthesized in a multigrain Fe+FeS sample at 90 GPa and 2400 K and establishes the stability of C 23 Fe 2 S at these conditions. A previous powder diffraction study reports an orthorhombic Fe 2 S phase with a C 37, Co 2 Si-like unit cell above 190 GPa. A C 23– C 37 structural transition is inferred to explain the previously observed unit-cell parameters at higher pressures and temperatures. These results highlight the utility of applying single-crystal X‑ray diffraction techniques to high P-T multigrain samples to explore the structural properties of iron-rich phases in Earth and planetary cores.

Grain boundaries in mantle minerals are of critical importance to geophysical and geochemical processes of the Earth’s interior. One of the fundamental issues is to understand how the water (H 2 O) component influences the properties of grain boundaries in silicate materials. Here, we report the results of the structure and stability of several tilt grain boundaries in Mg 2 SiO 4 forsterite over the pressure range 0 to 15 GPa using density functional theory-based first-principles simulations. The results suggest greater energetic stability and hydration-driven volume collapse (negative excess volume) at zero pressure for the majority of hydrous grain boundaries relative to the anhydrous (dry) ones. All the hydrous grain boundaries become increasingly favorable at elevated pressures as the calculated hydration enthalpy systematically decreases with increasing pressure. The hydrous components at the interfacial regions are predominantly in the hydroxyl form and, to a lesser extent, in the molecular H 2 O form. Their calculated ratio ranges from 1.6 to 8.7 among the different grain boundary configurations. Our structural analysis also reveals that the hydroxyls are bound to either both Mg and Si or to Mg only. In comparison, the molecular species are bound only to Mg sites. Besides direct oxygen-hydrogen bonding, intermolecular hydrogen bonding becomes important with compression. On the basis of our results, we suggest that local atomic rearrangements caused by dissociative adsorption of water facilitate efficient compaction of the boundary interfaces, which, in turn, results in greater relative stability of hydrous grain boundaries. This means that water prefers to be incorporated within the grain boundaries over the bulk of silicate materials.

In this work, we address the kinetics of dehydrogenation occurring at high temperatures (HT) in riebeckite, a sodic amphibole with the ideal composition Na2Fe23+Fe32+Si8O22(OH)2.${\rm{N}}{{\rm{a}}_2}{\rm{Fe}}_2^{3 + }{\rm{Fe}}_3^{2 + }{\rm{S}}{{\rm{i}}_8}{{\rm{O}}_{22}}{({\rm{OH}})_2}.$We performed isothermal experiments on both powders and single-crystals up to 560 °C and monitored the O-H stretching signal by Fourier transform infrared (FTIR) spectroscopy. Single-crystals show an initial increase in IR absorption intensity due to increasing vibrational amplitudes of the O-H bond stretching, not observed for powders. The OH-intensities vs. time were fitted using the formalism for first-order reactions. The calculated activation energies for H + diffusion in riebeckite are 159 ± 15 kJ/mol for powders and 216 ± 20 kJ/mol for single crystals, respectively. The exponential factor m in the Avrami-Erofeev equation obtained for crystals ranges between 1.02 and 1.31, suggesting that, unlike powders, the dehydration process in crystals is not a purely first-order reaction. This implies that a second energy barrier must be considered, i.e., diffusion of H + through the crystal. FTIR imaging showed that H + diffusion occurs mainly perpendicular to the silicate double-chain. Our results confirm that the release of H + from riebeckite occurs after the irreversible Fe 2+ -to-Fe 3+ exchange, thus at temperatures >550 °C. To be effective, the process needs the presence of external oxygen that, by interacting with H + at the crystal surface, triggers the release of H 2 O molecules. This implies that oxidizing conditions are required for the amphibole to be an efficient water source at depth.

Silician magnetite within ~1.85 Ga lithologies hosting the ~1.6 Ga Wirrda Well iron oxide copper gold (IOCG) prospect, South Australia, was examined at the nanoscale. The magnetite is oscillatory-zoned with respect to the density and orientation of nanometer-scale inclusions, among which Si-Fe-nanorods and Al-rich amphibole (as much as hundreds of nanometers long and tens of nanometers wide) form swarms along <111> directions in magnetite. The amphibole is identified as ferro-tschermakite (Ftsk) with the crystal-chemical formula: A   K 0.06 N a 0.01 0.07 B C a 1.65 N a 0.35 2 C F e 2.07 2 A l 1.64 M g 1.15 T i 0.06 F e 0.04 33 M n 0.04 5 T S i 6.48 A l 1.52 8 O 22 W ( O H ) 2 . ${ }^{A}\left(\mathrm{~K}_{0.06} \mathrm{Na}_{0.01}\right)_{0.07}{ }^{B}\left(\mathrm{Ca}_{1.65} \mathrm{Na}_{0.35}\right)_{2}^{C}\left(\mathrm{Fe}_{2.07}^{2} \mathrm{Al}_{1.64} \mathrm{Mg}_{1.15} \mathrm{Ti}_{0.06} \mathrm{Fe}_{0.04}^{33} \mathrm{Mn}_{0.04}\right)_{5}^{T}\left(\mathrm{Si}_{6.48} \mathrm{Al}_{1.52}\right)_{8} \mathrm{O}_{22}{ }^{W}(\mathrm{OH})_{2}.$This contains single and double rows of a triple-chain silicate attributed to clinojimthompsonite (Cjt) as coherently intergrown (010) zippers along the entire length of the grains. High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) imaging and simulation of Ftsk and Cjt on the [001] zone axis provide direct visualization of crystal structures. These are defined by the 7- and 10-atom octahedron strips ( B + C sites) and flanked by double- and triple-pairs of Si atoms ( T sites). Remarkably, the sites for light cations and/or vacancies are clearly imaged as single and double, darkest, diamond-shaped motifs separating the octahedron strips showing that A cavities known in amphibole are readily depicted in the wider-chain silicate. I-beam models show that nanoscale intergrowths among the two silicates are coherent along zigzag chains of cations at the edges of the octahedron strips, with single and double rows of the triple-chain silicate corresponding to 1 and 1.5 unit cells of Cjt (27 and 41 Å intervals along the b axis). This type of polysomatic chain-width disorder is widely reported in Mg-rich pyriboles but is shown here in an Al-Fe-rich amphibole. The lack of planar deffects and/or reaction fronts at mutual contacts between three-chain zippers and host amphibole indicates primary co-crystallization growth, promoted by the formation of the Si-Fe-nanorods. Co-crystallizing plagioclase is also preserved in close vicinity to the amphibole hosted by magnetite (from a few nanometers to micrometers apart). In contrast, the replacement of amphibole by phyllosilicates is recognizable as irregular swells along the (010) zippers and results in extensive chloritization of the amphibole during an overprinting event. Pressures of ~11.5 kbar are estimated using Al-in-hornblende nano-geobarometry and calculated Al content in Ftsk (3.16 apfu). Assuming the amphibole-plagioclase association buffered by host magnetite fulfills the textural equilibration criteria required for application of this barometer, we interpret the Ftsk nanoinclusions in magnetite as preserved evidence for amphibolite facies metamorphism afecting host lithologies at Wirrda Well with subsequent retrograde alteration during the ~1.6 Ga IOCG mineralizing event. Magnetite records petrogenetic processes by accommodating variable ranges of nanomineral inclusions and preserving them over geological time scales. HAADF STEM imaging is ideally suited to the depiction of crystal-structural modularity and also provides insights into the evolution of geological terranes with protracted histories.

In this issue of New Mineral Names, a thematic approach is used to help provide context for advances and discoveries in mineralogy. Recovered high-pressure minerals are not commonplace, rare platinum group elements tend to be recovered from important ore bodies, and then there are the minerals/collections that are culturally important and significant to the history of science. Here we investigate the newly discovered high-pressure minerals and highly precious minerals: davemaoite, kaitianite, elgoresyite, oberthüite, torryweiserite, palladothallite, fleetite, tamuraite, marathonite, palladogermanite, and kernowite.