Volume 99, Issue 8-9

Transmission electron microscopy in combination with in situ high-pressure and high-temperature measurements is uniquely able to provide high-resolution data about materials under conditions resembling those in Earth’s interior. By using nanocontainers made of graphitized carbon, it is possible to achieve pressures and temperatures up to at least 40 GPa and 1500 °C, respectively. A wide range of relatively simple minerals have been studied using this approach. Results to date show the influence of crystallographic defects in concentrating and storing carbon within analogs to minerals occurring deep inside Earth.

A collection of 35 diamondite samples (polycrystalline diamond aggregates, sometimes referred to as framesites), assumed to be from southern Africa, have been studied to investigate their infrared (IR) spectroscopic characteristics. Due to the abundance of sub-micrometer, interlocking diamonds (polycrystalline) with mineral and fluid inclusions within the diamond material affecting their transparency, only fragments from 10 of the samples provided high-quality data. The IR spectra showed a wide range of generally high-nitrogen concentrations (386-2677 ppm), with a full range of nitrogen aggregation states, from pure IaA to pure IaB. Platelet characteristics were interpreted as being regular (i.e., not having been affected by deformation and/or heating events), meaning the nitrogen aggregation data could be interpreted with confidence. Surprisingly, the platelet data showed a positive correlation between their intensity (integrated area) and peak position. The primary hydrogen band (at 3107 cm -1 ) and secondary band (at 1405 cm) are both often present in the samples’ spectra, but show no correlation with any other characteristic. There is also no correlation between the samples’ paragenesis (as defined by their garnet chemistry) and any of the IR characteristics. While we have no independent determination of the samples mantle residence age, nor the temperature they resided at, we infer that diamondite formation has occurred episodically over a large time frame in single and distinct growth events (as opposed to over a short time frame but over a large depth/temperature range). This idea is more in keeping with the theory that C-O-H diamond- (and diamondite-) forming fluids are the result of localized small volume processes. Interestingly, one sample contained fluid inclusions that exhibited a water:carbonate molar ratio (~0.8), similar to the saline and silicic end-members of the monocrystalline diamond-forming fluid chemical spectrum.

Pressure-temperature melting curves in two carbonate-bearing systems of relevance to Earth’s mantle have been determined using the laser-heated diamond-anvil cell (LH-DAC). The solidus along the MgSiO 3 -MgCO 3 join in the MgO-SiO 2 -CO 2 system (MS-CO 2 ) was defined from 15 to 80 GPa using in situ melting criteria, reaching a maximum temperature of ~2340 K at 80 GPa. The occurrence of melting has been confirmed with ex-situ textural and chemical analysis of recovered samples. The melting curve has a negative dT/dP slope at pressures between ~15 and 23 GPa resulting from the subsolidus phase transition of ilmenite- to perovskite-structured MgSiO 3 . The shallow slope of the melting curve at pressures higher than this transition indicate that for plausible mantle geotherms carbonate-bearing silicate lithologies will melt throughout the lower mantle. The solidus of a mixture along the MgCO 3 -CaCO 3 join was determined as a proxy for alkali-free carbonate lithologies. Melting temperatures increase from 1860 K at 16 GPa to ~2100 K above 35 GPa, where the melting curve flattens. The melting reaction magnesite + post-aragonite (high-pressure CaCO 3 ) = melt was confirmed using an in situ experiment. We conclude that crystalline Mg and Ca carbonate mixtures are unstable with respect to molten carbonate at conditions of the convective lower mantle. The flat melting curves at high pressures in both systems suggests that subducted carbonates will undergo melting at lower mantle conditions, a process that may be important for superdeep diamond formation and carbon storage in the deep mantle.

Valence, spin states, and crystallographic sites of Fe in (Mg,Fe)SiO 3 perovskite were investigated using energy-domain 57 Fe-synchrotron Mössbauer spectroscopy and powder X-ray diffraction up to 86 GPa. The volumes of Fe 3+ -bearing perovskite in this study are slightly smaller than those of Mg endmember perovskite. Our Mössbauer data suggest that Fe 3+ prefers A sites coupled with Mg vacancies, which is consistent with previous data at ambient conditions. Fe 3+ in the A site remains in a high-spin state up to 86 GPa, and some fraction of the A site is occupied by Fe 2+ at pressures above 30 GPa. Fe 2+ in the A sites is also in a high-spin state up to 86 GPa. The coupled substitution from Mg 2+ to a highspin state of Fe 3+ and Mg 2+ vacancy would make the volume of perovskite smaller than that of Mg end-member perovskite. If the lower mantle is saturated in silica, perovskite containing high-spin Fe 3+ in A site has a higher density. Such silica oversaturated regions could sink by the density difference.

At lower mantle conditions, subducted mid oceanic ridge basalts (MORB) will crystallize more than 20 vol% of an aluminum-rich phase, which is referred to generally as the new aluminum (NAL) phase. Given that a significant proportion of the lower mantle may be comprised of subducted crust, the NAL phase may contribute to the bulk elastic properties of the lower mantle. In this study we report for the first time the structure, Raman spectrum and elasticity of single crystals of Na 0.41 [Na 0.125 Mg 0.79 Al 0.085 ] 2 [Al 0.79 Si 0.21 ] 6 O 12 NAL phase, synthesized at 2260 °C and 20 GPa. The single-crystal structure refinement of NAL, which is consistent with the space group P6 3 /m, reveals dynamic disorder of Na atoms along channels within the structure. The elastic tensor was experimentally determined at ambient conditions by Brillouin scattering spectroscopy. The elastic modulii obtained from the Voigt- Reuss-Hill approximation using the elastic constants determined in this study are K S = 206 GPa and μ = 129 GPa, whereas the isotropic compressional and shear sound velocities are v P = 9.9 km/s and v S = 5.8 km/s. The NAL phase is elastically anisotropic, displaying 13.9% compressional and shear wave anisotropy. Elastic constants as well as Raman active modes of NAL have also been calculated using density-functional theory and density-functional perturbation theory.

We have built three multivariate analysis mathematical models based on principal component analysis (PCA), partial least squares (PLS), and artificial neural networks (ANNs) to detect sulfate minerals in geological samples from laser Raman spectral data. We have critically assessed the potential of the models to automatically detect and quantify the abundance of selected Ca-, Fe-, Na-, and Mg-sulfates in binary mixtures. Samples were analyzed using a laboratory version of the Raman laser spectrometer (RLS) instrument onboard the European Space Agency 2018 ExoMars mission. Our results show that PCA and PLS, can be used to quantify to some extent the abundance of mineral phases. PCA separated hydrated from dehydrated mixtures and classified mixtures depending on the phase abundances. PLS provided relatively good calibration curves for these mixtures. Upon spectral pre-processing, ANNs provided the most precise qualitative and quantitative results. The detection of mineral phases was 100% accurate for pure samples, as was for binary mixtures where the abundance of mineral phases was >10%. The outputs of the ANN were proportional to the phase abundance of the mixture, thus demonstrating the ability of ANNs to quantify the abundance of different phases without the need for calibration. Taken together, our findings demonstrate that multivariate analysis provides critical qualitative and quantitative information about the studied sulfate minerals

K + , Na + , Ca 2+ , Mg 2+ , Fe 2+ , Fe 3+ , and Al 3+ perchlorate salts were studied to provide spectral and thermal data for detecting and characterizing their possible presence on Mars. Spectral and thermal analyses are coordinated with structural analyses to understand how different cations and different hydration levels affect the mineral system. Near-infrared (NIR) spectral features for perchlorates are dominated by H 2 O bands that occur at 0.978-1.01, 1.17-1.19, 1.42-1.48, 1.93-1.99, and 2.40-2.45 μm. Mid-IR spectral features are observed for vibrations of the tetrahedral ClO 4 - ion and occur as reflectance peaks at 1105-1130 cm -1 ( ~ 8.6-9 μm), 760-825 cm -1 ( ~ 12-13 μm), 630 cm -1 ( ~ 15.9 μm), 460-495 ( ~ 20-22 μm), and 130-215 ( ~ 50-75 μm). The spectral bands in both regions are sensitive to the type of cation present because the polarizing power is related to the band center for many of the spectral features. Band assignments were confirmed for many of the spectral features due to opposing trends in vibrational energies for the ClO 4 - and H 2 O groups connected to different octahedral cations. Differential scanning calorimetry (DSC) data show variable patterns of water loss and thermal decomposition temperatures for perchlorates with different cations, consistent with changes in spectral features measured under varying hydration conditions. Results of the DSC analyses indicate that the bond energies of H 2 O in perchlorates are different for each cation and hydration state. Structural parameters are available for Mg perchlorates (Robertson and Bish 2010) and the changes in structure due to hydration state are consistent with DSC parameters and spectral features. Analyses of changes in the Mg perchlorate structures with H2O content inform our understanding of the effects of hydration on other perchlorates, for which the specific structures are less well defined. Spectra of the hydrated Fe 2+ and Fe 3+ perchlorates changed significantly upon heating to 100 °C or measurement under low-moisture conditions indicating that they are less stable than other perchlorates under dehydrated conditions. The perchlorate abundances observed by Phoenix and MSL are likely too low to be identified from orbit by CRISM, but may be sufficient to be identifiable by a VNIR imager on a future rover.

Visible and near-infrared wavelength (VNIR, λ = 0.35-5 mm) laboratory diffuse reflectance spectra and corresponding optical functions (real and imaginary refractive indices) for several iron sulfates (natural K- and Na-jarosite, szomolnokite, rhomboclase) are presented. On Mars, jarosite has been identified in Meridiani Planum, Mawrth Vallis, Melas Chasma, and Eridania Basin; szomolnokite has been found as distinct layers at Columbus Crater and as outcrops at Juventae Chasma, and rhomboclase has been identified at Gusev Crater. Constraining the mineralogy and chemistry (Fe- vs. Mg-rich) of the sulfates on Mars may contribute to our understanding of the environmental and aqueous conditions present on Mars during their formation. The data presented here will help to constrain the mineralogy, abundance, and distribution of sulfates on the martian surface, which will lead to improvements in understanding the pressure, temperature, and humidity conditions and how active frost, groundwater, and atmospheric processes once were on Mars.

Oxygen fugacity (ƒ O2 ) is a fundamental parameter that controls carbon mobility in aqueous fluids in geological environments such as subduction zones, where reduced serpentinite fluids have the potential to infiltrate oxidized carbonate-bearing lithologies. Using experiments and calculations, we describe how mineral-fluid equilibria evolve as ƒ O2 decreases in the model Ca- C-O-H system at forearc conditions (300-700 °C and 2-10 kbar). Experimental calcite solubility was constant at ƒ O2 values from quartz-fayalite-magnetite (QFM) to hematite-magnetite (HM). At lower ƒ O2 values of iron-magnetite (IM) or wüstite-magnetite (WM), calcite reacted with H 2 to form methane plus portlandite or melt. These results were consistent with thermodynamic calculations and indicate that carbon mobility, as parameterized by total aqueous carbon ([CTOT]), is strongly dependent on ƒ O2 . At constant pressure and temperature, carbon mobility is minimized at oxidizing conditions, where [CTOT] is controlled by calcite solubility. Carbon mobility is maximized at the most reducing conditions because all the carbon in the system is present as CH 4 . An intermediate region of carbon mobility exists in which calcite is stable with a CH 4 -bearing fluid. As pressure increases from 2 to 10 kbar, the ƒ O2 range over which calcite is stable with a methane-rich fluid shifts to more reducing conditions. The variety of geological conditions with the potential for redox enhancement of carbon mobility becomes more restricted with depth. Reduction melting was observed at 700 °C and 6 kbar, and at 650 °C and 10 kbar, due to the partial reaction of calcite to portlandite at conditions above the hydrous melting curve of calcite+portlandite. Although likely metastable in the present experiments, reduction melting may occur in nature whenever H 2 partially reduces carbonate minerals at pressures and temperatures above the hydrous melting curve of calcite+portlandite. Whether it causes melting or not, calcite reduction is likely an important mechanism for abiotic methanogenesis in natural systems such as subduction zone forearcs or similar environments with the potential for interaction of reduced fluids with carbonate minerals. Because calcite solubility at oxidized conditions is already known to increase substantially with pressure, the additional increase in carbon mobility provided by calcite reduction implies that subduction zones may host some of the most carbon-rich aqueous fluids on Earth.

The partitioning of Zr between high P-T aqueous fluids and melts has been investigated in situ in the haplogranite-H 2 O and haplogranite-(F)-H 2 O systems to assess the mobilization of high field strength elements (HFSE) in magmatic-hydrothermal processes in subduction zones. The partition coefficients D f/Z mr were determined from Zr concentrations measured in situ by synchrotron X-ray fluorescence (SXRF) in both aqueous fluids and F-free or F-bearing hydrous haplogranite melts equilibrated in diamond-anvil cells at 575 to 800 °C and 0.3 to 2.4 GPa. This experimental approach eliminates the need for internal or external calibrations of the SXRF signal and/or post-mortem analysis of the melt phase, hence decreasing the total uncertainties on D f/Z mr below 16%. Above 0.6 GPa, Zr partitions favorably into the hydrous silicate melt in both F-free and F-bearing systems, with D f/Z mr that range between 0.19 ± 0.02 and 0.38 ± 0.03. However, the relatively high D f/Z mr values indicate that alkali-silica rich aqueous fluids generated by metamorphic devolatilization may contribute significantly to the recycling of HFSE in subduction zones. The efficient uptake of Zr (and likely other HFSE) by subduction zone fluids, regardless of their nature (aqueous fluid, hydrous melt, or supercritical fluid), supports the idea that the typical HFSE depletion recorded in arc magmas does not result from their incompatibility in water-rich slab-derived fluids but most probably originates from complex fluid-melt-rock interactions occurring at the slab interface and within the mantle wedge. At shallow crustal pressure conditions (800 °C and 0.3 GPa), Zr partitions reversely into the aqueous fluid in the presence of fluorine (D f/Z mr = 1.40 ± 0.10) as observed for Nb at similar conditions by Webster et al. (1989). The enrichment of the aqueous phase in HFSE (Zr, Nb) at shallow crustal conditions is likely related to the enhanced peralkalinity of low pressure, F-bearing aqueous fluid with temperature, that provides the favorable conditions for their mobilization via the formation of HFSE-O-Si/Na clusters. This mechanism may control the enrichment in HFSE (and plausibly other rare metals such as REE) in early magmatic fluids exsolved from granitic melts, leading to the formation of HFSE-enriched aggregates in shallow magmatic-hydrothermal environments (e.g., Strange Lake and Thor Lake Nechalacho deposit, Canada; Galineiro complex, Spain).

This study explores the petrogenesis of Shişr 161, an immature lunar regolith breccia meteorite with low abundances of incompatible elements, a feldspathic affinity, and a significant magnesian component. Our approach was to identify all clasts >0.5 mm in size in a thin section, characterize their mineral and melt components, and reconstruct their bulk major and minor element compositions. Trace element concentrations in representative clasts of different textural and compositional types indicate that the clast inventory of Shişr 161 is dominated by impact melts that include slowly cooled cumulate melt rocks with mafic magnesian mineral assemblages. Minor exotic components are incompatible-element-rich melt spherules and glass fragments, and a gas-associated spheroidal precipitate. Our hypothesis for the petrologic setting of Shişr 161 is that the crystallized melt clasts originate from the upper ~1 km of the melt sheet of a 300 to 500 km diameter lunar impact basin in the Moon’s feldspathic highlands. This hypothesis is based on size requirements for cumulate impact melts and the incorporation of magnesian components that we interpret to be mantle-derived. The glassy melts likely formed during the excavation of the melt sheet assemblage, by an impact that produced a >15 km diameter crater. The assembly of Shişr 161 occurred in a proximal ejecta deposit of this excavation event. A later impact into this ejecta deposit then launched Shişr 161 from the Moon. Our geochemical modeling of remote sensing data combined with the petrographic and chemical characterization of Shişr 161 reveals a preferred provenance on the Moon’s surface that is close to pre-Nectarian Riemann-Fabry basin.

Water is dissolved in silicate glasses and melts as hydroxyl groups and molecular water, with mostly hydroxyl groups at low water contents and mostly molecular water at high water contents. However, we recently predicted that water will be dissociated nearly completely in potassium aluminosilicate glasses with more alumina than silica because of the strong aluminum avoidance and the strong tendency for Al-O-Al linkages to hydrolyze in such glasses. In the present study, I test this prediction on hydrous K 2 Al 2 SiO 6 glasses: the Raman and infrared absorption spectra show that water is indeed predominantly present as hydroxyl groups, even for glasses with more than 7 wt% water. This observation validates the previously proposed speciation reactions, demonstrates that variations in water speciation are related to the nature of the cations to which the hydroxyl groups are bonded, and indicates that the classical picture of water dissolution, with predominantly molecular water at high water contents, may not apply near compositional extremes.

A detailed microscopic study of sepiolite and palygorskite natural samples reveals that independently of the fiber length they all are composed of other minor width fibers until the minor units or the true crystals. They are prismatic crystals elongated along [001] with rhombus-like morphology in cross section, designated laths. Although their length can vary greatly, their width is always nanometric (~10-30 nm). The laths are grouped in a crystallographic arrangement, sharing edges or faces forming rods. Several rods, parallel to the c-axis of the fiber, form bundles. The laths are the smallest stable nucleated crystals. After the nucleation, the process of growth continues via a nonclassic crystal growth mechanism (aggregation of the nanolaths). Subsequently, the aggregated sepiolite and palygorskite natural crystals can continue growing, along the c-axis at the expense of the diffusion of molecular scale species throughout the solution. The morphology of the faces is controlled by the highly different attachment energy of the faces.

Geologic disposal of spent nuclear fuel in high-capacity metal canisters may reduce the repository footprint, but it may yield high-thermal loads (up to 300 °C). The focus of this experimental work is to expand our understanding of the hydrothermal stability of bentonite clay barriers interacting with metallic phases under different geochemical, mineralogical, and engineering conditions. The hydrothermal experiments were performed using flexible Au/Ti Dickson reaction cells mounted in an externally heated pressure vessel at 150-160 bars and temperatures up to 300 °C for five to six weeks. Unprocessed Wyoming bentonite, containing primarily montmorillonite with minor amount of clinoptilolite, was saturated with a K-Ca-Na-Cl-bearing water (~1900 mg/L total dissolved solids) at a 9:1 water:rock mass ratio. The bentonite and solution combination contained either steel plates or Cu-foils and were buffered to low Eh using magnetite and metallic iron. During reactions, pH, K + , and Ca 2+ concentrations decreased, whereas SiO 2(aq) , Na + , and SO 4 2- concentrations increased throughout the experiments. Pyrite decomposition was first observed at ~210 °C, generating H 2 S (aq,g) that interacted with metal plates or evolves as a gas. The aqueous concentrations of alkali and alkaline earth cations appear to be buffered via montmorillonite and clinoptilolite exchange reactions. Illite or illite/smectite mixed-layer formation was significantly retarded in the closed system due to a limited K + supply along with high Na + and SiO 2(aq) concentrations. Precursor clinoptilolite underwent extensive recrystallization during the six weeks, 300 °C experiments producing a Si-rich analcime in addition to authigenic silica phases (i.e., opal, cristobalite). Analcime and feldspar formation partially sequester aqueous Al 3+ , thereby potentially inhibiting illitization. Associated with the zeolite alteration is a ~17% volume decrease (quartz formation) that translates into ~2% volume loss in the bulk bentonite. These results provide chemical information that can be utilized in extending the bentonite barriers’ lifetime and thermal stability. Zeolite alteration mineralogy and illitization retardation under these experimental conditions is important for the evaluation of clay barrier long-term stability in a spent nuclear fuel repository.

Numerical simulation of stranded diffusion profiles in partially resorbed garnet crystals from the aureole of the Makhavinekh Lake Pluton (Labrador, Canada) yields quantitative rates of intracrystalline diffusion for Li in garnet. Diffusion coefficients for Li at 700-900 °C and 0.53 GPa are 0.5-1.5 log10 units lower than those for divalent cations in the same samples, and match those for Y and Yb. This correspondence likely stems from coupled substitution of Li and Y (or REE) ions in the garnet structure: the requirement of local electroneutrality limits Li mobility to be no faster than that of Y+REEs. Because of this coupling, Li zoning in garnet is retained to comparatively high temperatures, making garnet a valuable monitor of the behavior of Li in deep crustal systems.

Djerfisherite is a K-Cl-bearing sulfide that is present in both ultra-reduced extraterrestrial enstatite meteorites (enstatite chondrites or achondrites) and reduced terrestrial alkaline intrusions, kimberlites, ore deposits, and skarns. Major element chemistry of two terrestrial occurrences of djerfisherite (from the Ilímaussaq and Khibina alkaline igneous suites) and three extraterrestrial examples of djerfisherite have been determined and combined with petrographic characterization and element mapping to unravel three discrete modes of djerfisherite formation. High Fe/Cu is characteristic of extraterrestrial djerfisherite and low Fe/Cu is typical of terrestrial djerfisherite. Ilímaussaq djerfisherite, which has high-Fe contents (~55 wt%) is the exception. Low Ni contents are typical of terrestrial djerfisherite due to preferential incorporation of Fe and/or Cu over Ni, but Ni contents of up to 2.2 wt% are measured in extraterrestrial djerfisherite. Extensive interchange between K and Na is evident in extraterrestrial samples, though Na is limited (<0.15 wt%) in terrestrial djerfisherite. We propose three setting-dependent mechanisms of djerfisherite formation: primitive djerfisherite as a product of nebula condensation in the unequilibrated E chondrites; formation by extensive K-metasomatism in Khibina djerfisherite; and as a product of primary “unmixing” due to silicate-sulfide immiscibility for Ilímaussaq djerfisherite. There are several important reasons why a deeper understanding of the petrogenesis of this rare and unusual mineral is valuable: (1) its anomalously high K-contents make it a potential target for Ar-Ar geochronology to constrain the timing of metasomatic alteration; (2) typically high Cl-contents (~1.1 wt%) mean it can be used as a valuable tracer of fluid evolution during metasomatic alteration; and (3) it may be a potential source of K and magmatic Cl in the sub-continental lithospheric mantle (SCLM), which has implications for metal solubility and the generation of ore deposits.

Extensive negative aeromagnetic anomalies in the Modum area, south Norway, derive from rocks containing ilmenite with hematite exsolution, or hematite with ilmenite exsolution, carrying strong/stable reversed remanence. Here we describe a 2.5 cm thick high-temperature metamorphic vein of exsolved titanohematite. Reflected-light and EMP analyses show it contains three types of exsolution: spinel plates on (001); rutile blade satellites on spinel oriented at angles of ~60-90° to titanohematite (001); and lamellae 0.1-0.3 mm thick too fine for EMP analyses, also parallel to (001). Powder XRD gave a = 5.0393 Å, c = 13.7687 Å, V = 302.81 Å 3 for titanohematite (≈Ilm9), and unrefined reflections of rutile and geikielite. Overlap EMP analyses showed enrichment in MgO, TiO 2 , and lack of Al 2 O 3 , indicating a mixture of titanohematite and geikielite. Non-overlap analyses showed the titanohematite is 6%Fe2+TiO 3 , 2%MgTiO 3 , 92% Fe 2 O 3 , generally confirmed by TEM-EDX analyses that also showed the geikielite is 30%Fe 2+ TiO 3 , 70%MgTiO 3 . Orientation and interface relationships between exsolutions and host titanohematite were characterized with TEM, using conventional and high-resolution imaging complemented by selected-area electron diffraction. Spinel shares (111) with (001) of titanohematite and geikielite (001) the same. The epitactic relationship between rutile and titanohematite, previously not well constrained, was estimated from reflected-light and TEM images and lattice-fit studies. The a 1 axis of rutile is parallel to a 1 of hematite and c of rutile is normal to a 2 of hematite, all in the hematite basal plane, which, however is not a phase interface. The rutile appears to occur in blades within prism planes in titanohematite located ~69° from a axes of hematite, with long axes of the blades oriented in a minimum strain direction within the planes at ~63° from the (001) basal plane. Spinel and rutile, analyzed by EMP, exsolved first. Spinel gave 96%MgAl 2 O 4 , 3%FeFe 2 O 4 , Mg/total R 2+ = 0.98. Magnesian/aluminous spinel lacking Ti exsolved from titanohematite in coupled exsolution with ferrian rutile, where combined components were dissolved as corundum/geikielite components in high-T aluminous magnesian titanohematite. Early exsolution lowered geikielite, and eliminated the corundum component. Later fine exsolution of ferroan geikielite moved the titanohematite closer to Fe 2 O 3 . Mg 2+ has no magnetic moment, but breaks up linkages between Fe atoms, lowers Néel Ts, and produces unusual low-T properties. This titanohematite has Néel T, 873 K (600 °C). Geikielite at 70%MgTiO 3 , is far beyond its theoretical nearest-neighbor percolation threshold at 30.3%MgTiO 3 . However, the sample shows a negative magnetic exchange bias below 25 K and low-T remanence lost above ~40 K. Such properties are reported in samples containing thin ilmenite lamellae in titanohematite, in theory with odd numbers of Fe layers, where exchange bias is linked to lamellar magnetism at the phase interfaces, when the ilmenite becomes a high-anisotropy magnet in a magnetically softer host. Potential explanations for the behavior of ferroan geikileite are discussed.

Incorporation mechanisms of Cl in peralkaline and peraluminous Na 2 O-CaO-Al 2 O 3 -SiO 2 glasses as a model system for phonolitic melts were investigated using 35 Cl, 23 Na, 27 Al, and 29 Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The size and large distribution of electric field gradients for 35 Cl causes loss of signal in the MAS NMR experiment and this, in combination with the low concentration of Cl and the large chemical shift dispersion, means that even at the highest available fields we are at the limits of MAS NMR. Nevertheless clear differences in the Cl environment in peralkaline and peraluminous glasses can readily be seen. In both glass types Cl exists in relatively symmetric Na-Ca-Cl environments. The 35 Cl chemical shift indicates that the Cl environment is dominated by the presence of Na cations, consistent with the Na/Ca ratio of 5/1 in the glasses. 35 Cl MAS NMR spectra of the peraluminous glasses show a larger chemical shift distribution and a more positive isotropic chemical shift, ~-75 ppm, than the peralkaline glasses, ~-100 ppm. They also have a larger quadrupole coupling constant with a larger distribution, indicating greater disorder in the peraluminous glasses. It is likely that there are more Ca cations present in the Cl environments in the peraluminous glasses than in the peralkaline glasses despite their having the same Na/Ca ratio. In the peralkaline glasses the formation of Na-Ca-Cl environments leads to a decrease in the number of network-modifying cations, which causes a polymerization of the glass network. No effect on the glass polymerization was observed in the peraluminous glasses. Some 35 Cl signal is also lost in the static spectra indicating that ~20% of Cl for a peralkaline glass and more than ~70% for a peraluminous glass must be in environments where there is a large enough electric field gradient that the resulting very broad line is unobservable. These environments could be simply Na-Ca-Cl with higher electric field gradients than those producing the observed 35 Cl signal or non-bridging Cl environments like for example Al-Cl. The Cl environment in the present mixed Na 2 O-CaO aluminosilicate glasses appears to be more disordered than was to be expected from previous NMR spectroscopic studies on simpler glass compositions.

The present study aims primarily at refining a structure model for interlayer cations and H 2 O molecules in tri-hydrated (3W) smectite (d 001 = 18-19 Å). The <2 mm fraction of the SWy-2 source clay (low-charge montmorillonite) was saturated by Mg 2+ , Ca 2+ , Ba 2+ , or Na + cations, before collection of X-ray diffraction (XRD) patterns at 98% relative humidity. Experimental d 001 values derived for the essentially homogeneous 3W hydrates provided volume constraints for Grand Canonical Monte Carlo (GCMC) simulations. Computed atomic density distribution of interlayer species were used in turn to calculate XRD intensities of 00l reflections. The agreement between calculated and experimental 00l intensities allowed validating the GCMC results of both interlayer H 2 O content and distribution of interlayer species (cations and H 2 O molecules). Computed atomic density profiles do not correspond to the usual model of three discrete planes of H 2 O molecules but rather exhibit two sharp planes of H 2 O molecules wetting the clay surfaces (at ~2.7 Å from the clay layer surface). Additional H 2 O molecules belong to cation hydration shells or define a poorly organized ensemble filling internal voids. This alternative model suggests that the stability of the 3W hydrate results from the dual interaction of some H 2 O molecules with interlayer cation, through their second hydration shell, and with the 2:1 clay surface. Computed atomic density profiles were approximated to propose an interlayer structure model for 3W smectite. This simplified model includes two sets of two planes (symmetrical relative to the interlayer mid-plane) for H 2 O molecules and one set for interlayer cations. This model allows reproducing experimental XRD patterns for the different samples investigated and thus represents a valid set of parameters for routine quantitative analysis of XRD profiles in an effort to determine smectite reactivity close to water-saturated conditions. Implications of such studies are crucial to provide experimental constraints on the behavior of the main vector of element transfer under conditions common in surficial environments and prevailing in waste repositories. In addition, the present study provides an experimental validation of structure models derived from the widely used ClayFF model, and thus allows its use to predict the fate of water in clayey systems close to water-saturated conditions.

The Co 2+ ion in fourfold coordination provides d-d electronic transitions with the strongest optical density among oxides and silicates. For this reason, it is widely used in pigments and dyes to get blue shades detectable down to a very low cobalt concentration. Such a low-detection limit turns the Co 2+ ion into a suitable probe to disclose the local ligand environment in a wide range of materials by means of optical spectroscopy. Even if extensively studied in organometallic complexes, an in-depth investigation of optical properties of Co 2+ in tetrahedral coordination into oxidic structures is limited to some case-study in minerals and synthetic analogs (spinel, zincite, gahnite, willemite, calcium cobalt selenite). The present study represents an attempt to outline crystal structural (long-range metal-oxygen distances, O-T-O bond angles, and distortion parameters by XRD) and optical parameters (10Dq, Racah B and C, band splitting by EAS) in 13 samples of oxides and silicates providing a wide set of different local fourfold coordination around Co 2+ added as a dopant. Subtle variations of crystal field strength and interelectronic repulsion can be appreciated in gahnite, Ca-Sr-hardystonite, Ca-Sr-Baåkermanite, willemite, Ba 2 MgSi 2 O 7 melilite-related (where Co 2+ substitutes Mg 2+ or Zn 2+ by 0.25-0.3 apfu) as well as in gehlenite and fresnoite (where Co 2+ substitutes Al 3+ and Ti 4+ , respectively, by 0.2 apfu due to charge mismatch). Results are compared with literature data about hibonite, spinel s.s., staurolite, yttrium garnets, and zincite. Spectral interpretation is not straightforward owing to the occurrence of different Co 2+ bands: spin-allowed and spin-forbidden electronic transitions, two- or threefold split due to both lowering of point symmetry at the tetrahedron and spin-orbit coupling plus presumably vibronic transitions. Optical spectra vary significantly even for apparently small changes in the long-range CoO 4 arrangement as measured by XRD. The expected relationship between 10Dq and the mean Co-O distance is fulfilled, but the accommodation into small AlO 4 sites in gehlenite (YAG and hibonite) implies a significant structural relaxation around the Co 2+ ion. The threefold splitting of the spin-allowed 4 T 1 (F) and 4 T 1 (P) bands can be related to the angular distortion of the CoO 4 tetrahedra. Overall, changes of spectral features of tetrahedrally coordinated Co 2+ can be attributed to different local arrangement of ligands with an effect correlated to the second nearest neighbors by the bond valence theory. This was disclosed contrasting 10Dq with the ratio of the observed and ideal bond valence sum for the polyhedra sharing oxygen with the Co-centered tetrahedron.

A variable-temperature powder neutron diffraction study of serandite, Na(Mn,Ca) 2 Si 3 O 8 (OH), has been undertaken over the temperature range 4-800 K to investigate the behavior of the very strong hydrogen bond in this mineral. At 4 K the O(D)…O(A) distance in serandite has been determined to be extremely short at 2.413(10) Å. The distribution of hydrogen along the O3…O4 direction at low temperatures confirms that reported previously at room temperature with one site, bonded to O3, strongly, but not exclusively, favored; the origin of the occupation of this preferred site has been assigned to additional weak hydrogen bonding interactions. At higher temperatures the hydrogen distribution along the O3…O4 direction becomes increasingly random as the thermal energy and motion outweigh the weak hydrogen bonding. The data also show that calcium substitutes only on one manganese site, Mn2 in the mineral structure.

A new biogenic, struvite-related phosphate, the ammonium analog of hazenite (AAH), ideally (NH 4 ) NaMg 2 (PO 4 ) 2 ·14H 2 O, has been found in cultures containing the bacterial strain Virgibacillus sp.NOT1 (GenBank Accession Number: JX417495.1) isolated from an XVII Century document made of parchment. The chemical composition of AAH, determined from the combination of electron microprobe and X-ray structural analyses, is [(NH 4 ) 0.78 K 0.22 ]NaMg 2 (PO 4 ) 2 ·14H 2 O. Single-crystal X-ray diffraction shows that AAH is orthorhombic with space group Pmnb and unit-cell parameters a = 6.9661(6), b = 25.236(3), c = 11.292(1) Å, and V = 1985.0(3) Å 3 . Compared with hazenite, the substitution of NH 4 + for K + results in a noticeable increase of the average A-O (A = NH 4 + +K + ) bond length and the unit-cell volume for AAH, as also observed for struvite vs. struvite-K. Both infrared and Raman spectra of AAH resemble those of hazenite, as well as struvite. Our study reveals that AAH forms only in cultures with Na-bearing solutions and pH below 10.0. No AAH or hazenite was found in experiments with the K-bearing solutions, suggesting the necessity of a Na-bearing solution for AAH formation.

Chromo-alumino-povondraite, NaCr 3 (Al 4 Mg 2 )(Si 6 O 18 )(BO 3 ) 3 (OH) 3 O, is a new mineral of the tourmaline supergroup. It is found in metaquartzites of the Pereval marble quarry (Sludyanka, Lake Baikal, Russia) in association with dravite, oxy-chromium-dravite, oxy-dravite, quartz, calcite, chromphyllite, eskolaite, chromite, uvarovite, chromian phlogopite, and pyroxenes of the diopside-kosmochlor series, Cr-bearing tremolite, Cr-bearing titanite, Cr-bearing rutile, and pyrite. Crystals are green and transparent with a vitreous luster, and exhibit a pale-green streak and conchoidal fracture. Chromo-alumino-povondraite has a Mohs hardness of approximately 7½, and a calculated density of 3.227 g/cm 3 . In plane-polarized light, chromo-alumino-povondraite is pleochroic (O = emerald green and E = pale yellowish green) and uniaxial negative: ω = 1.745(5), ε = 1.685(5). Chromo-alumino-povondraite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.0277(2), c = 7.3085(1) Å, V = 1625.93(5) Å 3 , Z = 3. Crystal-chemical analysis resulted in the empirical structural formula: X (Na 0.87 Ca 0.07 □ 0.04 K 0.02 ) Σ 1.00 Y (Cr 3+ 2.29 Mg 0.71 ) Σ 3.00 Z (Al 3.04 Mg 1.54 Cr 3+ 1.18 V 3+ 0.22 Fe 3+ 0.01 ) Σ 6.00 [ T (Si 5.96 Al 0.04 )O 18 ] ( B BO 3 ) 3 V (OH) 3 W [O 0.73 F 0.25 (OH) 0.02 ] Σ 1.00 The crystal structure of chromo-alumino-povondraite was refined to an R1 index of 1.68% using 1803 unique reflections collected with MoKα X-radiation. Ideally, chromo-alumino-povondraite is related to oxy-dravite and oxy-chromium-dravite by the homovalent substitution Cr 3+ ↔ Al 3+ . Tourmaline with chemical compositions classified as chromo-alumino-povondraite can be either Al-dominant or Cr-dominant as a result of the compositional boundaries along the solid solution between Al and Cr 3+ that are determined at Y+Z (Cr 1.5 Al 5.5 ), corresponding to Na Y (Cr 1.5 Al 1.5 ) Z (Al 4 Mg 2 )Si 6 O 18 (BO 3 ) 3 (OH) 3 O, and Y+Z (Cr 5 Al 2 ), corresponding to Na Y (Cr 3 ) Z (Cr 2 Al 2 Mg 2 )Si 6 O 18 (BO 3 ) 3 (OH) 3 O.

Two samples from the enigmatic Ni-oxide body of Bon Accord (Barberton greenstone belt, South Africa) have been investigated with the hydroseparation technique to obtain heavy mineral concentrates. The concentrates contain abundant Pt, Pd, and gold minerals never reported before from the Bon Accord Ni-oxide body. The grains occur as: (1) minute inclusions (<3 μm) in trevorite (ideally NiFe 3+ O 4 ) and (2) larger (5-70 μm) free aggregates liberated from the host phase. The first group comprises several PGM compounds of Pd-Sb, Pd-Sb-As, Pd-Cu-Sb, Pt-Sb, Pt-As-S, Ru-As-S, Ru-S, along with free grains of Ni-Fe-As. The second consists of sperrylite (PtAs 2 ), members of the sobolevskite-kotulskite series, and electrum. These results are in good agreement with previous analyses of PGE-Au in bulk rock. Paragenetic relationships indicate that the PGM and electrum are of secondary origin, probably generated during low-temperature metamorphism of the Ni-rich mineralization. They have a terrestrial origin and are related with a low-sulfidation regime that usually accompanies hydrothermally driven serpentinization of mafic-ultramafic bodies. The ligands in the newly formed PGM (As, Sb, Bi, Te, and O) probably proceed from the same source of the hydrothermal solutions. In this model, the metals Ni-PGE-Au were original components of the primary mineral assemblage of the Bon Accord precursor, whereas As, Sb, Bi, Te, and O might have been contributed by the metasomatizing fluids, during nearsurface evolution of the ore body. The data on the high-grade heavy mineral concentrates, obtained by hydroseparation, have provided new knowledge about the mineral deportment of Pd, Pt, and Au.

A new mineral beshtauite, (NH 4 ) 2 (UO 2 )(SO 4 ) 2 ·2H 2 O, was found in the oxidation zone of the Beshtau uranium deposit, Mount Beshtau, Stavropol region, Northern Caucasus, Russia, and named after the locality. It is associated with rozenite, gypsum, lermontovite, and older marcasite, pyrite, halloysite, and opal. Beshtauite occurs as well-shaped short-prismatic crystals up to 0.1 × 0.15 × 0.2 mm, their clusters and crusts up to 0.5 mm across growing on marcasite. Beshtauite is transparent, light green. The luster is vitreous. The mineral fluoresces strongly yellow-green under both short- and long-wave UV irradiation. It is brittle. The Mohs hardness is ca. 2. Cleavage was not observed. D calc is 3.046 g/cm 3 . Beshtauite is optically biaxial (+), α = 1.566(3), β = 1.566(3), γ = 1.592(3), 2V meas < 10°. The chemical composition (wt%, electron microprobe data, H 2 O by difference) is: (NH 4 ) 2 O 10.33, UO 3 53.21, SO 3 29.40, H 2 O calc 7.06, total 100.00. Content of (NH 4 ) 2 O was calculated from measured nitrogen content: 5.56 wt% N. The empirical formula, calculated on the basis of 12 O apfu, is (NH 4)2.12 U 0.99 S 1.96 O 9.91 (H 2 O) 2.09 . Beshtauite is monoclinic, P2 1 /c, a = 7.7360(8), b = 7.3712(5), c = 20.856(2) Å, β = 102.123(8)°, V = 1162.76(19) Å 3 , Z = 4 (from single-crystal X-ray diffraction data). The strongest reflections of the X-ray powder pattern [d (Å), I(hkl)] are: 6.86, 100(011, 102̅); 5.997, 19(012); 5.558, 15(102); 5.307, 36(111̅,110); 5.005, 35(013,112̅); 3.410, 38(114,204̅,106̅); 3.081, 24(016); 2.881, 20(106,123). The crystal structure was solved by direct methods and refined on the basis of 2677 independent reflections with I > 4σ(I) to R 1 = 0.093. The structure is based upon [UO 2 (SO 4 ) 2 (H 2 O)] 2- layers consisting of corner-sharing UO 6 (H 2 O) pentagonal bipyramids and SO 4 tetrahedra. The layers are coplanar to (1̅02) and are linked via hydrogen bonding that involve interlayer NH 4 + ions and H 2 O molecules. Beshtauite is important indicator mineral: its presence can be considered as an evidence of transportation of U 6+ in nature in forms of mobile complexes of uranyl cation with ammonia or polyamines.

We determined phase relations in FeCr 2 O 4 at 12-28 GPa and 800-1600 °C using a multi-anvil apparatus. At 12-16 GPa, FeCr 2 O 4 spinel (chromite) first dissociates into two phases: a new Fe 2 Cr 2 O 5 phase + Cr 2 O 3 with the corundum structure. At 17-18 GPa, the two phases combine into CaFe 2 O 4 - type and CaTi 2 O 4 -type FeCr 2 O 4 below and above 1300 °C, respectively. Structure refinements using synchrotron X-ray powder diffraction data confirmed the CaTi 2 O 4 -structured FeCr 2 O 4 (Cmcm), and indicated that the Fe 2 Cr 2 O 5 phase is isostructural to a modified ludwigite-type Mg 2 Al 2 O 5 (Pbam). In situ high-pressure high-temperature X-ray diffraction experiments showed that CaFe 2 O 4 -type FeCr 2 O 4 is unquenchable and is converted into another FeCr 2 O 4 phase on decompression. Structural analysis based on synchrotron X-ray powder diffraction data with transmission electron microscopic observation clarified that the recovered FeCr 2 O 4 phase has a new structure related to CaFe 2 O 4 -type. The high-pressure phase relations in FeCr 2 O 4 reveal that natural FeCr 2 O 4 -rich phases of CaFe 2 O 4 - and CaTi 2 O 4 -type structures found in the shocked Suizhou meteorite were formed above about 18 GPa at temperature below and above 1300 °C, respectively. The phase relations also suggest that the natural chromitites in the Luobusa ophiolite previously interpreted as formed in the deep-mantle were formed at pressure below 12-16 GPa.

We report the first occurrence of kumdykolite, a high-temperature analog of albite, in the European Variscan belt. It was discovered in an ultrahigh-pressure, diamond-bearing quartzofeldspathic granulite from the northern Bohemian Massif. It is associated with phlogopite and quartz in a multiphase solid inclusion within garnet, considered to represent a trapped fluid or melt phase. Micro-Raman analysis and mapping along with BSE revealed the presence of a sub-equant, elongated grain of kumdykolite reaching 20 mm in length. WDX analysis has shown that kumdykolite contains 2 wt% CaO, probably indicating significant miscibility with the Ca-end-member svyatoslavite. Similar to the case of microdiamond inclusions, the kumdykolite-bearing multiphase inclusion is located in the Ca- and Mg-rich central part of the garnet and thus must have been trapped at P > 4 GPa. The inclusion minerals, however, crystallized upon decompression and cooling during the exhumation. Kumdykolite preservation thus provides independent evidence for high temperature of the original trapped fluid, or melt, crystallization, and rapid cooling of the rocks. Our results imply that kumdykolite and other feldspar modifications stable at elevated pressures and temperatures may be common phases in quartzofeldspathic granulites and need to be searched for.

The crystal structure of the dense hydrous magnesium silicate phase H, MgSiH 2 O 4 , synthesized at 45 GPa and 1000 °C, was investigated by single-crystal X-ray diffraction. Although showing a deterioration process under the X-ray beam, the compound was found to be orthorhombic, space group Pnnm (CaCl 2 - type structure), with lattice parameters a = 4.733(2), b = 4.3250(10), c = 2.8420(10) Å, V = 58.18(3) Å 3 , and Z = 1. The structure was refined to R 1 = 0.0387 using 53 observed reflections [2s(I) level]. Magnesium and silicon were found to be disordered at the same octahedral site (with a mean bond distance of 1.957 Å). Hydrogen was not located in the difference Fourier maps, but it is very likely disordered at a half-occupied 4g position. The centrosymmetric nature of the structure of phase H is examined in relation to that reported for pure d-AlOOH at ambient conditions (non-centrosymmetric, P2 1 nm), and the possibility that these two compounds can form a solid solution at least at high pressure is discussed.