Showing papers in "American Mineralogist in 2006"

Structural model for the biogenic Mn oxide produced by Pseudomonas putida

Abstract: X-ray diffraction (XRD) and Mn K-edge extended X-ray absorption Þ ne structure (EXAFS) spectroscopy were combined to elaborate a structural model for phyllomanganates (layer-type Mn oxides) lacking 3D ordering (turbostratic stacking). These techniques were applied to a sample produced by a common soil and freshwater bacterium (Pseudomonas putida), and to two synthetic analogs, δ-MnO2 and acid birnessite, obtained by the reduction of potassium permanganate with MnCl2 and HCl, respectively. To interpret the diffraction and spectroscopic data, we applied an XRD simulation technique utilized previously for well-crystallized birnessite varieties, complementing this approach with single-scattering-path simulations of the Mn K-edge EXAFS spectra. Our structural analyses revealed that all three Mn oxides have an hexagonal layer symmetry with layers comprising edgesharing Mn 4+ O6 octahedra and cation vacancies, but no layer Mn 3+ O6 octahedra. The proportion of cation vacancies in the layers ranged from 6 to 17%, these vacancies being charge-compensated in the interlayer by protons, alkali metals, and Mn atoms, in amounts that vary with the phyllomanganate species and synthesis medium. Both vacancies and interlayer Mn were most abundant in the biogenic oxide. The diffracting crystallites contained three to six randomly stacked layers and have coherent scattering domains of 19–42 A in the c* direction, and of 60–85 A in the a-b plane. Thus, the Mn oxides investigated here are nanoparticles that bear signiÞ cant permanent structural charge resulting from cation layer vacancies and variable surface charge from unsaturated O atoms at layer edges.

Hydrogen incorporation in olivine from 2–12 GPa

Abstract: We performed new experiments on incorporation of hydrogen in olivine at high pressures (2–12 GPa) and temperatures (1000–1300°C). OH concentrations were calculated using the Bell et al. (2003) calibration applied to principal-axis infrared absorption spectra synthesized from polarized measurements on randomly oriented grains. Starting materials for the experiments included both fi ne-grained powders and larger single crystals. Hydrogen was incorporated during grain growth in the former case and by volume diffusion in the latter. The spectra of Fe-bearing olivines exhibit similar structure regardless of the starting material, and are dominated by bands in the wavenumber range from about 3500 to 3650 cm^(–1). We do not observe bands at 3525 and 3573 cm^(–1), which are predominant in many natural olivines as well as olivines annealed in experiments at lower pressures, and are attributed to humite-related defects. Furthermore, bands between 3300 and 3400 cm^(–1), attributed to high silica activity or high oxygen fugacity, are weak or non-existent. Our measurements indicate that OH solubility in Fe-bearing olivine is 2.5–4 times higher than that measured by Kohlstedt et al. (1996). Although this is largely due to the use of a new calibration in our study, correction of previous values is not straightforward. In the pure Mg-system, in contrast to Fe-bearing olivine, order-of-magnitude apparent differences in OH solubility can be obtained using different experimental procedures. This raises questions about attainment of equilibrium in experimental studies of hydrogen incorporation in nominally anhydrous minerals, particularly when crystals are grown from a hydrous melt.

Rates of Fe, Mg, Mn, and Ca diffusion in garnet

Abstract: Extraction of the thermal and chemical histories preserved by modified compositional zoning in garnet is compromised by large uncertainties in experimentally determined diffusion coefficients for principal divalent cations. Some of this uncertainty derives from long down-temperature extrapolations, but much of it arises from an inability to account accurately for large variations in diffusivity as a function of garnet composition. Numerical simulation of stranded diffusion profiles in partially resorbed natural garnets constrains diffusivities at low temperature, and reveals a systematic dependence of diffusion rates on host garnet composition. A comprehensive model incorporating a simple compositional dependence allows accurate prediction of diffusion rates for Fe, Mg, Mn, and Ca for nearly all garnet compositions across the full range of geologically relevant temperatures, pressures, and oxygen fugacities.

Microhardness, toughness, and modulus of Mohs scale minerals

Abstract: We report new results of microhardness and depth-sensing indentation (DSI) experiments for the first nine minerals in the Mohs scale: talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, and corundum. The Mohs scale is based on a relative measure of scratch resistance, but because scratching involves both loading and shearing, scratch resistance is not equivalent to hardness as measured by modern loading (indentation) methods; scratch resistance is also related to other material properties (fracture toughness, elastic modulus). To better understand the relationship of hardness to scratch resistance, we systematically determined hardness, fracture toughness, and elastic modulus for Mohs minerals. We measured hardness and toughness using microindentation, and modulus and hardness with DSI (“nanoindentation”) experiments. None of the measured properties increases consistently or linearly with Mohs number for the entire scale.

The rate of ferrihydrite transformation to goethite via the Fe(II) pathway

Abstract: In this study, we quantified the rate of ferrihydrite conversion to goethite via the Fe(II) pathway using synchrotron radiation-based energy dispersive X-ray diffraction (ED-XRD). Ferrihydrite transformation experiments were conducted in oxygen-free solutions at neutral pH with synthetic 2-line ferrihydrite reacting with 100 mM Fe(II). The kinetics of goethite crystallization was measured in situ at temperatures ranging from 21 to 90 ??C. The results showed that in the presence of ferrous iron, the transformation of poorly ordered ferrihydrite into crystalline goethite is rapid and highly dependent on temperature. The time-resolved peak area data fitted using a Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetic model yielded rate constants of 4.0 ?? 10 -5, 1.3 ?? 10 -4, 3.3 ?? 10 -4, 2.27 ?? 10 -3, and 3.14 ?? 10 -3 l/s at reaction temperatures of 21, 45, 60, 85, and 90 ??C respectively. The activation energy for the transformation was determined to be 56 ?? 4 kJ/mol. Comparison with the activation energy predicted for the phase conversion in the absence of ferrous iron indicates that Fe(II) acts as a catalyst that decreases the activation energy barrier by approximately 38 kJ/mol. The kinetic parameters derived from the experimental data suggest that goethite crystallization is controlled by a 1-D phase boundary growth mechanism with a constant nucleation rate occurring during the reaction.

Verifying and quantifying carbon fixation in minerals from serpentine-rich mine tailings using the Rietveld method with X-ray powder diffraction data

Abstract: Most carbon on Earth is bound within minerals, and increasing the uptake of atmospheric carbon dioxide into minerals may reduce the greenhouse gas content of the atmosphere. We document carbon disposal through the mineralization of mine tailings at Clinton Creek, Yukon Territory, and Cassiar, British Columbia. We confirm crystallographic binding of carbon in these tailings and quantify carbon dioxide uptake using quantitative phase analysis with the Rietveld method for X-ray powder diffraction data. Planar disorder in the structures of the kaolinite-serpentine group minerals makes Rietveld refinements of X-ray powder diffraction data for serpentinites problematic. Using structureless pattern fitting and with the addition of a known quantity of a well-crystallized material, the problem of structural disorder is overcome by considering the serpentine minerals as amorphous phases. We test the accuracy and precision of this refinement method using synthetic serpentine-rich mine tailings of known composition. Estimates of the abundance of hydrated magnesium carbonates in these tailings have a precision of approximately 5% relative for mineral species present in amounts greater than 10 wt%. Precise estimates of carbonate mineral content and crystallographically bound atmospheric CO 2 are made for samples of serpentine-rich tailings from Clinton Creek and Cassiar. Results for mine tailings are also compared to mineralogically similar samples from a carbonate playa at Atlin, British Columbia. The potential for decomposition of metastable hydrated magnesium carbonate phases to geologically stable magnesite may represent long-term stability of the products of mineral sequestration in mine tailings.

Intensity of quartz cathodoluminescence and trace-element content in quartz from the porphyry copper deposit at Butte, Montana

Abstract: Textures of hydrothermal quartz revealed by cathodoluminescence using a scanning electron microscope (SEM-CL) refl ect the physical and chemical environment of quartz formation. Variations in intensity of SEM-CL can be used to distinguish among quartz from superimposed mineralization events in a single vein. In this study, we present a technique to quantify the cathodoluminescent intensity of quartz within individual and among multiple samples to relate luminescence intensity to specific mineralizing events. This technique has been applied to plutonic quartz and three generations of hydrothermal veins at the porphyry copper deposit in Butte, Montana. Analyzed veins include early quartz-molybdenite veins with potassic alteration, pyrite-quartz veins with sericitic alteration, and Main Stage veins with intense sericitic alteration. CL intensity of quartz is diagnostic of each mineralizing event and can be used to fingerprint quartz and its fluid inclusions, isotopes, trace elements, etc., from specific mineralizing episodes. Furthermore, CL intensity increases proportional to temperature of quartz formation, such that plutonic quartz from the Butte quartz monzonite (BQM) that crystallized at temperatures near 750 °C luminesces with the highest intensity, whereas quartz that precipitated at ~250 °C in Main Stage veins luminesces with the least intensity Trace-element analyses via electron microprobe and laser ablation-ICP-MS indicate that plutonic quartz and each generation of hydrothermal quartz from Butte is dominated by characteristic trace amounts of Al, P, Ti, and Fe. Thus, in addition to CL intensity, each generation of quartz can be distinguished based on its unique trace-element content. Aluminum is generally the most abundant element in all generations of quartz, typically between 50 and 200 ppm, but low-temperature, Main Stage quartz containing 400 to 3600 ppm Al is enriched by an order of magnitude relative to all other quartz generations. Phosphorous is present in abundances between 25 and 75 ppm, and P concentrations in quartz show little variation among quartz generations. Iron is the least abundant of these elements in most quartz types and is slightly enriched in CL-dark quartz in pyrite-quartz veins with sericitic alteration. Titanium is directly correlated with both temperature of quartz precipitation, and intensity of quartz luminescence, such that BQM quartz contains hundreds of ppm Ti, whereas Main Stage quartz contains less than 10 ppm Ti. Our results suggest that Ti concentration in quartz is controlled by temperature of quartz precipitation and that increased Ti concentrations in quartz may be responsible for increased CL intensities.

First-principles study of the OH-stretching modes of gibbsite

Abstract: The theoretical infrared (IR) and Raman spectra of gibbsite [α-Al(OH) 3 ] were computed using ab initio quantum mechanical calculations. The low-frequency dielectric tensor and the Raman tensors of gibbsite were determined using linear response theory. The transmission powder IR spectrum was found to strongly depend on the shape of the gibbsite particles. In the region of the OH-stretching bands, an excellent agreement between theory and experiment was obtained, providing an unambiguous interpretation of the OH bands in terms of vibrational modes. In contrast, the assignment of the bands observed at lower frequency is complicated by the significant overlap between neighboring bands together with their sensitivity to particle shape.

Presidential Address to the Mineralogical Society of America, Salt Lake City, October 18, 2005: Mineral surfaces and the prebiotic selection and organization of biomolecules

Abstract: One of the most enigmatic steps in Earth’s ancient transition from a lifeless planet to a living world was the process or processes by which prebiotic organic molecules were selected, concentrated, and organized into the essential macromolecules of life. More than a half-century of theory and experiment points to the critical roles of mineral surfaces in the assembly of proteins, lipid bilayers, and genetic polymers. This review considers three aspects of this problem: (1) the self-assembly of lipids, which may be enhanced in the presence of minerals; (2) the role of minerals in polymerization of amino acids and nucleic acids; and (3) the selective adsorption of organic species, including chiral molecules, onto mineral surfaces.

Lithium isotopic systematics of granites and pegmatites from the Black Hills, South Dakota

Abstract: To study Li isotopic fractionation during granite differentiation and late-stage pegmatite evolution, Li isotopic compositions and concentrations have been measured for the S-type Harney Peak Granite, the spatially associated Tin Mountain pegmatite, and possible metasedimentary source rocks in the Black Hills, South Dakota. The Harney Peak Granite is isotopically heterogeneous, with δ 7 Li varying from –3.1 to +6.6. The δ 7 Li values of Proterozoic metasedimentary rocks that are possible sources of the Harney Peak Granite range from –3.1 to +2.5 and overlap with post-Archean shales and the Harney Peak Granite. For the granite suite, there is no correlation between δ 7 Li and elements indicative of degrees of granite differentiation (SiO2, Li, Rb, etc.). The Li isotopic composition of the Harney Peak Granite, therefore, appears to res ect the source composition. Minerals from the zoned Tin Mountain pegmatite have extremely high Li contents and heavier Li isotopic compositions than the granite or surrounding Black Hills metasedimentary rocks. The heavier compositions may res ect Li isotopic fractionation resulting from extensive crystal-melt fractionation. Lithium concentrations decrease in the order: spodumene (~3.7 wt%), muscovite (0.2 to 2.0 wt%), plagioclase (100–1100 ppm), quartz (30–140 ppm). Plagioclase, muscovite, and spodumene in all zones display a relatively narrow range in δ 7 Li of +7.9 to +11.4. In contrast, quartz is isotopically heavier and more variable (+14.7 to +21.3), with δ 7 Li showing an inverse correlation with Li concentration. This correlation res ects the mixing of isotopically heavy Li in quartz and lighter Li in s uid inclusions, as documented by s uid inclusion compositions (δ

Crystal chemistry of lunar merrillite and comparison to other meteoritic and planetary suites of whitlockite and merrillite

Abstract: Merrillite, also known as “whitlockite,” is one of the main phosphate minerals, along with apatite, that occur in lunar rocks, martian meteorites, and in many other groups of meteorites. SigniÞ cant structural differences between terrestrial whitlockite and lunar (and meteoritic) varieties warrant the use of “merrillite” for the H-free extraterrestrial material, and the systematic enrichment of REE in lunar merrillite warrants the use of “RE-merrillite.” Laser Raman spectroscopy of extraterrestrial merrillite and terrestrial whitlockite conÞ rms the absence of H in the former and presence of H in the latter. Lunar merrillite, ideally (Mg, Fe 2+ , Mn 2+ )2[Ca18–x (Y,REE)x](Na2–x)(P,Si)14O56, contains high concentrations of Y+REE, reaching just over 3 atoms per 56 O atoms, or up to ~18 wt% as (Y,RE)2O3. In the absence of extensive Si↔P substitution, the “availability” of the Na site limits Y+REE substitution to ~2 atoms per 56 O atoms. Higher concentrations of Y+REE, with coupled substitution of Si for P to balance charge are possible, but rare in lunar material. Intrinsically low Na concentrations in lunar rocks, combined with the typical formation of merrillite in late-stage basaltic mesostasis or residual, intercumulus melt pockets, produce these high REE concentrations. Lunar merrillite typically contains 0.1–0.4 Na atoms per 56 O atoms. For comparison, martian merrillite contains signiÞ cantly higher Na concentrations (up to 1.7 Na atoms per 56 O atoms) and much lower REE concentrations. Meteoritic merrillite has relatively low REE contents, but exists in both Ca-rich and Na-rich varieties. Concentrations of Fe and Mg in all varieties sum to near 2 atoms per 56 O atoms. Merrillite in lunar crustal lithologies typically has Mg >> Fe; however, Fe-rich mare basalts contain up to 1.8 Fe 2+

TEM imaging and analysis of microinclusions in diamonds: A close look at diamond-growing fluids

Abstract: Fluid-bearing microinclusions in diamonds (<1 μm) provide a unique source of information on the diamond-forming medium. Transmission electron microscopy (TEM) investigation of such microinclusions enables the detailed study of their size, external habit, internal morphology, and mineralogy, and yields information on the chemical composition and crystallography of the included phases. Here we present a detailed TEM examination of microinclusions in four fibrous diamonds from Canada and Siberia, each with a distinctive inferred original fluid composition. Most microinclusions contain multi-phase assemblages that include carbonate, halide, apatite, possible pyroxene, and high-silica mica (6.8–7.7 Si atoms per formula unit) whose composition lies along the phlogopite–Al-celadonite join. The TEM results, together with the tight range of composition detected by electron probe microanalysis (EPMA) and the volatiles detected by infrared (IR) spectroscopy, suggest that the microinclusions trapped a uniform, dense, supercritical fluid and that the crystallized minerals grew as secondary phases during cooling. Carbonates appear in all assemblages, together with either halides or silicates, indicative of the importance of carbonatitic high-density fluid during diamond growth and fluid evolution. The presence of halide-carbonate or silicate-carbonate assemblages is in agreement with the bulk composition of the microinclusions as detected by EPMA. The high K content of some microinclusions detected by EPMA cannot be accounted for by the solid phases analyzed by TEM. This discrepancy suggests that K is concentrated in the residual fluid that is lost during TEM sample preparation. In addition to microinclusions, large cavities containing amorphous phases were found in the inner parts of one Siberian and one Canadian diamond. An Al-rich phase is the most abundant, and it is accompanied by Ca-rich and Si-rich phases. These phases may be explained by amorphization of crystalline phases. A breakdown of a single melt into three immiscible components is less likely.

The effect of composition on Cr2+/Cr3+ in silicate melts

Abstract: Chromium K -edge X-ray absorption near-edge structure (XANES) spectra were recorded at room temperature for 27 CaO-MgO-Al 2 O 3 -SiO 2 (CMAS) glass compositions quenched from melts equilibrated at various oxygen fugacities ( f O 2 ) at 1400 °C. Values of Cr 2+ /∑Cr were determined from the intensity of a shoulder on the main absorption edge, attributed to the 1s → 4s transition, which is characteristic of Cr 2+ in these glasses. For each composition, Cr 2+ /∑Cr could be quantified as a function of f O 2 , using a theoretical expression, from as few as three samples (Cr 2+ /∑Cr 0, 0.5, and 1). This allowed log K ′, or the reduction potential of the Cr 3+/2+ half-reaction, and hence the relative change in the ratio γ melt Cr 3+ O 1.5 /γ melt Cr 2+ O , to be determined for each composition. At constant f O 2 , log[Cr 2+ /Cr 3+ ] was found to decrease linearly with increasing optical basicity. The variation in log K ′ with composition is controlled by γ melt Cr 3+ O 1.5 , corresponding to the capacity of the melt to stabilize both the charge and the preferred solvation site of Cr 3+ . The method was then applied to spectra recorded in situ at 1400 °C for a synthetic mid-ocean ridge basalt (MORB) composition, allowing Cr 2+ /∑Cr to be quantified in a Fe-bearing melt for the first time. Cr 2+ /∑Cr was found to vary from ~0.45 at the nickel-nickel oxide (NNO) f O 2 buffer to ~0.90 at iron-wustite (IW). This indicates that Cr 2+ is likely to be the dominant oxidation state in terrestrial basaltic melts.

Experiments on CaCO3-MgCO3 solid solutions at high pressure and temperature

Abstract: Multi-anvil experiments have been performed in the system CaCO3-MgCO3 at pressures of 5.0–7.0 GPa and temperatures of 800–1600 °C. The reaction dolomite = aragonite + magnesite has been reversed and located near 750 °C at 5.0 GPa, 900 °C at 6.0 GPa, and 1000 °C at 7.0 GPa. Between 5 and 6 GPa, the reaction boundary is strongly curved and its d P/ d T slope increases from 2 to 12 MPa/°C, expanding the stability field of dolomite. This increase is attributed to increasing Ca-Mg disorder in the dolomite. In addition to the reaction boundary, an isobaric section of CaCO3-MgCO3 has been determined at 6.0 GPa. There is a two-phase field of aragonite + dolomite on the Ca-rich side, which closes before minimum melting temperatures of 1350 °C are reached. The two-phase field on the Mg-rich side, where dolomite + magnesite coexist, intersects with the solidus. Inferred minimum melts are close to dolomite composition suggesting congruent melting of dolomite at 6 GPa. The melt compositions and temperatures in the pure carbonate system are surprisingly similar to solidus phase relations in the CMAS-CO2 system, implying that minimum melting conditions in carbonated peridotite at high pressures are dominated and controlled by the carbonate component.

A numerical model for steady-state temperature distributions in solid-medium high-pressure cell assemblies

Abstract: We present a numerical model for calculating the temperature distribution inside resistance-heated high-pressure solid-medium axi-symmetric cell assemblies that incorporates both composition- and temperature-dependent thermal conductivity. The code was validated using both analytic solutions of simplified thermal diffusion problems and comparisons to actual laboratory experiments and was found to be reliable in matching the temperature characteristics of multi-anvil experiments. Calculations for various cell assembly designs resulted in temperature fields that are consistent with experimental measurements of thermal gradients. These calculations also illustrated the influence of temperature-dependence of thermal conductivity, an important and often-overlooked property, on the thermal profiles. This model may be used to fine-tune the design of cell assemblies, either to minimize thermal gradients or to produce a desired temperature distribution. The four "typical" multi-anvil cells that we used to demonstrate this technique have temperature profiles across the sample that range from 25 to 75 °C when the thermocouple temperature is 1200 °C. The thermocouple in all four is in a region where the temperature gradient is on the order of 100 °C per millimeter, which could lead to experimental temperature uncertainties that are correlated with the thermocouple location.

Thermal diffusivity of olivine-group minerals at high temperature

Abstract: Thermal diffusivity ( D ) data from 12 oriented single crystals and seven polycrystalline samples of olivine group minerals were acquired with the laser-flash method at temperatures ( T ) of up to ~1500 °C. Samples included forsterite, Fe-Mg binary olivines, sinhalite, and chrysoberyl; specimens were characterized using infrared spectroscopy and electron microprobe analysis. Crystal orientation and chemistry both affect D . For our single crystals, D [100] > D [001] > D [010] at all temperatures. Thermal diffusivity decreases with increasing T and becomes virtually constant at high temperatures. At room temperature, D [001] of pure forsterite has the highest observed values, but substitution of a small amount of Co in forsterite (0.3 wt% CoO) lowers D by ~20%. Substitution of ~10% Fe for Mg in forsterite, as in typical mantle olivine, lowers D by ~50%. At room temperature, mantle olivine has D = 3.25, 1.66, and 2.59 mm2/s for the [100], [010], and [001] orientations, respectively. The values decrease to 0.93–0.87 mm2/s at 790–985 °C for [100], 0.54–51 mm2/s at 590–740 °C for [010] and 0.83–0.79 mm2/s at 740–890 °C for [001]. Two dunite samples have D of 0.55–0.56 mm2/s at 890–1080 °C, showing the effect of preferred orientation of grains dominated by [010]. Thermal diffusivity of polycrystalline samples is controlled by the large amounts of olivine present; minor phases offset the curves for D ( T ) from the value of the olivine mineral. Our laser-flash measurements isolate the phonon component of heat transfer from radiative transfer and show that the phonon contribution becomes nearly constant for the high temperatures expected in the mantle. The other microscopic component (diffusive radiative transfer) depends strongly on temperature and this temperature dependence likely exerts greater control on mantle convection.

SIMS investigation of electron-beam damage to hydrous, rhyolitic glasses: Implications for melt inclusion analysis

Abstract: Electron-beam irradiation causes permanent damage to hydrous, silica-rich glasses. The extent of electron-beam damage is quantiÞ ed using data generated by SIMS analysis of points subjected to previous electron microprobe analysis (EPMA). Even optimum EPMA conditions cause damage to the glass, manifest as a marked depletion in alkali ions at the surface of an irradiated sample. Deeper in the sample, an enrichment in alkali ions to above-baseline levels is followed by a decay back to baseline. The depth of the Þ nal decay correlates with species diffusivity and increases in the order K-Li-Na. H-bearing species are also affected by electron beam irradiation, but in the opposite sense to the alkalis, i.e., they are enriched at the surface. Migration of alkaline earth cations is not observed because of their low diffusivities. Ion depletion or enrichment results from simple migration of ions toward or away from electrons implanted by the beam. Migration depth depends on species diffusivity and heating caused by the electron beam, and therefore increases with increasing electron beam current. Because of the reverse behavior of H, the mobile hydrous species in the presence of an electric Þ eld is probably OH – . The extent of electron beam damage to glasses may increase with total water content. Critically, SIMS measurements of H, Li, Na, D/H, and 6 Li/ 7 Li after electron-probe analysis are compromised by the damage. Despite the damage caused by the electron beam, use of appropriate electron-beam conditions (e.g., 2 nA, 15 kV) gives volatiles by difference accurate to ~0.6 wt%.

Water in the interlayer region of birnessite: Importance in cation exchange and structural stability

Abstract: Birnessite is an important scavenger of trace metals in soils and aqueous environments. The basic birnessite-type structure consists of sheets of Mn octahedra separated by ~7 or ~10 A (“buserite”) interlayer regions filled with cations and water. Synthetic birnessite-like structures were produced through cation exchange reactions with synthetic Na-birnessite. The unheated, synthetic Mg2+, Ca2+, and Ni2+ layer structures have an ~10 A interlayer spacing, whereas the other cation-exchanged synthetic birnessites and the related mineral chalcophanite have an interlayer spacing of ~7 A. The Li+, Na+, K+, Cs+, and Pb2+ synthetic birnessites each contain two to three structurally different water sites, as evidenced by multiple H2O bending and stretching modes in the infrared spectra. The complexity of the water bands in these spectra is likely related to disordering of cations on the interlayer sites. H-birnessite contains structural water and either hydroxyl, hydronium (H3O+), or both. The small difference in the width of the water stretching modes between room temperature and −180 °C indicates that the water molecules in birnessite-like structures are predominantly structurally, rather than dynamically, disordered. Most of the synthetic birnessites, including Na- and K-birnessite, undergo significant water loss at temperatures below 100 °C. There is a linear relationship between the temperature at which most of the water is lost from a given cation-exchanged birnessite and the heat of hydration of the interlayer cation. This finding implies that the interlayer water is strongly bound to the interlayer cations, and plays an important role in the thermal stability of birnessite-like structures.

Estimation of volume fractions of liquid and vapor phases in fluid inclusions, and definition of inclusion shapes

Abstract: The molar volume ( V m ) and chemical composition ( x ) of saline aqueous inclusions and gas inclusions in minerals can be calculated satisfactorily from microthermometric and other analytical data For complex gas-bearing aqueous inclusions, however, calculation of V m – x properties requires additional input of the volume-fractions of the inclusion phases (φ) Traditional estimation of φ in non-fluorescing inclusions involves measuring area-fractions of the phases projected in the microscope and then making rough corrections for the third dimension The uncertainties in the results are unknown and therefore the accuracies of the calculated V m – x properties are also unknown To alleviate this problem we present a new, routine method to estimate φ using the petrographic microscope in conjunction with a spindle-stage Inclusions in normal thick-sections are rotated stepwise and their projected areas and area-fractions are plotted against rotation angle The resulting data arrays are systematically related to inclusion orientation, to inclusion shape, and to φ The dependency on orientation is minimized when area fractions are measured at the position where the inclusions project their largest total areas The shape dependency is accounted for using a new objective classification of inclusion projections, based on parameters from digital image processing The method has been verified with synthetic fluid inclusions of known φ For individual liquid + vapor inclusions with regular (not “negative-crystal”) shapes, the new procedure yields φ with a relative accuracy of ± 4% This degree of accuracy permits V m – x properties of gas-bearing, aqueous fluid inclusions to be calculated with sufficient certainty for many geochemical applications Even better accuracy (eg, down to ± 06%) can be obtained by combining results from several inclusions in the same homogeneously trapped petrographic assemblage

An experimental study of the oxidation state of vanadium in spinel and basaltic melt with implications for the origin of planetary basalt

Abstract: The distribution of V in magmatic rocks is controlled primarily by spinel stability. Extensive previous experimental work at oxidized conditions on doped (V-rich) compositions has led to the recognition of the importance of temperature, oxygen fugacity, and spinel composition, but also left ambiguity with respect to the relative importance of these variables in controlling D Vspinel/melt. One major uncertainty has been the valence of V in the spinel and glass. Spinel-melt pairs were equilibrated at low and variable oxygen fugacities, with a range of V and Ti contents. XANES spectra were measured on the spinel and glass products, and pre-edge peaks measured and calibrated against valence with the use of glass and oxide standards. The valence of V is always greater in the glass than in the spinels. In spinel, V is dominantly 3+ at oxygen fugacities near the FMQ (fayalite magnetite quartz) buffer, but we find evidence for mixed 3+, 4+, and 5+ at oxidized conditions (FMQ to air), and 2+ and 3+ at very reduced conditions [FMQ to IW-1 (1 log f O2 unit below the iron wustite buffer)]. Increased V contents in spinels are correlated with increased D Vspinel-melt, at constant temperature and oxygen fugacity. However, increased Ti content causes only a slight decrease in D Vspinel-melt and a shift to more reduced V (smaller pre-edge peak), which may be related to Fe-V exchange equilibria. Using the new partition coefficients, together with published results and valence information, expressions have been derived to predict D Vspinel/melt for basaltic systems. Application of these expressions to natural suites illustrate their utility and also the great range of D Vspinel/melt values relevant to natural systems. Calculation of V depletions in planetary mantles from basalt suites must take silicate, oxide, and metal fractionation into account, as is demonstrated using terrestrial, lunar, martian, and eucritic samples.

On the structure of palygorskite by mid- and near-infrared spectroscopy

Abstract: The OH-structural characteristics of an iron-rich palygorskite from Western Macedonia, Greece (Gr-1) and an aluminous palygorskite from Florida (PFl-1) were examined by combined Fourier-transform near-infrared reflectance (NIR) and mid-infrared attenuated total reflectance (ATR) spectroscopy. Analyses of samples heated from ambient to 130 °C allowed for the development of a self-consistent set of band assignments for the structural and surface OH and H2O species of both the hydrated and dehydrated forms. The inner octahedral sites of both samples are largely accounted for by dioctahedral AlAlOH,AlFe3+OH, and Fe3+Fe3+OH pairs. Band intensities for these pairs are consistent with variations in the concentration of octahedral Fe and Al in the two samples. In addition, both samples display a trace trioctahedral signature in NIR, which may be related to local trioctahedral domains, or the presence of sepiolite in trace amounts, or as intergrowths. A surface H2O species typical of the hydrated phase was identified via its NIR combination mode at 5317 cm−1. The desorption of this species by heating revealed distinct silanol groups with overtone and combination modes at 7255 and 4575 cm−1, respectively. Mg-coordinated and zeolitic H2O species are strongly coupled in the hydrated phase and give rise to NIR combination modes at 5190 and 5240 cm−1. The removal of zeolitic H2O causes the blue shift of the three dioctahedral OH overtones by ca. 20 cm−1 and the rearrangement of the coordinated H2O manifested by the growth of sharp combination modes at ca. 5215 and 5120 cm−1.

Transformations of Mg- and Ca-sulfate hydrates in Mars regolith

Abstract: Salt hydrates have an active role in regolith development on Mars. The Mg-sulfate system, with highly variable values of n in the formula MgSO 4 · n H 2 O, is particularly subject to transformations among several crystalline and amorphous forms. The Ca-sulfate system, CaSO 4 · n H 2 O, is likely to be associated with the Mg-sulfates in most occurrences, but is less susceptible to transformations in n . Desiccation of MgSO 4 · n H 2 O occurs in exposed soils at the martian equator in summer where higher daytime temperatures at low relative humidity prevail against sluggish nighttime rehydration at high relative humidity. Desiccation and rehydration are both accelerated in the finest size fractions, particularly in silt-size aeolian particles subject to global redistribution by dust storms. This redistribution and periodic excursions into long-term episodes of high obliquity work to rehydrate desiccated MgSO 4 · n H 2 O to form epsomite, MgSO 4 ·7H 2 O, at higher latitudes in the first case and more globally in the latter. Kieserite, a monohydrate form of MgSO 4 · n H 2 O resistant to desiccation, can survive equatorial summer conditions, but not protracted high relative humidity; preservation of kieserite at the surface may place limits on the equatorial distribution of ice during past episodes of high obliquity. Deeper horizons in equatorial regolith may preserve hydrated phases through repeated obliquity episodes, raising the possibility of an ancient regolith archive of past hydration. At shallower depths in the regolith, in situ determination of the hydration states of the Mg-sulfates, and possibly the Ca-sulfates, may be used to constrain regolith dynamics if rates and modes of transitions in n can be fully characterized.

An experimental study of trace-element partitioning between Ti-Al-clinopyroxene and melt: Equilibrium and kinetic effects including sector zoning

Abstract: Equilibrium and dynamic crystallization experiments were used to determine distribution coefficients (D-values) for the REE and the trace elements Sc, V, Rb, Sr, Y, Zr, Nb, Ba, Hf, Th, and U between Ti-Al-clinopyroxene and melt. Equilibrium values for D correlate well with previous studies where values have been determined. The D-values were also studied as a function of cooling rate. Increased cooling rates produce higher growth rates. At growth rates that exceed the diffusion rates in the crystals, the D-values increase to near unity and the HREE D-values exceed unity only at the most rapid crystal growth rates. The increase in D-values is the result of the inclusion of components into the growing pyroxene from the boundary layer that develops at the crystal-melt interface because of the disparity between the growth and diffusion rates. The origin of sector zoning is best explained as an interplay of primarily crystallographic control with kinetic effects.

Insights into the redox history of the NWA 1068/1110 martian basalt from mineral equilibria and vanadium oxybarometry

Abstract: The NWA 1068/1110 martian meteorite contains megacrystic olivine and associated chromite set in a basaltic groundmass of pyroxene, maskelynite (shock-isotropized plagioclase), phosphates, ilmenite, and ulvospinel. Spinel is compositionally zoned from chromian cores to titanian rims, and lacks the chromite-ulvospinel miscibility gap. Co-crystallizing olivine + pyroxene + spinel and ulvospinel + ilmenite assemblages were identified on the basis of compositional and textural criteria, and were used to determine the oxygen fugacity from the earliest to the latest stages of crystallization. Megacrystic olivine with earliest chromite and low-Ca pyroxene yield an oxygen fugacity 2.5 log units below the Quartz-Fayalite-Magnetite (QFM) buffer, whereas titanian-spinel rims, olivine rims, and co-crystallizing pyroxene yield QFM + 0.3 and groundmass ulvospinel + ilmenite assemblages yield QFM + 0.5. Vanadium in spinel was examined to determine whether it would be sensitive to the changing redox conditions of the NWA 1068/1110 melt. Only small changes in V concentration were observed between spinel cores and rims; however, the relationship between V and Ti in spinel is consistent with an increase in the valence state of V from V 4+ to V 5+ in accordance with the increase in oxygen fugacity. Application of a quantitative version of the “V-in-chromite” oxybarometer results in an oxygen fugacity of QFM − 3, consistent with mineral equilibria results, although assumptions and uncertainties limit its application to circumstances where the parental melt V concentration is known independently of the redox conditions. Oxygen fugacity results and textural observations are strongly indicative of a xenocrystic origin for the olivine megacrysts and associated chromite, and lend support to a model in which martian basalt oxygen fugacity is determined by mantle-source redox conditions as opposed to assimilation of oxidized crustal material.

Heat capacities and entropies of mixing of pyrope-grossular (Mg3Al2Si3O12-Ca3Al2Si3O12) garnet solid solutions: A low-temperature calorimetric and a thermodynamic investigation

Abstract: The low-temperature heat capacities for a series of synthetic garnets along the pyrope-grossular (Py-Gr) join were measured with the heat capacity option of the Physical Properties Measurement System (PPMS) produced by Quantum Design. The measurements were performed between 5 and 300 K on milligram-sized polycrystalline garnets that have been well characterized in previous studies. The C-p measurements indicate positive excess heat capacities (Delta C-P(xs)) for all solid-solution compositions at temperatures 150 K, Delta C-P(xs) values scatter around zero for all compositions and the experimental error is too large to permit a clear determination of whether Delta C-P(xs) is different from zero within 2 sigma uncertainty. Excess entropies (Delta S-ex) at 298.15 K, calculated from the Cp data of the various solid-solution members, are asymmetric in nature with the largest positive deviations in pyrope-rich compositions. An asymmetric Margules mixing model was found to be inappropriate for modeling the Delta S-xs-X data and, thus, a two-parameter Redlich-Kister model was used to describe the excess entropy-composition relationships. Using this macroscopic mixing model for the excess entropy, a T-X diagram for Py-Gr garnets was calculated using different published values for the excess enthalpies of mixing. The effect of short range Ca-Mg order in the solid solution also was considered in the calculations. The calculations give a solvus for the pyrope-grossular join with a higher critical temperature in the range 850-1330 degrees C at X-Gr = 0.35 compared to previous thermodynamic models (T-crit < 600 degrees C) that use symmetric mixing models to describe the excess entropy. Unmixing of garnets in nature, as documented from occurrences in ultramatic diatremes may, therefore, have occurred at higher temperatures than previously thought. The atomistic and lattice-dynamic properties of Py-Gr garnets are reviewed and compared to the macroscopic Cp data. Published IR and Raman spectra are consistent with the occurrence of positive Delta C-P(xs) values at low temperatures.

A model to predict phase equilibrium of CH4 and CO2 clathrate hydrate in aqueous electrolyte solutions

Abstract: A thermodynamic model to predict phase equilibrium of methane and carbon dioxide hydrate in aqueous electrolyte solutions is presented. Using the Pitzer model to account for the variation of water activity due to electrolytes and dissolved gas in aqueous solutions, we extended the model based on ab initio molecular potential developed recently by us for the CH 4 -H 2 O and CO 2 -H 2 O binary systems to the CH 4 (or CO 2 )-H 2 O-salts system. Comparison of the model with extensive experimental data indicates that this model can accurately predict the phase equilibrium of CH 4 hydrate and CO 2 hydrate in various electrolyte solutions (such as aqueous NaCl, KCl, CaCl 2 , NaCl + KCl, NaCl + CaCl 2 solutions, and seawater) from zero to high ionic strength (about 6 m ) and from low to high pressures.

The origin of jadeitite-forming subduction-zone fluids: CL-guided SIMS oxygen-isotope and trace-element evidence

Abstract: Jadeitite, a rare high P/T rock, is associated spatially with blueschist and/or eclogite terranes. Scanning electron microscope (SEM) and cathodoluminescence (CL) petrography of jadeitite samples from several major occurrences [in Burma (Myanmar), Guatemala, Japan, Kazakhstan, and the U.S.A.] show that grains were deposited from s uids. Jadeite grain compositions indicate these s uid compositions changed with time. CL imagery guided the acquisition of oxygen-isotope and trace-element analyses with the ion microprobe. Jadeite grains in each rock grew in cycles that began with red- and/or blue-luminescent and ended with green-luminescent zones. The CL images were used to order the data into crystallization sequences. These data and electron-microprobe, major-element analyses document the association of green CL with increases in Ca, Mg, and Cr: (1) toward grain exteriors; (2) in Þ ne-grained matrix around porphyroblasts; (3) in shear zones that cut grains; (4) in former open spaces now Þ lled with jadeite; or (5) in veins. Abundances of many trace elements are greater in green-CL jadeitite compared with the red- or blue-CL zones. Some of these elements—in particular Li, Rb, Sr, Ti, Hf, Zr, Y, and REE—are unlikely to have been derived from serpentinite. Although crystal-chemical effects may explain some of the trace-element systematics (e.g., preferential incorporation of REE into Ca-richer jadeite), some kinetic control is suggested by sector-zoned, rhythmically zoned grains. The oxygen-isotope data suggest that jadeitite-depositing s uids either had multiple sources or evolved in composition along their s ow paths (or both).

Electrical conductivities of pyrope-almandine garnets up to 19 GPa and 1700 C

Abstract: Electrical conductivities of polycrystalline garnets ranging in chemical composition from almandine (Fe3Al2Si3O12) to pyrope (Mg3Al2Si3O12) were measured at 10 GPa and 19 GPa at temperatures ranging from 300 to 1700 °C using complex impedance spectroscopy in a multianvil device. Mossbauer spectroscopy of each sample was carried out both before and after the electrical measurements to characterize the oxidation state of Fe in the almandine bearing garnets. Similar to the behavior of other ferromagnesian silicates, the substitution of Fe for Mg along this compositional join dramatically increases electrical conductivity, but this compositional effect is reduced with increasing temperature. Conductivities increase with increasing total Fe content, as the average Fe2+-Fe3+ distance decreases. At 10 GPa, activation energies for conductivity vary smoothly with composition and increase rapidly toward the pyrope end-member composition, where it reaches a value of 2.5 eV. The results are consistent with an electrical conductivity mechanism involving small polaron mobility in the Fe-bearing garnets at 10 GPa. At 19 GPa, however, there is virtually no change in the activation energy as a function of Fe-Mg substitution for the pyrope-rich garnets. These higher pressure measurements reflect a mechanism involving oxygen related point defects, as conductivities increase with pressure at constant T for each garnet, and the effect of pressure is greater for the more Mg-rich garnets. The data also allow for a more quantitative evaluation of the effect of chemical composition, specifically Fe-Mg substitution, on the electrical conductivity profile of the mantle, using a recently developed laboratory-derived model. We apply the model using these data to a portion of the transition zone between 520 and 660 km, in which we vary the garnet composition from Py100 to Py85Alm15. Although only a minor effect on bulk mantle conductivity results, we conclude that the overall garnet composition may, however, be important in characterizing the magnitude of any EC discontinuity with respect to the above-lying mantle.

Quantitative polarized infrared analysis of trace OH in populations of randomly oriented mineral grains

Abstract: Use of infrared spectroscopy as an accurate, quantitative method to measure concentrations of hydrous species in minerals requires consideration of the interactions of anisotropic crystals with infrared light. Ensuring that contributions are identified from species at all orientations in the crystal requires combining three measurements, taken with the electric field polarized along three mutually perpendicular directions. This is typically accomplished by determining the orientation of a crystal in advance, and then sectioning it perpendicular to its principal axes. In many instances, however, natural or experimental samples are not suitable for such handling. Here we demonstrate a method that instead uses at least three randomly sectioned grains, considered to be multiple samples of a homogeneous population. We explain the theory whereby: (1) the orientations of the polarization vectors of measurements taken on these grains are determined by comparison to oriented standards of the same mineral, and (2) the principal-axis spectra of the sample are synthesized from the randomly oriented spectra. By comparison to complementary electron backscatter diffraction (EBSD) data, we demonstrate that determination of orientations using the silicate overtone bands in Fourier-Transform infrared (FTIR) spectra is accurate and precise, with typical angular errors of 6°. We show that this precision is sufficient for the synthetic principal-axis spectra to be essentially indistinguishable from X-ray oriented standard spectra. We demonstrate the application of this technique to determining the OH concentrations in a population of hydrated olivine grains recovered from a high-pressure, high-temperature multi-anvil experiment.

High-pressure proton disorder in brucite

Abstract: In this paper we explore the structure and physical properties of brucite over a wide range of pressures with density functional theory using the variable cell-shape plane wave pseudopotential method in the local density (LDA) and generalized gradient (GGA) approximations. We probe the energetics underlying the structure and dynamics of the proton sub-lattice by performing a series of constrained and unconstrained static calculations based on an energetically stable root 3x root 3x1 super-cell wherein proton locations are related to the 6i Wyckoff sites as opposed to the ideal 2d site. The displacement of the hydrogen atom from the threefold axis increases with increasing pressure. This means that even in the absence of thermal energy, the protons are frustrated and would be expected to exhibit long-range disorder akin to a spin glass. To shed light on the dynamic nature of the proton hopping between the 6i-like sites, we determined the activation energy barrier for such jumps. We found that the energy barrier increases with compression, possibly indicating a transition from dynamic proton disorder at lower pressures to static disorder at higher pressure. We have also investigated the possibility of proton jumps across the interlayer, by determining the potential energy well along the O center dot center dot center dot O vector. We infer that proton jumps across the interlayer are either severely limited or highly cooperative since we did not find any evidence for a double well along the O center dot center dot center dot O vector. The absence of a double well along the O center dot center dot center dot O vector, the evolution of O-H center dot center dot center dot O distances with compression, and the gradual transition to a symmetric O-H center dot center dot center dot O configuration, all argue for weak hydrogen bonding in brucite.