Showing papers in "American Mineralogist in 2012"

Nomenclature of the amphibole supergroup

Abstract: A new classification and nomenclature scheme for the amphibole-supergroup minerals is described, based on the general formula AB 2 C 5 T 8 O 22 W 2 , where A = □, Na, K, Ca, Pb, Li; B = Na, Ca, Mn 2+ , Fe 2+ , Mg, Li; C = Mg, Fe 2+ , Mn 2+ , Al, Fe 3+ , Mn 3+ , Ti 4+ , Li; T = Si, Al, Ti 4+ , Be; W = (OH), F, Cl, O 2− . Distinct arrangements of formal charges at the sites (or groups of sites) in the amphibole structure warrant distinct root names , and are, by implication, distinct species; for a specific root name, different homovalent cations (e.g., Mg vs. Fe 2+ ) or anions (e.g., OH vs. F) are indicated by prefixes (e.g., ferro-, fluoro-). The classification is based on the A, B, and C groups of cations and the W group of anions, as these groups show the maximum compositional variability in the amphibole structure. The amphibole supergroup is divided into two groups according to the dominant W species: W (OH,F,Cl)-dominant amphiboles and W O-dominant amphiboles (oxo-amphiboles). Amphiboles with (OH, F, Cl) dominant at W are divided into eight subgroups according to the dominant charge-arrangements and type of B-group cations: magnesium-iron-manganese amphiboles, calcium amphiboles, sodium-calcium amphiboles, sodium amphiboles, lithium amphiboles, sodium-(magnesium-iron-manganese) amphiboles, lithium-(magnesium-iron-manganese) amphiboles and lithium-calcium amphiboles. Within each of these subgroups, the A- and C-group cations are used to assign specific names to specific compositional ranges and root compositions. Root names are assigned to distinct arrangements of formal charges at the sites, and prefixes are assigned to describe homovalent variation in the dominant ion of the root composition. For amphiboles with O dominant at W, distinct root-compositions are currently known for four (calcium and sodium) amphiboles, and homovalent variation in the dominant cation is handled as for the W (OH,F,Cl)-dominant amphiboles. With this classification, we attempt to recognize the concerns of each constituent community interested in amphiboles and incorporate these into this classification scheme. Where such concerns conflict, we have attempted to act in accord with the more important concerns of each community.

Determination of Mn valence states in mixed-valent manganates by XANES spectroscopy

Abstract: The valence states of Mn in mixed-valent layer and tunnel structure manganese dioxides (MnO2), usually referred to as phyllomanganates and tectomanganates, can be measured by X-ray absorption near-edge structure (XANES) spectroscopy with a precision and accuracy that are difficult to estimate owing to the paucity of well-characterized standards. A compilation of the Mn K -edge XANES spectra of most naturally occurring manganates, synthetic analogs of known structure and chemical composition, and pure-valence phase species is presented and made available as an open source. We intend this compilation to serve as a basis for the spectroscopic determination of the fractions of the Mn 2+, 3+, and 4+ valences in mixed-valent manganates and phase mixtures. The XANES derivatives of tectomanganates and phyllomanganates with no or little Mn3+ in the MnO2 layer exhibit intensities, shapes, and relative energy positions of the main features characteristics of a particular valence composition. For these compounds, valence fractions can be derived using linear combination fitting analysis. Best quantitative results are obtained when the unknown spectrum is fit to a weighted sum of all reference spectra in the database with the fractions of species constrained to be non-negative (Combo method). The accuracy of the average valence is estimated to 0.04 v.u. in the range of 3+ to 4+, and decreases when the proportion of divalent Mn is higher than 15%. The accuracy of the method is also lower in (layer Mn3+, Mn4+) manganates, because the XANES features are affected non-additively by the amount and distribution of the Jahn-Teller Mn3+ cations. The merit of the Combo method for the determination of manganese valence sums relative to the methods based on calibration curves is discussed.

A revised diamond-graphite transition curve

Abstract: The transition from diamond to graphite is a key equilibrium for interpreting ultrahigh-pressure metamorphic rocks. Despite widespread interest, there remain significant systematic differences between the best available experimental determinations of P and T (Kennedy and Kennedy 1976) and numerous thermodynamic calculations of the transition. At temperatures below 1400 K, calculated equilibrium pressures are lower than extrapolations of the experiments by as much as 5 kbar. At 3000 K, calculated pressures vary from more than 8 kbar above to almost 20 kbar below the position of the extrapolated transition. A revised curve based on a critical review of the experimental and thermodynamic data is consistent with expanded experimental brackets and the preferred calorimetric data. It is steeper than the transition proposed by Kennedy and Kennedy (1976) and previous calculations and passes through 16.2 kbar, 298 K; 33.9 kbar, 1000 K; 63.5 kbar, 2000 K; and 98.4 kbar, 3000 K. The revised curve implies that the minimum pressure for formation of diamond-bearing crustal rocks is 3–4 kbar less than implied by extrapolation of the experiments. Because the revised transition is steeper than most previous calculations, the triple point among graphite, diamond, and liquid carbon may be as much as 40 kbar higher than previously estimated.

Characterization of fluor-chlorapatites by electron probe microanalysis with a focus on time-dependent intensity variation of halogens

Abstract: Prior research has shown that fluorine and chlorine X-ray count rates vary with exposure to the electron beam during electron probe microanalysis (EPMA) of apatite. Stormer et al. (1993) and Stormer and Pierson (1993) demonstrate that the EPMA-operating conditions affect the halogen intensities in F-rich natural Durango and Wilberforce apatites and in a Cl-rich apatite. Following these studies, we investigated the effects of operating conditions on time-dependent X-ray intensity variations of F and Cl in a broad range of anhydrous fluor-chlorapatites. We tested 7, 10, and 15 kV accelerating voltages; 4, 10, and 15 nA beam currents; 2, 5, and 10 µm diameter fixed spot sizes; and the influence of 2 distinct crystal orientations under the electron beam. We find that the halogen X-ray intensity variations fluctuate strongly with operating conditions and the bulk F and Cl contents of apatite. We determined the optimal EPMA operating conditions for these anhydrous fluor-chlorapatites to be: 10 kV accelerating voltage, 4 nA beam current (measured at the Faraday cup), 10 µm diameter fixed spot, and the apatite crystals oriented with their c-axes perpendicular to the incident electron beam. This EPMA technique was tested on a suite of 19 synthetic anhydrous apatites that covers the fluorapatite-chlorapatite solid-solution series. The results of these analyses are highly accurate; the F and Cl EPMA data agree extremely well with wet-chemical analyses and have an R 2 value >0.99.

Gold-telluride nanoparticles revealed in arsenic-free pyrite

Abstract: Pyrite, the most abundant sulfide on Earth and a common component of gold deposits, can be a significant host for refractory gold. This is the first documentation of pore-attached, composite Au-telluride nanoparticles in “arsenic-free” pyrite. Trace elements mapping in pyrite from an intrusion-hosted Au deposit with orogenic overprint (Dongping, China) shows trails of tellurides overlapping Co-Ni-zonation. Intragranular microfracturing, anomalous anisotropy, and high porosity are all features consistent with devolatilization attributable to the orogenic event. The pyrite-hosted nanoparticles are likely the “frozen,” solid expression of Te-rich, Au-Ag-Pb-bearing vapors discharged at this stage. Nanoparticle formation, as presented here, provides the “smallest-scale” tool to fingerprint Au-trapping during crustal metamorphism

Rates and mechanism of Y, REE, and Cr diffusion in garnet

Abstract: Numerical simulation of the evolution of stranded diffusion profiles in partially resorbed garnet crystals from the aureole of the Makhavinekh Lake Pluton, Labrador, yields quantitative determinations of rates of diffusion of Y, rare earth elements (REEs), and Cr at ~700–900 °C, 0.53 GPa. Diffusion coefficients for these trivalent cations are 0.5–1.5 log 10 units smaller than those for major divalent cations measured in the same crystals, but diffusivities for trivalent cations are all equal to one another to within ±0.25 log 10 unit. Integration of these new data with previously published results resolves some prior inconsistencies, and defines the dependence of diffusivities for Y, the REEs, and Cr on temperature, pressure, and oxygen fugacity, while accounting for minor effects of ionic radius and host-crystal composition. Nd, Sm, and Eu—elements that are strongly depleted in rims of relict garnet crystals due to preferential partitioning out of garnet during resorption—evolve small but distinct maxima from initially nearly flat profiles; this “uphill diffusion” results from cross-coupling with Y and the other REEs, which are strongly concentrated in relict garnet rims by resorption. The weak dependence of diffusivity on ionic radius and host-crystal composition, the near-equivalence of diffusivities of Y+REEs with that of Cr, and the strong positive cross-coupling among Y+REEs are all explained by a diffusion mechanism that links the mobility of VIII Y+REEs with that of VI Al; this is likely a consequence of the dominance of the menzerite-(Y) component as a means of incorporating Y+REEs in the garnet structure. However, the variety of possible substitution mechanisms that may enable Y+REE incorporation into garnet, and the degree to which each may be favored in different regimes of temperature, pressure, and oxygen fugacity, imply a potential for great complexity, as the net diffusional flux may result from the superposed effect of multiple diffusion mechanisms, each with different kinetics.

Polysaccharide-catalyzed nucleation and growth of disordered dolomite: A potential precursor of sedimentary dolomite

Abstract: The origin of dolomite is a long-standing enigma in sedimentary geology. It has been proposed that microorganisms, especially anaerobic microorganisms, can overcome kinetic barriers to facilitate dolomite precipitation, although their specific role in dolomite formation is still unclear. Our experimental results demonstrate that disordered dolomite can be synthesized at room temperature abiotically from solutions containing polysaccharides such as carboxymethyl cellulose or agar. We propose that when dissolved in solution, polysaccharides can be strongly adsorbed on Ca-Mg carbonate surfaces through hydrogen bonding. The adsorbed polysaccharides may help weaken the chemical bonding between surface Mg2+ ions and water molecules, which can lower the energy barrier to the desolvation of surface Mg2+-water complexes, enhance Mg2+ incorporation into the precipitating carbonate, and thereby promote disordered dolomite formation. In natural environments, it is possible that polysaccharides produced by microorganisms, e.g., extracellular polysaccharides, may play a key role in promoting disordered dolomite nucleation and crystallization. In marine sediments, the accumulated dissolved carbohydrates produced from organic matter degradation during early diagenesis may also serve as catalysts for disordered dolomite formation.

Cell assemblies for reproducible multi-anvil experiments (the COMPRES assemblies)

Abstract: The multi-anvil high-pressure technique is an important tool in high-pressure mineralogy and petrology, as well as in chemical synthesis, allowing the treatment of large (millimeter-size) samples of minerals, rocks, and other materials at pressures of a few GPa to over 25 GPa and simultaneous uniform temperatures up to 2500 °C and higher. A series of cell assemblies specially designed and implemented for interlaboratory use are described here. In terms of the size of the pressure medium and the anvil truncation size, the five sizes of assemblies developed here are an 8/3, 10/5, 14/8, 18/12, and 25/15 assembly. As of this writing, these assemblies are in widespread use at many laboratories. The details of design, construction, and materials developed or used for the assemblies are presented here.

Determination of the oxidation state of Cu in substituted Cu-In-Fe-bearing sphalerite via μ-XANES spectroscopy

Abstract: In situ spatially resolved X-ray absorption near edge structure (μ-XANES) spectra are obtained for natural Cu-In-bearing sphalerite. Copper K -edge spectral data show that, in Cu-In-bearing sphalerite, in which an excellent correlation between the Cu and In contents is noted, Cu is present in the Cu + state. This offers indirect proof for the coupled substitution 2 Zn 2+ ↔ Cu + + In 3+ , which allows indium to enter the sphalerite structure. The study clearly demonstrates the utility of synchrotron radiation to accurately determine oxidation state in small volumes of mineral in which the concentration of the element of interest is low or very low. The study also demonstrates that good quality μ-XANES spectra can be collected on TEM foils prepared in situ at a chosen position on the surface of a polished sample using the focused ion beam–scanning electron microscope method.

Determination of water content in silicate glasses using Raman spectrometry: Implications for the study of explosive volcanism

Abstract: Raman spectroscopy can measure water concentrations of hydrous silicate glasses with several advantages such as: (1) high-spatial resolution of 1–2 μm2; (2) non-destructive character; and (3) easy access, without any specific sample preparation or mounting techniques. The latter reasons render Raman highly suitable for studying natural products, such as volcanic pumice and scoriae fragments. Two spectral regions can be distinguished in Raman spectra of hydrated silicate glasses: a low-wavenumber region (15–1500 cm−1), which corresponds to vibrations of the silicate network, and a high-wavenumber region (3100–3750 cm−1), corresponding to the OH stretching vibrations of H2O molecules and OH groups. Behrens et al. (2006) have published empirical equations relating the area ratio between these two regions and the water content. However, the proposed internal calibrations depend on chemical composition of the glasses. In this paper, we reinvestigated the previous procedures to improve the background subtraction. Our results allow us to present a more general and linear calibration. Water concentrations up to 13 wt% can be measured for a broad range of natural silicate melts, from basalts to rhyolite (40 up to 80 wt% SiO2), using a single calibration curve with an absolute error of 0.2 wt%.

The mechanism of thermal decomposition of dolomite: New insights from 2D-XRD and TEM analyses

Abstract: Despite being studied for more than one century, no consensus exists regarding the ultimate mechanism(s) of the thermal decomposition of dolomite [(CaMg(CO 3 ) 2 ]. To shed light on such a reaction, dolomite single crystals were calcined in air between 500 and 1000 °C, and in situ, in a TEM (high vacuum), following irradiation with the electron beam. In situ TEM shows that the decomposition involves the initial formation of a face centered cubic mixed oxide (Ca 0.5 Mg 0.5 O) with reactant/product orientation relationships [001] dolomite // oxide , dolomite // oxide , {1120} dolomite //{110} oxide , {1128} dolomite //{110} oxide and {1014} dolomite ^{100} oxide ~12°. This phase undergoes de-mixing into oriented crystals of Mg-poor CaO and Ca-poor MgO solid solutions upon long-term e-beam exposure. Ex situ TEM, XRD, 2D-XRD, and FESEM analyses show the formation of porous pseudomorphs made up of oxide nanocrystals with similar parent/product orientation relationships, but with limited Ca/Mg substitution (up to ~9–11%) due to de-mixing (spinodal decomposition) of the metastable (Ca,Mg)O precursor. High ion diffusivity at T > 500 °C (ex situ experiments) favors the formation of pure CaO and MgO crystals during coarsening via oriented aggregation and sintering. These results show that the thermal decomposition of dolomite is topotactic and independent of p CO 2 . Formation of Mg-calcite nanocrystals (up to ~8 mol% Mg) during the so-called “half decomposition” is observed at 650–750 °C. This transient phase formed topotactically following the reaction of CaO nanocystals (solid solution with ~9 mol% Mg) with CO 2 present in the air and/or released upon further dolomite decomposition. With increasing T , Mg-calcite transformed into calcite, which underwent decomposition following the known topotactic relationship: {1014} calcite //{110} CaO and calcite // CaO . These observations solve the long standing controversy on the mechanism of the “two-stage” decomposition of dolomite, which assumed the direct formation of calcite during the so-called “half decomposition.”

Thermodynamics of manganese oxides: Effects of particle size and hydration on oxidation-reduction equilibria among hausmannite, bixbyite, and pyrolusite

Abstract: The surface enthalpies of manganese oxide phases, hausmannite (Mn3O4), bixbyite (Mn2O3), and pyrolusite (MnO2), were determined using high-temperature oxide melt solution calorimetry in conjunction with water adsorption calorimetry. The energy for the hydrous surface of Mn3O4 is 0.96 ± 0.08 J/m2, of Mn2O3 is 1.29 ± 0.10 J/m2, and of MnO2 is 1.64 ± 0.10 J/m2. The energy for the anhydrous surface of Mn3O4 is 1.62 ± 0.08 J/m2, of Mn2O3 is 1.77 ± 0.10 J/m2, and of MnO2 is 2.05 ± 0.10 J/m2. Supporting preliminary findings (Navrotsky et al. 2010), the spinel phase (hausmannite) has a lower surface energy than bixbyite, whereas the latter has a smaller surface energy than pyrolusite. Oxidation-reduction phase equilibria at the nanoscale are shifted to favor the phases of lower surface energy—Mn3O4 relative to Mn2O3 and Mn2O3 relative to MnO2. We also report rapidly reversible structural and phase changes associated with water adsorption/desorption for the nanophase manganese oxide assemblages.

Brittle-ductile microfabrics in naturally deformed zircon: Deformation mechanisms and consequences for U-Pb dating

Abstract: We present an electron backscatter diffraction, cathodoluminescence, and radiogenic U-Pb dating study of large zircon grains (0.8-1.5 mm) that show evidence of intracrystalline deformation, fractur ...

Precipitation of rutile and ilmenite needles in garnet: Implications for extreme metamorphic conditions in the Acadian Orogen, U.S.A.

Abstract: We report the discovery of oriented needles of rutile and ilmenite in garnet crystals from granulite facies metapelitic rocks of the Merrimack synclinorium, Connecticut, and present a precipitation model for their origin. The rocks were strongly metamorphosed and deformed during the Devonian Acadian orogeny. The needles are primarily elongated parallel to in garnet. Rutile has anomalous extinction angles as great as ~35° (cf. Griffin et al. 1971). Rutile and ilmenite needles are typically a few hundred nanometers to several micrometers in diameter and are several tens of micrometers to nearly a millimeter long. Other oxide inclusions that may be present include submicrometer- to micrometer-scale twinned rutile bicrystals, as well as srilankite and a crichtonite group mineral. Some garnet cores have unusual, box-shaped quartz inclusions, which coexist with Ti±Fe oxide needles and commonly contain micrometer-scale rods of F-OH-Cl apatite. Negative garnet crystal “pores” are also widespread. Ti±Fe oxide needles are restricted to garnet core regions; rims have a distinctly different inclusion population dominated by granulite facies minerals including sillimanite, spinel, cordierite, and K-feldspar. Consequently, the garnet core regions represent an earlier, distinct period of growth relative to the rims. Garnet cores contain ~25–35% pyrope, and a host of minor and trace constituents including TiO2 (0.07–0.6 wt%), Cr2O3 (0.01–0.10 wt%), Na2O (0.01–0.03 wt%), P2O5 (0.01–0.09 wt%), and ZrO2 (up to ~150 ppm). Na2O and ZrO2 correlate positively with TiO2. Titanium zoning is preserved in some garnets; zoning profiles and two-dimensional chemical mapping show that Ti and, to a lesser degree, Cr are depleted around Ti±Fe oxide inclusions. Therefore, we conclude that the needles are precipitates that formed from Ti-bearing garnet during exhumation and cooling. Garnet contained sufficient Ti to form precipitates; no Ti source external to garnet was necessary. Titanium-bearing garnets that contain oriented Ti±Fe oxide needles are known primarily from ultrahigh-pressure metamorphic rocks, mantle peridotites and pyroxenites, and high-pressure granulites. Thus, the presence of needle-bearing garnets in Connecticut strongly suggests that a previously unrecognized domain of extreme pressure and/or temperature metamorphism exists in the Acadian orogen.

Mercury (Hg) mineral evolution: A mineralogical record of supercontinent assembly, changing ocean geochemistry, and the emerging terrestrial biosphere

Abstract: Analyses of the temporal and geographic distribution of earliest recorded appearances of the 88 IMA-approved mercury minerals plus two potentially valid species exemplify principles of mineral evolution. Metacinnabar (HgS) and native Hg are the only two species reported from meteorites, specifically, the primitive H3 Tieschitz chondrite with an age of 4550 Ma. Since the first terrestrial appearance of cinnabar more than 3 billion years ago, mercury minerals have been present continuously at or near Earth’s surface. Mercury mineral evolution is characterized by episodic deposition and diversification, perhaps associated with the supercontinent cycle. We observe statistically significant increases in the number of reported Hg mineral localities and new Hg species at ~2.8–2.6, ~1.9–1.8, and ~0.43–0.25 Ga—intervals that correlate with episodes of presumed supercontinent assembly and associated orogenies of Kenorland (Superia), Columbia (Nuna), and Pangea, respectively. In constrast, few Hg deposits or new species of mercury minerals are reported from the intervals of supercontinent stability and breakup at ~2.5–1.9, ~1.8–1.2, and 1.1–0.8 Ga. The interval of Pangean supercontinent stability and breakup (~250–65 Ma) is also marked by a significant decline in reported mercury mineralization; however, rocks of the last 65 million years, during which Pangea has continued to diverge, is characterized by numerous ephemeral near-surface Hg deposits. The period ~1.2–1.0 Ga, during the assembly of the Rodinian supercontinent, is an exception because of the absence of new Hg minerals or deposits from this period. Episodes of Hg mineralization reflect metamorphism of Hg-enriched marine black shales at zones of continental convergence. We suggest that Hg was effectively sequestered as insoluble nanoparticles of cinnabar (HgS) or tiemannite (HgSe) during the period of the sulfidic “intermediate ocean” (~1.85–0.85 Ga); consequently, few Hg deposits formed during the aggregation of Rodinia, whereas several deposits date from 800–600 Ma, a period that overlaps with the rifting and breakup of Rodinia. Nearly all Hg mineral species (87 of 90 known), as well as all major economic Hg deposits, are known to occur in formations ≤400 million years old. This relatively recent diversification arises, in part, from the ephemeral nature of many Hg minerals. In addition, mercury mineralization is strongly enhanced by interactions with organic matter, so the relatively recent pulse of new Hg minerals may reflect the rise of a terrestrial biosphere at ~400 Ma.

Unraveling complex <2 μm clay mineralogy from soils using X-ray diffraction profile modeling on particle-size sub-fractions: Implications for soil pedogenesis and reactivity

Abstract: A specific methodology was developed to refine the complex clay mineralogy commonly encountered in soil environments. The soil examined was a Cambisol developed into a ferralitic paleosol. The sample was split into four sub-fractions of different particle sizes (<0.05, 0.05-0.1, 0.1-0.2, and 0.2-2 mu m), and their respective mass contributions to the overall <2 mu m clay fraction were determined. For each sub-fraction, X-ray diffraction (XRD) patterns were modeled using a trial-and-error approach based on the direct comparison of experimental and calculated profiles. Quantitative information derived from the fitting procedure for the different sub-fractions allowed for the determination of the complex mineralogy of the <2 mu m clay fraction through the identification and quantification of eight clay phases. The results show that the finest and most reactive clay fraction (<0.05 pm) was totally hidden in the XRD pattern of the <2 mu m fraction, the fraction commonly considered in soil mineralogical analyses. Similarly, this procedure revealed the presence of illite-smectite-chlorite and kaolinite-illite mixed-layer minerals seldom described in soil literature using classical methods. The use of this methodology improved our understanding of the pedogenesis of this soil through the identification and quantification of clay phases structural properties. The analysis of the evolution of structural parameters with particle size allowed for the detection of local modifications in the interlayer composition of expandable and hydroxy-interlayered vermiculite layers. Following this approach, key information can be derived to determine subtle changes in clay mineralogical composition that are related to microorganism and/or plant activity

Electronic spin states of ferric and ferrous iron in the lower-mantle silicate perovskite

Abstract: The electronic spin and valence states of iron in lower-mantle silicate perovskite have been previously investigated at high pressures using various experimental and theoretical techniques. However, experimental results and their interpretation remain highly debated. Here we have studied a well-characterized silicate perovskite starting sample [(Mg 0.9 ,Fe 0.1 )SiO 3 ] in a chemically inert Ne pressure medium at pressures up to 120 GPa using synchrotron Mossbauer spectra. Analyses of the Mossbauer spectra explicitly show a high-spin to low-spin transition of the octahedral-site Fe 3+ occurring at ~13–24 GPa, as evidenced from a significant increase in the hyperfine quadrupole splitting. Two quadrupole doublets of the A-site Fe 2+ , with extremely high-QS values of 4.1 and 3.1 mm/s, occur simultaneously with the spin transition of the octahedral-site Fe 3+ and continue to develop to 120 GPa. It is conceivable that the spin-pairing transition of the octahedral-site Fe 3+ causes a volume reduction and a change in the local atomic-site configurations that result in a significant increase of the quadrupole splitting in the dodecahedral-site Fe 2+ at 13–24 GPa. Our results here provide a coherent explanation for recent experimental and theoretical results on the spin and valence states of iron in perovskite, and assist in comprehending the effects of the spin and valence states of iron on the properties of the lower-mantle minerals.

Graphite in the martian meteorite Allan Hills 84001

Abstract: We use confocal Raman imaging spectroscopy and transmission electron microscopy to study the martian meteorite Allan Hills (ALH) 84001, reported to contain mineral assemblages within carbonate globules (carbonate + magnetite), interpreted as potential relict signatures of ancient martian biota Models for an abiologic origin for these assemblages required the presence of graphite, and this study is the first report of graphite within ALH 84001 The graphite occurs as hollow spheres (nano-onions), filaments, and highly crystalline particles in intimate association with magnetite in the carbonate globules In addition to supporting an abiologic origin for the carbonate globule assemblages in ALH 84001, this work proves that there is an inventory of reduced-carbon phases on Mars that has not yet been thoroughly investigated

Vibrational and elastic properties of ferromagnesite across the electronic spin-pairing transition of iron

Abstract: Ferromagnesite [(Mg,Fe)CO3] has been proposed as a candidate host mineral for carbon in the Earth’s mantle. Studying its physical and chemical properties at relevant pressures and temperatures helps our understanding of deep-carbon storage in the planet’s interior and on its surface. Here we have studied high-pressure vibrational and elastic properties of magnesian siderite [(Mg0.35Fe0.65)CO3] across the electronic spin transition by Raman and X-ray diffraction spectroscopies in a diamond-anvil cell. Our results show an increase in Raman shift of the observed lattice modes of magnesian siderite across the spin transition at 45 GPa as a result of an ∼8% unit-cell volume collapse and a 10% stiffer lattice (higher bulk modulus). C-O bond lengthening in the strong, rigid (CO3) 2– unit across the spin transition contributes to a competitive decrease in Raman shift, most evident in the Raman shift decrease of the symmetric stretching mode. Combined vibrational and elastic results are used to derive the mode Gruneisen parameter of each mode, which drops significantly across the transition. These results suggest that the low-spin state has distinctive vibrational and elastic properties compared to the high-spin state. Analyses of all recent experimental results on the (Mg,Fe)CO3 system show no appreciable compositional effect on the transition pressure, indicating weak iron-iron exchange interactions. Our results provide new insight into understanding the effects of the spin transition on the vibrational, elastic, and thermodynamic properties of (Mg,Fe)CO3 as a candidate carbon-host in the deep mantle.

Silician magnetite from the Dales Gorge Member of the Brockman Iron Formation, Hamersley Group, Western Australia

Abstract: We report silician magnetite from banded iron formation (BIF) in the Dales Gorge Member of the Brockman Iron Formation, Hamersley Group, Western Australia. Magnetite mesobands typically consisting of individual ~100 μm microlaminae are revealed to be composed of silician magnetite overgrowths on magnetite. Silician magnetite overgrowths contain from 1 to 3 wt% SiO 2 , whereas (low-Si) magnetite domains contain less than 1 wt% SiO 2 . Silicon solid solution is present in the magnetite crystal lattice as determined by in situ micro-X-ray diffraction and high-resolution transmission electron microscopy. Three textures are distinguished in magnetite mesobands: (1) magnetite sub-microlaminae with silician magnetite overgrowths, (2) recrystallized magnetite fragments with silician magnetite overgrowths, and (3) a complex intergrowth of magnetite and silician magnetite. All three textures are found in magnetite mesobands from the BIF4–5 and BIF12–16 macrobands of the Dales Gorge type-section drill core DDH-47A from Wittenoom, Western Australia. Magnetite domains contain numerous submicrometer-to-micrometer inclusions of quartz, carbonate, stilpnomelane, and apatite, whereas silician magnetite overgrowths are devoid of mineral inclusions. The presence of mineral inclusions in magnetite indicates the BIF oxide precipitate was not chemically pure iron oxyhydroxide/oxide. Magnetite domains display textures formed during soft sediment deformation that are the earliest and best preserved relict sedimentary structures in this BIF. Silician magnetite is the dominant iron oxide in the Dales Gorge BIF and is present in many other sub-greenschist facies BIFs worldwide. We suggest the former presence of organic matter creates reducing conditions necessary to stabilize silician magnetite. Thus, silician magnetite is a potential biosignature in BIFs.

The dehydroxylation of serpentine group minerals

Abstract: The thermal transformation, stability field, and reaction kinetics of serpentine minerals (antigorite, chrysotile, and lizardite) have been studied to draw a comprehensive model for their dehydroxylation and recrystallization reactions. In situ X-ray powder diffraction (XRPD) and kinetic studies were combined with transmission electron microscopy (TEM) observations to describe the mechanisms of dehydroxylation and later high-temperature crystallization. During dehydroxylation, a metastable transition phase with a characteristic peak around 9 A was observed in antigorite and, to a minor extent, in lizardite. Rietveld refinements confirmed that the 9 A phase actually possesses a talc-like structure. The appearance of this phase is controlled by structure and kinetic factors. The kinetic parameters and reaction mechanism for lizardite and antigorite dehydroxylation in air at ambient pressure were calculated using the Avrami models and compared to those of chrysotile. For both lizardite and antigorite, the kinetics of dehydroxylation is controlled by diffusion. Apparent activation energy of the reaction in the temperature range 612–708 °C was 221 and 255 kJ/mol for lizardite and antigorite, respectively. The reaction sequences of chrysotile, lizardite, and antigorite leading to the formation of stable high-temperature products (i.e., forsterite and enstatite) are described taking into account previous topotactic and dissolution-recrystallization models.

Morphological quantification of hierarchical geomaterials by X-ray nano-CT bridges the gap from nano to micro length scales

Abstract: Morphological quantification of the complex structure of hierarchical geomaterials is of great relevance for Earth science and environmental engineering, among others. To date, methods that quantify the 3D morphology on length scales ranging from a few tens of nanometers to several hundred nanometers have had limited success. We demonstrate, for the first time, that it is possible to go beyond visualization and to extract quantitative morphological information from X-ray images in the aforementioned length scales. As examples, two different hierarchical geomaterials exhibiting complex porous structures ranging from nanometer to macroscopic scale are studied: a flocculated clay water suspension and two hydrated cement pastes. We show that from a single projection image it is possible to perform a direct computation of the ultra-small angle-scattering spectra. The predictions matched very well the experimental data obtained by the best ultra-small angle-scattering experimental setups as observed for the cement paste. In this context, we demonstrate that the structure of flocculated clay suspension exhibit two well-distinct regimes of aggregation, a dense mass fractal aggregation at short distance and a more open structure at large distance, which can be generated by a 3D reaction limited cluster-cluster aggregation process. For the first time, a high-resolution 3D image of fibrillar cement paste cluster was obtained from limited angle nanotomography.

Crystal structure and thermal expansion of aragonite-group carbonates by single-crystal X-ray diffraction

Abstract: Crystal structures of four aragonite-group carbonates—aragonite (Ca0.997Sr0.003CO3), calcian strontianite (Ca0.147Sr0.853CO3), cerussite (Ca0.001Pb0.999CO3), and witherite (Sr0.019Ba0.981CO3)—have been refined at ambient conditions, and thermal expansion has been measured over a range of temperatures from 143 to 586 K by single-crystal X-ray diffraction. Average linear thermal expansion coefficients α( V ) are 58(2), 58.3(7), 64(2), and 57(2) (× 10−6 K−1) for aragonite, strontianite, cerussite, and witherite, respectively, throughout the experimental temperature range. Aragonite, strontianite, and witherite have very similar α( V ) values, whereas that of cerussite is significant larger, primarily due to the c -axis thermal expansion for cerussite being much larger than those of the other carbonates. There are no significant differences for α( a ) values among the four carbonates, whereas α( b ) values decrease in the order of aragonite > strontianite > cerussite ≈ witherite, and α( c ) values increase in the order of aragonite vs. T (K) is fitted linearly quite well, with a slope of 5.8(8) × 10−6 (A/K). Corrected for assumed rigid body motion, the CO3 groups showed no significant change in C-O distances over the temperature range.

Experimental calibration of the effect of H2O on plagioclase crystallization in basaltic melt at 200 MPa

Abstract: Crystallization experiments were conducted at 200 MPa to determine the effect of small amounts of H 2 O on the liquidus temperature of basaltic melts in which plagioclase is the liquidus phase. The H 2 O concentrations in the quenched glasses, determined by infrared spectroscopy and Karl-Fischer titration, ranged from 0.02 to 4.2 wt% H 2 O. The dry liquidus temperature at 200 MPa was estimated from experiments at 1 atm (H 2 O-free) and from the known pressure dependence of plagioclase crystallization temperature. The effect of water (expressed as wt% H 2 O) on the plagioclase liquidus temperature is nonlinear and diminishing with increasing melt H 2 O concentrations. According to our new experimental data, it can be empirically predicted with following equation: ( T DRY - T WET ) = 76.99 · C H 2 O 0.71 where C H 2 O is the water concentration in the melt (wt%), T DRY , and T WET are plagioclase crystallization temperatures in water-free and water-bearing systems, respectively. The relationship between C H 2 O and liquidus temperature worked out in this study is valid for a range of basaltic compositions, ranging from high-alumina basalts to basaltic andesites. The combination of the empirical equation predicting the liquidus depression of plagioclase with previous models predicting the olivine liquidus curve is useful to determine the liquidus temperature in various H 2 O-bearing basaltic systems in which either plagioclase or olivine is the liquidus phase.

Boron in natural type IIb blue diamonds: Chemical and spectroscopic measurements

Abstract: The presence of boron in the structure of diamond is rare in nature, and even when present, reported values are ≤0.5 ppm. This study used various spectroscopic methods and time-of-fight (ToF-) SIMS to characterize and analyze for boron in natural type IIb blue diamonds, including the well-known Hope and the Blue Heart diamonds, and on one high-pressure, high-temperature annealed natural stone. Infrared spectroscopy measurements reveal uncompensated boron values as large as 1.72 ± 0.15 ppm, which is significantly higher than the previously reported maximum of 0.5 ppm. ToF-SIMS analyses gave spot total boron concentrations as high as 8.4 ± 1.1 ppm for the Hope diamond to less than 0.08 ppm in other blue diamonds. By comparison, a type Ia diamond did not show detectable boron. ToF-SIMS analyses revealed strong zoning of boron in some diamonds, which was confirmed by mapping the uncompensated boron using synchrotron infrared spectroscopy. This greater range of boron concentrations compared to previous studies might be explained by the larger number of natural diamonds analyzed here, 78, compared to <10 samples reported in the literature. The samples in this study are all gem-quality diamonds, including some Intense to Fancy-Deep blue diamonds; color intensity, however, only loosely correlates with the boron content. Boron is also likely responsible for the phosphorescence emissions of type IIb diamonds, in the red at 660 nm and in the blue-green at 500 nm. Our results are consistent with previous work suggesting that the emissions are caused by donor-acceptor pair recombination processes involving boron and other defects. The exact nature of the phosphorescence processes is still not fully understood, but likely involves complex steps of charge carrier trapping and detrapping.

Implications of ferrous and ferric iron in antigorite

Abstract: Microprobe analyses of antigorite show that (Al+Cr) and inferred Fe 3+ correlate inversely with Si apfu in a Tschermaks substitution. This observation suggests that the uptake of Fe 3+ is not simply related to f O 2 . For Si = 1.95 apfu estimated Fe 3+ = 0.032 apfu (or 0.95 wt% Fe 2 O 3 ). Such estimates of Fe 3+ require high analytical accuracy and precision, and assume a fixed polysomatic formula (e.g., m = 17) and freedom from interlayer sheet-silicate impurities. In many cases the estimates appear to be high. An alternative measure of Fe 3+ is provided by the partitioning of total Fe and Mg between antigorite and olivine in well-equilibrated natural antigorite-olivine-magnetite parageneses. Extrapolation of Nernst and Roozeboom partition plots to Fe-free olivine permits an estimate of the Fe 3+ content of the average antigorite in this paragenesis, namely 0.42 or 0.64 wt% Fe 2 O 3 . The partition estimates are in good agreement with the results of Mossbauer spectroscopy performed here on 14 antigorites from metaperidotites, together with four from the literature. These spectra reveal a range of 0.16 to 1.94 in wt% Fe 2 O 3 in metaperidotite antigorite, with an average of 0.83. In two olivine-bearing rocks, antigorite has Fe 3+ /∑Fe ratios of 0.13 and 0.15, which corresponds to wt% Fe 2 O 3 = 0.47 and 0.54, respectively. Larger amounts of Fe 2 O 3 occur in some, but not all, vein antigorites. The prograde formation of antigorite in serpentinite from lizardite is accompanied by loss of some cronstedtite component and the precipitation of additional magnetite. The Roozeboom Mg/Fe partition plot is concave down rather than up; in other words the partition coefficient K D is a function of the X Mg of olivine. This behavior has been found in other olivine-mineral pairs. It can be interpreted to reflect strongly non-ideal solution behavior of MgFe-olivine at low temperatures, viz. W G ≈ 8.5 kJ assuming a symmetrical solution. MgFe-brucite appears to be similarly non-ideal.

Growth process and crystallographic properties of ammonia-induced vaterite

Abstract: Metastable vaterite crystals were synthesized by increasing the pH and consequently the saturation states of Ca2+- and CO32−-containing solutions using an ammonia diffusion method. SEM and TEM analyses indicate that vaterite grains produced by this method are polycrystalline aggregates with a final morphology that has a sixfold-symmetry. The primary structure develops within an hour and is almost a spherical assemblage of nanoparticles (5–10 nm) with random orientation, followed by the formation of hexagonal platelets (1–2 μm), which are first composed of nanoparticles and that develop further into single crystals. As determined using transmission electron microscopy, these hexagonal crystallites are terminated by (001) surfaces and are bounded by {110} edges. The hexagonal crystals subsequently stack to form the “petals” (20 μm wide, 1 μm thick) of the final “flower-like” vaterite morphology. The large flakes gradually tilt toward the center as growth progresses so that their positions become more and more vertical, which eventually leads to a depression in the center. Since this sequence encompasses several morphologies observed in previous studies (spheres, hexagons, flowers etc.), they may actually represent different stages of growth rather than equilibrium morphologies for specific growth conditions.

Density functional calculations of the enthalpies of formation of rare-earth orthophosphates

Abstract: Electronic structure calculations are carried out to estimate the enthalpies of formation of rareearth orthophosphates from their oxides. The calculated enthalpies of formation are systematically less exothermic than the measured values. The discrepancy is almost entirely in the electronic total energy calculated from density functional theory, and appears to be intrinsic to the generalized-gradient exchange-correlation functional used. However, comparison with electronic structure calculation of the enthalpies of formation of alkaline earth oxyacid carbonates, silicates, and sulfates suggests that near chemical accuracy can be obtained for the enthalpies of formation of most of the compounds in the phosphate system by applying a scaling factor obtained from the simpler alkaline earth oxyacids. The increasingly exothermic ∆H f ox with increasing ionic radius (i.e., LaPO4 is more exothermic than ScPO4) results from the higher charge localization on the oxide anion (O 2– ) relative to the phosphate anion (PO4 3– ), making it more favorable, in a relative sense, to pair the smaller cation with the oxide anion than with the phosphate anion. This effect is also manifested in ∆H f ox of the other oxyacids, such

Effect of alkalis on the Fe oxidation state and local environment in peralkaline rhyolitic glasses

Abstract: Iron oxidation state and coordination geometry have been determined by Fe K-edge X-ray absorption near edge spectroscopy (XANES) for three sets of silicate glasses of peralkaline rhyolitic composition with different peralkalinity values. These compositions were chosen to investigate the effect of alkali content (and oxygen fugacity) on the Fe oxidation state. The samples were produced by means of hydrothermal vessels at 800 °C with oxygen fugacity conditions ranging from NNO-1.61 to NNO+2.96 log units. Comparison of the pre-edge peak data with those of Fe model compounds of known oxidation state and coordination number allowed determination of the Fe oxidation state and coordination number in all glasses analyzed. Within each group of samples, Fe tends to oxidize with increasing oxygen fugacity as expected. However, alkali content is shown to have a strong effect on the Fe3+/(Fe3++Fe2+) ratio at constant oxygen fugacity: this ratio varies from 0.25 to 0.55 (±0.05) for the least peralkaline series, and from 0.45 to 0.80 (±0.05) for the most peralkaline series. Moreover, pre-edge peak data clearly indicate that Fe3+ is in fourfold coordination in the most peralkaline glasses. Extrapolation of pre-edge peak data suggests the presence of both fourfold and fivefold coordination for trivalent Fe in the other two series. Divalent Fe is suggested to be mainly in fivefold coordination in all the three glass series. The presence of minor amounts of sixfold- and fourfold-coordinated Fe cannot be ruled out by XANES data alone. XANES data suggest that the amount of alkalis also affects the Fe3+ coordination environment resulting in a decrease in the average coordination numbers. Extended X-ray absorption fine structure (EXAFS) data of the most oxidized and peralkaline sample indicate that Fe3+ is in tetrahedral coordination with = 1.85 A (±0.02). This value compares well with literature data for [4]Fe3+ in crystalline phases (e.g., in tetra-ferriphlogopite or rodolicoite) or in silicate glasses (e.g., phonolite glasses) supporting the XANES-determined coordination number obtained for the most peralkaline glasses. Calculated NBO/T ratios decrease slightly with Fe oxidation because of the higher fraction of network forming Fe, thus increasing the polymerization of the tetrahedral network.

The enigmatic iron oxyhydroxysulfate nanomineral schwertmannite: Morphology, structure, and composition

Abstract: Two sets of precipitates collected from stream sediments in the Monte Romero (MR) and Tinto Santa Rosa (TSR) abandoned mine sites—located in the Iberian Pyrite Belt (IPB) of Spain—were identified as the iron oxyhydroxysulfate nanomineral schwertmannite using X-ray diffraction (XRD) and bulk digestion and were further studied in great detail using analytical high-resolution transmission electron microscopy (HRTEM). Extensive HRTEM observations suggest that schwertmannite should not be described as a single-phase mineral with a repeating unit cell, but as a polyphasic nanomineral with crystalline areas spanning less than a few nanometers within an amorphous matrix. The d -spacings measured from lattice fringes within schwertmannite’s needles match with d -spacings of the known transformation products of schwertmannite (goethite and jarosite). This finding implies that the initial stages of schwertmannite transformation occur as a gradual structural reordering at the nanoscale. Energy-dispersive X-ray analysis applied across individual schwertmannite needles with ~3 nm spot size resolution reveal a decreasing ratio of sulfur to iron and silicon to iron from the surface of the needle to the core with the silicon to iron ratio consistently higher than the sulfur to iron ratio. Amorphous silicon-rich precipitates were identified on the surface of the TSR schwertmannite. All of these observations explain why the measured solubility product of schwertmannite is variable, resulting in calculated stability fields that differ greatly from sample to sample. Arsenic is the most abundant trace element in these samples [MR: 0.218(1) wt% and TSR: 0.53(2) wt%], keeping in mind that schwertmannite has been shown to be a key player in the cycling of this element on a global basis, particularly from the IPB. Furthermore, arsenic in the TSR schwertmannite is associated with crystalline areas within its needle matrix, implying that schwertmannite-derived goethite nanocrystals may be an important host of arsenic.