Volume 97, Issue 7

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.

First-principles calculations based on density-functional theory (DFT) and low-T IR spectroscopy were performed to gain more insight into the structure of the so-called 3.65 Å phase, a high-pressure phase of the composition MgSi(OH) 6 . DFT-calculations predict a monoclinic symmetry with ordered sixfold-coordinated Mg and Si and six unique hydrogen sites as the most stable structure. Adapting the structural parameters of the DFT-determined lowest-energy configuration and assuming (MgSi)- ordering, a new Rietveld refinement of the powder XRD pattern of the 3.65 Å phase was performed, which resulted in excellent refinement statistics and successful assignment of X-ray reflections that were unassigned in former structural models with orthorhombic symmetry. A configuration with ordered Mg and Si at the octahedral positions causes a small monoclinic distortion of the network of strongly tilted octahedra and thus leads to space group P2 1 . The structural refinement yields the following unit-cell parameters: a = 5.1131(3), b = 5.1898(3), c = 7.3303(4) Å, β = 90.03(1)°, V = 194.52(2) Å 3 , space group: P2 1 , Z = 2, ρ = 2.637 g/cm 3 . The structure of the 3.65 Å phase can be considered as a modified A-site defective perovskite with a unique network of corner-sharing alternating Mg(OH) 6 and Si(OH) 6 octahedra and is probably related to the structure of stottite group minerals. Low-T IR spectroscopy confirms the presence of 6 different H-positions in the proposed structure. Measured IR-spectra and computed spectra compare favorably, which further supports the computed structure as the correct model for the 3.65 Å phase.

Petrological and geochemical variations within plutons reflect their magmatic and emplacement histories. Here we present new magnetic susceptibility (K m ) data on the ~163 Ma Barcroft granodiorite pluton in eastern California, which is exceptionally well exposed, especially in the vertical dimension. The Barcroft pluton offers exposures over a total of 2560 m of elevation and is an appropriate target to investigate variations of magnetic susceptibility. In ferromagnetic plutonic rocks, K m reflects mainly the abundance of magnetite, whereas in paramagnetic plutonic rocks it reflects primarily the abundance of mafic silicates. Magnetic susceptibility is also determined by magmatic processes such as crystal fractionation and by intensive parameters such as oxygen fugacity. Other magmatic processes, including magma replenishment, hybridization, and host-rock assimilation, may also influence K m variations. A first data set is based on 622 core samples that were measured in the laboratory. Our second data set comes from 1960 field measurements collected at 196 stations between ~1600 and 4000 m elevation. Detailed surveys were performed at the outcrop scale to evaluate the impact of the ~100 Ma McAfee Creek intrusion on the Barcroft background magnetic susceptibility. The combined data sets display a broad positive correlation between K m and elevation. Pluton mineralogy also appears to vary with elevation but is more difficult to quantify. At the outcrop scale, small dikes of the McAfee Creek granite transect the pluton and are responsible for a decrease in Km of the host granodioritic rocks toward the dikes due to late-stage magmatic or hydrothermal alteration. A contour map of K m shows a high degree of correlation with local topographic features such as deep canyons. Magnetic susceptibility of the Barcroft mafic rocks varies at the outcrop scale as a result of presence of petrological heterogeneities. However, these small-scale variations are embedded in a broader magnetic susceptibility trend due primarily to elevation, which reflects petrologic stratification of the pluton. The late-magmatic and hydrothermal alterations described in previous studies do not affect the spatial distribution of magnetic susceptibility. We propose that vertical increase of K m was primarily caused by crystal fractionation or another magmatic differentiation mechanism rather than by an externally driven increase in oxygen fugacity toward the roof of the intrusion.

Thaumasite, Ca 3 Si(OH) 6 (CO 3 )(SO 4 )⋅12H 2 O, is recognized as a secondary-alteration mineral and indicator of sulfate attack in Portland cement in contact with sulfate-rich groundwater, especially in cold regions. The hydrogen positions in thaumasite have been determined from single-crystal neutron diffraction structure refinements at 300 and 22 K. No phase transitions occur within the temperature range investigated. The structure of thaumasite is largely held together by hydrogen bonding. The major structural units [CO 3 groups, SO 4 tetrahedra, Si(OH) 6 octahedra, and Ca(OH) 4 (H 2 O) 4 polyhedra] are interconnected via 10 distinct hydrogen bonds. Analysis of the difference-Fourier maps of the nuclear density reveals the positions of all 10 hydrogen atoms in the structure, and the hydrogen bonding becomes shorter (stronger) upon decreasing temperature to 22 K. The SO 4 tetrahedron expands upon decreasing temperature (i.e., negative thermal expansion at the molecular level), driven by shortening of the hydrogen bonding between [Ca 3 Si(OH) 6 (H 2 O) 12 ] 4+ columns. Polarized Raman spectra of thaumasite show that the ν 1 symmetric stretching modes of Si(OH) 6 , SO 4 , and CO 3 occur at 658, 983, and 1066 cm -1 , respectively. In addition, the out-of-plane bending mode (ν 2 ) and asymmetric stretching mode (ν 3 ) of the carbonate group are tentatively assigned to bands at 887 and 1400 cm -1 , respectively. Bands at 418 and 455 cm -1 (and possibly at 477 cm -1 ) are attributed to the symmetric bending modes (ν 2 ) of the sulfate group, and we observe a possible asymmetric stretching mode (ν 3 ) of SO 4 at 1090-1100 cm -1 . Splitting of some sulfate and carbonate vibrational modes may occur due to hydrogen bonding on all the oxygen sites. At 1685-1710 cm -1 we observe the H 2 O bending modes (H-O-H), and from 2900-3600 cm -1 there are 13 distinct bands associated with bending overtones and the O-H stretching vibrations corresponding to H-positions determined in the neutron diffraction study. The effect of the low-temperature stability of thaumasite on the pronounced “thaumasite sulfate attack” of Portland cements observed in cold regions is discussed.

Single-crystal X-ray structure refinements have been carried out on natural Fe-bearing orthoenstatite (OEN) at pressures up to 14.53 GPa. We report a new high-pressure phase transition from OEN to a monoclinic phase (HPCEN2) with space group P2 1 /c, with a density change of ~1.9(3)%. The HPCEN2 phase is crystallographically different from low-pressure clinoenstatite (LPCEN), which also has P2 1 /c symmetry. Upon release of pressure HPCEN2 reverts to OEN, and the transition pressure is bracketed between 9.96 and 14.26 GPa at room temperature. We find no evidence for a C2/c phase at high pressure. The lattice constants for the new phase at 14.26 GPa are a = 17.87(2), b = 8.526(9), c = 4.9485(10) Å, β = 92.88(4)° [ρ = 3.658(9) g/cm 3 ]. Refinement of the new structure indicates rotation of tetrahedral chain as the key characteristic of this transition. This experiment points to the possibility of OEN and HPCEN2 as the stable phases in Earth’s upper mantle.

Aluminum incorporation into TiO 2 has been studied in the TiO 2 -Al 2 O 3 system as a function of pressure at temperatures of 900 and 1300 °C using commercial Al 2 TiO 5 nanopowder as starting material. A new orthorhombic TiO 2 polymorph with the CaCl 2 structure has been observed in the recovered samples synthesized from 4.5 to 7 GPa and 900 °C and from 2.5 to 7 GPa at 1300 °C. The phase transition to the α-PbO 2 type TiO 2 phase takes place between 7 and 10 GPa at both temperatures. Two mechanisms of Al incorporation in TiO 2 rutile have been observed in the recovered samples. The substitution of Ti 4+ by Al 3+ on normal octahedral sites is dominant at lower pressures. High pressure induces the incorporation of Al 3+ into octahedral interstices of the rutile structure, which is responsible for an orthorhombic distortion of the TiO 2 rutile structure and gives rise to a (110) twinned CaCl 2 type structure. This phase is probably a result of temperature quench at high pressure. Aluminum solubility in TiO 2 increases with increasing pressure. TiO 2 is able to accommodate up to 9.8 wt% Al 2 O 3 at 7 GPa and 1300 °C. Temperature has a large effect on the aluminum incorporation in TiO 2 , especially at higher pressures. High pressure has a strong effect on both the chemistry and the microstructure of Al-doped TiO 2 . Enhanced aluminum concentration in TiO 2 rutile as well as TiO 2 grains with a microstructure consisting of twins are a clear indication of high-pressure conditions.

The Xitieshan terrane is one of four metamorphic terranes in the North Qaidam metamorphic belt, which is an early Paleozoic high-pressure to ultrahigh-pressure (HP-UHP) metamorphic belt in NW China. However, conclusive evidence and precise timing of UHP metamorphism in the Xitieshan terrane have not been well documented. In this study, we report an integrated study of zircon grains from an amphibolite in the Xitieshan terrane in terms of mineral inclusions, trace elements, and U-Pb age systematics. The first record of coesite is reported as an inclusion in a metamorphic zircon, which provides unambiguous evidence for the UHP metamorphism of the Xitieshan terrane. The metamorphic zircon domains have weak or no zoning, very low Th/U ratios, insignificant Eu anomalies, and flat HREE patterns. Zircon grains contain inclusions of garnet, omphacite and rutile, in addition to the coesite inclusion. These inclusions indicate that the metamorphic zircon grains formed under UHP metamorphic conditions. The metamorphic zircon grains were dated by the SIMS and LA-ICPMS methods and yield weighted mean ages of 432 ± 14 and 441 ± 9 Ma, respectively. Combined with previous results, the HP-UHP metamorphism of the Xitieshan terrane may have lasted 460-440 Ma with the peak UHP metamorphism at 441 ± 9 Ma. A compilation of the reported geochronological data reveals that all four terranes of the North Qaidam metamorphic belt might have experienced coeval UHP metamorphism during the early Paleozoic (420-450 Ma), and thus may have suffered a coherent subduction, UHP metamorphism, and exhumation cycle.

Lead-doped zircon crystals, which were synthesized under three different conditions (Watson et al. 1997): dry at 1430 °C at atmospheric pressure without P 2 O 5 ; wet at 900 °C at 1.5 GPa in the presence of P 2 O 5 ; and wet at 800 °C at 1.0 GPa without P 2 O 5 , have been investigated to understand the mechanisms of Pb incorporation into zircon at the sub-micrometer scale, using various electron microscopy techniques including high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Four different mechanisms in which Pb may be incorporated into zircon have been identified. In the P-free synthesis, Pb-oxide hydrate particles, 50-200 nm in size, are embedded in zircon. Each Pb-particle is associated with a single vesicle, ~50 nm. Lead in the zircon structure is possibly incorporated under the detection limit value of energy-dispersive X-ray analysis (EDX) by means of: (1) Zr 4+ = Pb 2+ + 2H + at less than ~0.1 wt%. In the system with P, Pb-phases occur in three different forms: Pb-rich domains concentrated along cleavage planes or grain boundaries without any evident crystal form; numerous Pb-phosphate particles, as large as 100 nm, embedded heterogeneously in the zircon crystal; and homogeneous distribution of Pb in the zircon structure at less than ~1 wt% as determined by EDX. These results suggest that charge balance is maintained by the xenotime-type coupled substitution: (2) Zr 4+ + 2Si 4+ = Pb 2+ + 2P 5+ with a possible minor contribution from mechanism 1. The apparent solubility limit of Pb, <1 wt%, is constrained mainly by the xenotime-type coupled substitution mechanism, which is probably due to increasing strain at higher Pb-concentrations. The presence of Pb 2+ in natural zircon is consistent with the low-level Pb allowed by substitution mechanism 2, with only a minor contribution from substitution mechanism 1, the latter of which causes distortion in the local structure.

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.

Measurements of volatile abundances in igneous apatites can provide information regarding the abundances and evolution of volatiles in magmas, with applications to terrestrial volcanism and planetary evolution. But measurement of volatiles in apatite (especially H) is challenging, and the most commonly utilized method- the electron probe-is often inadequate in precision and accuracy. Secondary ion mass spectrometry (SIMS) measurements can produce accurate and precise measurements of H and other volatiles in many materials including apatite. SIMS standardization generally makes use of empirical linear transfer functions that relate measured ion ratios (i.e., of an element of interest normalized to a reference element) to independently known concentrations or abundance ratios. However, this approach is often limited by the lack of compositionally diverse, well-characterized, homogeneous standards. In general, SIMS calibrations are developed for minor and trace elements, and any two calibrations are treated as independent of one another. However, in crystalline materials, additional stoichiometric constraints may apply. In the case of apatite, the sum of concentrations of abundant volatile elements (H, Cl, and F) must closely approach 100% occupancy of their collective structural site (recognizing the possibility of trace substitutions or vacancies and other elemental species). Here we propose and document the efficacy of a method for standardizing SIMS analyses of abundant volatiles in apatites that takes advantage of this stoichiometric constraint. The principle advantage of this method is that it requires no independently known homogeneous reference standards, and under specific conditions it is effectively self-standardizing. We define a system of independent linear equations relating measured ion ratios (OH - /P - , Cl - /P - , F - /P - ) and unknown calibration slopes (i.e., the empirical transfer functions described above). Given sufficient range in the concentrations of the different elements among apatites measured in a single analytical session, solving this system of equations allows for the calibration slope for each element to be determined without standards, using only blank-corrected ion ratios. In the case that a data set of this kind lacks sufficient range in measured compositions of one or more of the relevant ion ratios, one can employ measurements of additional apatites of a variety of compositions to increase the statistical range and make the inversion more accurate and precise. These additional non-standard apatites need only be wide-ranging in composition. They need not be homogenous nor have known H, F, or Cl concentrations. Tests utilizing synthetic data and data generated in the laboratory indicate that this method should yield satisfactory results for a wide range of compositions provided apatites have “halogen” sites dominated by OH, F, and Cl. The inversion method is able to reproduce conventional calibration curves to within <2.5%, a level of accuracy comparable to-or even better than-the uncertainty of the conventional calibration, and one that includes both error in the inversion method as well as any true error in the independently determined values of the standards. Uncertainties in the inversion calibrations range from 0.1-1.7% (2σ), typically an order of magnitude smaller than the uncertainties in conventional calibrations (~4-5% for H 2 O, 1-19% for F and Cl). However, potential systematic errors stem from the model assumption of 100% occupancy of this site by the measured elements. To a first order, each percent of deviation from 100% occupancy of this site by the measured elements results in a systematic error of 1% (relative) in all species. Carbonate-rich apatites appear to violate the assumptions of the model, and should not be used for calibration purposes-though their volatile contents can be accurately and precisely determined by an independent inversion calibration that omits the high-C data. Use of this method simplifies analysis of H, F, and Cl in apatites by SIMS, and may also be amenable to other stoichiometrically limited substitution groups, including P+As+S+Si in low-C apatite, and Zr+Hf+U+Th in non-metamict zircon.

High-resolution transects across amphiboles from the layered mafic-felsic Halfmoon Pluton suggest that amphibole compositions in the outermost ~40 μm of grains analyzed fluctuate in response to P-T-P H ₂ O conditions during the final stages of pluton emplacement and crystallization. Detailed transects (~10-15 μm point spacing) across 15 crystals from six representative rock types within the compositionally stratified Halfmoon Pluton revealed that in almost all crystals Al tot increased over at least the outer 40 μm, coupled with a decrease in temperature and Ti-content. P-T estimates for Halfmoon Pluton amphiboles using the Al-in-hornblende geobarometer and the hornblende-plagioclase geothermometer are consistent with upper crustal emplacement depths. Amphibole Al tot within the core and midsections is controlled by the temperature-dependent edenite and Ti-Tschermak exchanges, and is interpreted to reflect the influence of repeated injections of hot mafic magma into the magma reservoir during crystallization. Increases in Al tot at all amphibole rims arise from a notable increase in VI Al and small increases in IV Al, indicating that, unlike the core and midsections that exhibit a strong temperature dependence, rim-Al tot is a function of the pressure-dependent Al-Tschermak exchange. Pressure estimates calculated for amphibole rims record a rim-ward increase in P equivalent to between 0.2 and 1.9 kbar, which corresponds to increases in depth of crystallization of 0.7 and 6.7 km, respectively. This observed increase in pressure at the rims of all amphibole grains analyzed is not consistent with increasing lithostatic pressure as a result of pluton burial, but to an increase in P H ₂ O in response to second boiling, a common process during late-stage crystallization of H 2 O-rich intermediate-silicic magmas. The presence of water escape structures throughout the pluton, and the predominance of biotite-rich mafic enclaves that commonly contain amygdules toward the inferred top of the magmatic “stratigraphy” provide further evidence to support the process of second boiling. These results indicate that caution is required in choosing suitable analytical sites when applying the Al-in-hornblende geobarometer, particularly within plutonic bodies where there is evidence for vesiculation and water escape structures, as the outer rim compositions of amphiboles may not be a function of lithostatic pressure alone.

Laser-flash analyses (LFA) of oriented sections of six natural plagioclase crystals provide thermal diffusivity (D) as function of temperature (to ~1300-1500 K) and composition (An 5-95 ). Plagioclase has low-thermal diffusivity; our measurements indicate that plagioclase is more insulating than other major igneous rock-forming minerals. Over much of the solid solution, room-temperature D ranges from 0.751 to 0.979 mm 2 /s along c, 0.722 to 0.919 mm 2 /s along b, and 0.632 to 0.868 mm 2 /s perpendicular to b and c. The directionally averaged D is 30-45% lower than D of Amelia albite. Thermal conductivities calculated using measured D values are almost the same for all samples with 18 ≤ An ≤ 65, ranging from 1.5 to 1.9 W/m/K and changing little with temperature. Increasing Al-Si disorder causes D to decrease with increased An content, although sample structure causes more ordered samples to have higher D than more disordered samples. Anorthite is a special case. Although ordered, the larger unit cell provides many lattice modes, leading to low diffusivity. Structure dictates whether D along the b-axis is greater or less than that along the c-axis, possibly because ordering in An-like domains increases D more along c relative to b. Inflections in 1/D(T) are connected with lattice distortion during heating, and occur near temperatures expected for phase transitions; for example, the lattice stretch occurring at the temperature of the transition to C1̅ structure lowers diffusivity. Likewise, lattice distortion during heating decreases D in albite along c but has little impact on D in the other directions. The anharmonic lattice effects that dictate both thermal expansivity and D are masked by effects of disorder; the latter plays a major role in heat transport in plagioclase.

Empirical potential molecular dynamics (EPMD) simulations of 1-bar eutectic composition liquid in the system CaAl 2 Si 2 O 8 -CaMgSi 2 O 6 have been conducted using the interatomic pair-potential of Matsui (1998). Simulations using ~10 000 atoms over a wide range of conditions (r: 2200-5000 kg/m3; T: 1600-5500 K; P: 0-170 GPa) were used to derive an equation of state, determine self-diffusivities for all atoms, calculate melt viscosity, and investigate melt structures by coordination statistics. EOS results compare well to laboratory shock wave data up to ~25 GPa, diverging at higher pressure. Based on simulations of the end-member compositions of the join using the same potential, non-ideality in the volume of mixing at pressures below 10 GPa disappears at higher pressures. Ideal volume mixing at elevated pressure is consistent with inferences from laboratory shock wave studies of liquids in this system. The non-ideal volume of mixing at low pressure is directly correlated to structural differences between the end-member liquids and the mixing of cation-anion coordination polyhedra of differing volume. Self-diffusivities show reasonable agreement with laboratory values, with activation energies and activation volumes in the range 90-100 kJ/mol and 1-3 cm 3 /mol, respectively. Shear viscosities at 3500 K span from 1.8 × 10 -3 Pa·s at low P to ~4.4 × 10 -3 Pa·s at ~14 GPa.

A high-pressure single-crystal X-ray diffraction (XRD) study has been carried out on two natural zoisite samples Ca 2 Al 3-x FexSi 3 O 12 OH, one Fe-free (x = 0) and one Fe-rich (x = 0.12). The unit-cell parameters were determined for the Fe-free sample at 18 different pressures up to 7.76 GPa and for the Fe-rich sample at 13 different pressures up to 7.63 GPa. The P(V) data for both of the samples were fitted by a third-order Birch-Murnaghan equation of state (BM3 EoS). The equation of state coefficients are: V 0 = 903.39(5) Å 3 , K T0 = 122.1(7) GPa, and K 0 ′ = 6.8(2) for the Fe-free sample and V 0 = 906.95(5) Å 3 , K T0 = 119.1(7) GPa, and K 0 ′ = 7.3(2) for the Fe-rich sample. This shows that the addition of Fe in to the crystal structure of zoisite leads to a slight softening of the structure. Both compositions exhibit axial compressibilities β c > β a >> β b , with the compressibilities of the a and b axes of the two samples being indistinguishable. The softening of the bulk modulus of zoisite with Fe content follows from softening of the c-axis of the structure. A high-pressure structural study of the Fe-free sample showed that the main compression mechanisms in the structure are the compression of soft inter-octahedral distance along [001] and soft intra-octahedral distances along [010] directions, while along [100] the main compression occurs because of the compression of stiff intra-octahedral distances. The substitution of Fe on to the M3 octahedral site of the structure leads to an increase of the intra-octahedral distance of the M3 that triggers the rotation of M12 and therefore leads to the softening of the M12 inter-octahedral distances that accounts for the softening of the c-axis of the structure.

The structure of antigorite (m = 17) has been studied by density functional theory from 0 to 30 GPa. The fourth-order Birch-Murnaghan equation of state fit of the thermally corrected LDA results yields an equilibrium volume (V 0 = 2853.13 Å 3 ), bulk modulus (K 0 = 64.6 GPa), and its pressure derivative (K 0 ′ = 6.94) in good agreement with experimental results. Two changes in compression mechanism occur at 6.1 and 20.5 GPa, individuating three pressure ranges: (1) in the low-pressure range, the antigorite wave flattens and the interlayer thickness decreases rapidly; (2) in the intermediate-pressure range, in-plane rotations of tetrahedra (ditrigonalization) and then wave-bending become the dominant compression mechanisms; (3) in the extreme-pressure range, the mechanism of wave-bending becomes prevalent. The first change reveals the origin of softening found experimentally near 6 GPa: the change in compression mechanism occurs after the minimal mismatch between T- and O-sheets is achieved and is accompanied by an apparent symmetry breaking: accidental degeneracies of structural parameters between short and long halfwaves are lifted, including T-sheet thicknesses and Si-O bond lengths. In the extreme-pressure range, Si-O-Si angles decrease below 122°, which may be the origin of amorphization found experimentally at similar pressure.

The new mineral reynoldsite, Pb 2 Mn 4+ 2 O 5 (CrO 4 ), occurs at the Blue Bell claims, near Baker, San Bernardino County, California, U.S.A., and at the Red Lead mine, Dundas, Tasmania, Australia. At the Blue Bell claims, reynoldsite occurs in subparallel growths and divergent sprays of thin prisms with a square cross section. At the Red Lead mine, it occurs as thin rectangular blades. At both occurrences, crystals are small (≤0.2 mm), and ubiquitously and multiply twinned. At both deposits, reynoldsite formed as a secondary mineral derived from the weathering of primary minerals including oxides and sulfides in the presence of acidic groundwater. Reynoldsite is dark orange-brown to black in color and has a dark orange-brown streak. Its luster is subadamantine and its Mohs hardness is about 4½. The mineral is brittle with irregular to splintery fracture and a poorly developed {001} cleavage. The calculated density is 6.574 g/cm 3 (Red Lead mine). The very high indices of refraction and dark color permitted only partial determination of the transmitted light optical properties. Electron microprobe analyses of Blue Bell and Red Lead reynoldsite provided the empirical formulas (based on nine O atoms): Pb 1.97 Mn 2.01 O 5 (Cr 1.01 O 4 ) and (Pb 2.07 Sr 0.04 ) Σ2.11 Mn 2.15 O 5 (Cr 0.87 O 4 ), respectively. The strongest powder X-ray diffraction lines for Red Lead reynoldsite are [d(hkl)I]: 3.427(02̅1,110)52, 3.254(021,11̅2,12̅1)85, 3.052(1̅ 1̅2,111,02̅2,1̅03)100, 2.923(013,1̅22)40, 2.5015(004,2̅11,1̅30)47, 1.9818(01̅5,1̅05,202,23̅1)42, 1.7694(1̅15,13̅4,203,1̅42,1̅ 3̅3)36, and 1.6368(2̅2̅3,04̅3,221,124,22̅4)36. Reynoldsite is triclinic with space group P1 and unit-cell parameters: a = 5.0278(7), b = 7.5865(11), c = 10.2808(15) Å, α = 91.968(12), β = 99.405(12), γ = 109.159(10)°, V = 363.81(9) Å 3 , and Z = 2 (for a Red Lead mine crystal). The crystal structure of reynoldsite (R 1 = 10.2% for 902 reflections with Fo > 4σF for a Red Lead crystal) contains close-packed layers of edge-sharing Mn 4+ O 6 octahedra parallel to {001}. These layers are composed of edge-sharing double chains of octahedra extending along [100], which in turn are linked to one another by sharing edges in the [010] direction. The thick interlayer region contains Pb 2+ cations and CrO 4 tetrahedra. The 6s 2 lone-electron pair of the Pb 2+ is stereochemically active, resulting in a one-sided Pb-O coordination arrangement. The structure bears strong similarities to those of the phyllomanganates, such as chalcophanite and birnessite.

A new cubic perovskite polymorph of PbGeO 3 (Phase II) was synthesized by laser heating in the diamond-anvil cell (DAC) at the pressure of 36 GPa. Fitting the Birch-Murnaghan equation of state against its observed P-V data yields a bulk modulus K 0 of 196(6) GPa and the volume V 0 of 56.70(13) Å 3 when K 0 ′ is assumed being 4. After the pressure is released, the PbGeO 3 Phase II changes gradually into an amorphous phase, which contains mainly fourfold-coordinated germanium. It indicates that the PbGeO 3 Phase II with a GeO 6 octahedron framework transforms to a GeO 4 tetrahedron network during the amorphization. The existence of PbGeO 3 cubic perovskite Phase II at high pressures indicates that the polarized character of the Pb 2+ ion induced by its 6s 2 lone pair electrons would be totally reduced in the environment of major silicate perovskites inside the lower mantle, and thus the Pb atom would substitute the Ca atom to enter the CaSiO 3 perovskite.

Manganoquadratite, ideally AgMnAsS3, is a new mineral from the Uchucchacua polymetallic deposit, Oyon district, Catajambo, Lima Department, Peru. It occurs as dark gray, anhedral to subhedral grains up 0.5 mm across, closely associated with alabandite, Mn-rich calcite, Mn-rich sphalerite, proustite, pyrite, pyrrhotite, tennantite, argentotennantite, stannite, and other unnamed minerals of the system Pb-Ag-Sb-Mn-As-S. Manganoquadratite is opaque with a metallic luster and possesses a reddish-brown streak. It is brittle, the Vickers microhardness (VHN 10 ) is 81 kg/mm 2 (range 75-96) (corresponding Mohs hardness of 2-2½). The calculated density is 4.680 g/cm 3 (on the basis of the empirical formula). In plane-polarized reflected light, manganoquadratite is moderately bireflectant and very weakly pleochroic from dark gray to a blue gray. Internal reflections are absent. Between crossed polars, the mineral is anisotropic, without characteristic rotation tints. Reflectance percentages (R min and R max ) for the four standard COM wavelengths are 29.5, 31.8 (471.1 nm), 28.1, 30.5 (548.3 nm), 27.3, 29.3 (586.6 nm), and 26.0, 28.2 (652.3 nm), respectively. Manganoquadratite is tetragonal, space group P4 3 22, with unit-cell parameters: a = 5.4496(5), c = 32.949(1) Å, V = 978.5(1) Å 3 , c:a = 6.046, Z = 8. The structure, refined to R 1 = 0.0863 for 907 reflections with F o > 4σ(F o ), consists of a stacking along [001] of alabandite-like Mn 2 S 2 layers connected to each to other by a couple of AgAsS 2 sheets where As 3+ forms typical AsS 3 groups, whereas Ag + cations are fivefold coordinated. The six strongest lines in the observed X-ray powder-diffraction pattern [d in Å (I/I 0 ) (hkl)] are: 3.14 (60) (116), 2.739 (50) (0 0 12), 2.710 (100) (200), 1.927(70) (2 0 12 + 220), 1.645 (25) (3 0 16), and 1.573 (20) (2 2 12). Electron microprobe analyses gave the chemical formula (on the basis of six atoms) (Ag 0.95 Cu 0.05 ) Σ=1.00 (Mn 0.96 Pb 0.04 ) Σ=1.00 (As 0.87 Sb 0.14 ) Σ=1.01 S 2.99 , leading to the simplified formula AgMnAsS 3 . The name was chosen to indicate the close analogy of the formula and unit-cell dimensions with quadratite, Ag(Cd,Pb)(As,Sb)S 3 . The new mineral and mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA 2011-008.

Leucostaurite is a new nanoporous lead borate discovered in samples from the Mina Asunción, Sierra Gorda, Atacama Desert, Chile, preserved since 1912 in the collections of the Natural History Museum of Bern, Switzerland. Leucostaurite formed via the oxidation of base-metal ores in the presence of B-rich brines. The mineral name is derived from the Greek “leukos” (white) and “stauros” (cross), and alludes to the white or transparent, colorless cruciform twinned crystals. Leucostaurite forms thin-tabular {010}, striated //[100], interpenetrated twinned crystals, and sheaf-like aggregates up to 0.8 mm on a paralaurionite and boleite matrix. The streak is white and the luster adamantine. Leucostaurite shows a weak, light-yellow fluorescence under short-wavelength UV but no fluorescence under long-wavelength UV light. The mineral is brittle, Mohs hardness ~4, with perfect cleavage parallel to {010} and good cleavage parallel to {100}; calculated density is 5.071 g/cm 3 . Leucostaurite is biaxial, 2V (meas) ~30°, dispersion: r > v, strong. Optic sign and refractive indices could not be measured, but the average index calculated from the Gladstone-Dale relationship is 1.849. The empirical formula based on Pb + Sr + Ca = 2 atoms per formula unit (apfu), 1 H apfu and B + Si = 5 apfu, is (Pb 1.967 Sr 0.026 Ca 0.007 ) Σ2.000 (B 4.983 Si 0.017 ) Σ5.000 (Cl 1.073 I 0.004 ) Σ1.077 O 8.971 ⋅0.5H 2 O, which simplifies to Pb2[B 5 O 9 ]Cl⋅0.5H 2 O. The boron content was measured on two crystal fragments using proton-induced γ-ray emission spectroscopy; the analytical value [26.7(3) wt% B 2 O 3 ] is within error of the stoichiometric value of 26.5 wt% B 2 O 3 . Leucostaurite is orthorhombic, space group Pnn2, a = 11.376(2), b = 11.505(2), c = 6.5558(7) Å, V = 858.1(2) Å 3 , Z = 4. The seven strongest lines measured in the X-ray powder diffraction pattern are [d in Å/I rel in %]: 4.04/100; 2.84/100; 5.71/80; 2.019/70; 3.29/40; 2.55/40; 1.877/40. The crystal structure of leucostaurite (R 1 = 6.2%) contains a hilgardite-type three-dimensional [B 5 O 9 ] 3 - framework. Leucostaurite is the first mineral of the hilgardite group with orthorhombic (Pnn2) symmetry. However, several borates synthesized for their non-linear optical properties are structurally and chemically closely related to leucostaurite. For example in Na 0.5 Pb 2 (B 5 O 9 )Cl(OH) 0.5 , one type of channels contains Cl − ions, the other contains OH − , Cl − , and Na + ions; in leucostaurite these channels are occupied by Cl − ions, and Cl − ions + H 2 O groups, respectively.

The structure determination of synthetic LiAlGe 2 O 6 [Z = 4, space group P2 1 /n, a = 9.8892(5), b = 8.3929(5), c = 5.3995(3) Å, b = 110.646(3)°], is a new pyroxene structure-type, and represents the first structural example of P2 1 /n pyroxene symmetry. The crystal structure of the Ge-analog phase of spodumene was solved from single-crystal X-ray diffraction data by classical Patterson methods with a subsequent structure refinement converging to R 1 = 0.0169. The new P2 1 /n pyroxene polymorph was found to consist of a single S-type rotated tetrahedral chain type, which is-similar to ordered P2 1 /n omphacite-composed of alternating Ge1O 4 and Ge2O 4 tetrahedra located at two distinct sites within a single chain. This (Ge1Ge2O 6 ) n chain is S-rotated, strongly bent (O4-O2-O4 = 154.8°) compared to that of the C2/c spodumene structure, and assumes angles comparable to those of the two chains of spodumene at 3.3 GPa within the P2 1 /c symmetry. As a consequence of the interplay between the M1 and M2 sites, the new polymorph reveals a larger angular distortion for the AlO 6 octahedra, and the Li coordination is reduced from sixfold to fivefold coordination. This establishes Li [5] Al [6] (Ge1 [4] Ge2 [4] O 6 ) as the corresponding crystallochemical formula for the new P2 1 /n representative within the monoclinic (clino)pyroxene family.

Panguite (IMA 2010-057), (Ti 4+ ,Sc,Al,Mg,Zr,Ca) 1.8 O 3 , is a new titania, occurring as fine-grained crystals with Ti-rich davisite in an ultra-refractory inclusion within an amoeboid olivine inclusion from the Allende CV3 carbonaceous chondrite. The phase was characterized by SEM, EBSD, synchrotron micro-diffraction, micro-Raman spectroscopy, and EPMA. The mean chemical composition of the type panguite is (wt%) TiO 2 47.97, ZrO 2 14.61, Sc 2 O 3 10.67, Al 2 O 3 7.58, MgO 5.54, Y 2 O 3 5.38, CaO 3.34, SiO 2 1.89, FeO 1.81, V 2 O 3 0.95, Cr 2 O 3 0.54, HfO 2 0.28, sum 100.56 with a corresponding empirical formula calculated on the basis of 3 O atoms of [(Ti 0.79 Zr 0.16 Si 0.04 ) 4+ Σ0.99 (Sc 0.20 Al 0.20 Y 0.06 V 0.02 Cr 0.01 ) 3+ Σ0.49 (Mg 0.18 Ca 0.08 Fe 0.03 ) 2+ Σ0.29 ] Σ1.77 O 3 . Synchrotron micro-Laue diffraction (i.e., an energy scan by a high-flux X-ray monochromatic beam and white beam diffraction) on one type domain at sub-micrometer resolution revealed that panguite is an orthorhombic mineral in space group Pbca. The structure is a subgroup of the Ia3 bixbyite-type. The cell parameters are a = 9.781(1), b = 9.778(2), and c = 9.815(1) Å, yielding V = 938.7(1) Å 3 , Z = 16, and a calculated density of 3.746 g/cm 3 . Panguite is not only a new mineral, but also a new titania material, likely formed by condensation. It is one of the oldest minerals in the solar system.

Buseckite (IMA 2011-070), (Fe,Zn,Mn)S, is the Fe-dominant analog of wurtzite, a new member of the wurtzite group discovered in Zakłodzie, and an ungrouped enstatite-rich achondrite. The type material occurs as single-crystal grains (4-20 μm in size) in contact with two or more of enstatite, plagioclase, troilite, tridymite, quartz, and sinoite. Low-Ni iron, martensitic iron, schreibersite, keilite, cristobalite, and graphite, which are also present in the type sample, are not observed to be in contact with buseckite. Buseckite is black under diffuse illumination and nearly opaque grayish brown in transmitted light. The mean chemical composition of buseckite, as determined by electron microprobe analysis of the type material, is (wt%) S 35.84, Fe 28.68, Zn 23.54, Mn 10.04, Mg 1.18, sum 99.28, leading to an empirical formula calculated on the basis of 2 atoms of (Fe 0.46 Zn 0.32 Mn 0.16 Mg 0.04 ) Σ0.99 S 1.01 . Electron backscatter diffraction patterns of buseckite are a good match to that of synthetic (Zn 0.55 8Fe 0.442 )S with the P6 3 mc structure, showing a = 3.8357, c = 6.3002 Å, V = 80.27 Å 3 , and Z = 2. Buseckite is likely derived from the breakdown of high-temperature pyrrhotite to form troilite and buseckite following the solidification of sulfide-rich liquids produced during impact melting of an enstatite-rich rock.

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·CH 0.71 H ₂ O 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.

In the system Na 2 O-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 , the behavior of Ti-containing structural complexes has been determined in H 2 O-saturated silicate melts and in coexisting silicate-saturated aqueous fluids as well as in silicate-rich supercritical fluids to 900 °C and 2225 MPa. Titanium speciation in aqueous fluids in the system TiO 2 -H 2 O was also characterized. All measurements were carried out in situ at the desired temperature and pressure using confocal microRaman and microFTIR spectroscopy. The experiments were carried out in an Ir-gasketed hydrothermal diamond-anvil cell (HDAC) with K-type thermocouples for temperature measurement and the Raman shift of 13 C synthetic diamond to monitor pressure. In the system Na 2 O-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 , four or five O atoms surround a central Ti 4+ cation in melts, fluids, and supercritical fluids. In this environment, the titanium solubility mechanism is the same for melt, fluid, and single-phase liquid and can be described with the equation, 4Q 1 Si (Na)4H 2 O + TiO 2 ↔ 4Q 0 Si (HNa) + Q 0 Ti (Na). Subscripts denote the nature of central cation, superscripts, the number of bridging oxygens, and the symbol(s) in parentheses the type(s) of cation(s) that form bonds with nonbridging oxygen in the Q-species. The ΔH of this reaction is on the order several tens of kJ/mol. Because of the structural similarity of Ti-complexes in hydrous silicate melts and silicate-rich aqueous fluid, the fluid/melt partition coefficients also resemble one another. The partition coefficient is between 0.1 and 1 and is positively correlated with temperature, pressure, and Al/(Al+Si) of the silicate melt or aqueous fluid. In the chemically simpler system TiO 2 -H 2 O, titanium in aqueous fluid occupies the central position in oxygen polyhedra surrounded by approximately six O atoms. Here, the enthalpy change for the solution reaction is between 50 and 60 kJ/mol absent pressure corrections for volume differences between TiO 2 in rutile and in aqueous solution. The Ti concentration in aqueous fluid is on the order of fractions to a few tens of parts per million. This solubility is orders of magnitude lower than in Ti solubility in silicate-saturated aqueous fluid in the system Na 2 O-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 at similar pressures and temperatures. In natural environments such as high-grade metamorphic terranes and subduction zones, aqueous fluids are silicate-saturated. Such fluids are more efficient solvent refractory oxides, perhaps by 2-3 orders of magnitude for an oxide such as TiO 2 , than inferred from their solubility in pure H 2 O fluids.

The low-temperature heat capacity behavior of synthetic katoite, Ca 3 Al 2 H 12 O 12 , was investigated for the first time using microcalorimetry. The sample was synthesized hydrothermally in Au capsules at 250 °C and 3 kb water pressure. X-ray powder measurements show that about 98-99% katoite was obtained. Heat capacities were measured with a commercially designed relaxation calorimeter between 5 and 300 K on a milligram-sized sample and around ambient temperatures with a differential scanning calorimeter. The heat capacity data are well behaved at T < 300 K and show a monotonic decrease in magnitude with decreasing temperature. There is no evidence for any phase transition. A standard third-law entropy value of S° = 421.7 ± 1.6 J/(mol·K) was calculated. Published experimentally based S° values for katoite are slightly lower than this value. Estimations of S° based on empirical corresponding state schemes give S° values that are much too low. This is ultimately attributed to an inability to account for the vibration behavior of the OH groups in katoite that have very weak or no H-bonding. Using this new calorimetric-based S° value and published standard enthalpy of formation data for katoite, a calorimetric-based Gibbs free energy of formation at 298 K can be obtained as ΔG° f = -5021.2 ± 16.5 kJ/mol.

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.