Volume 91, Issue 11-12

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

A rare lithology consisting of garnet-tourmaline-sillimanite-biotite-ilmenite-quartz has been found within the granulite-facies region of south-central Massachusetts. The homogeneous, Ti-rich oxy-dravitic tourmaline, X Mg = Mg/(Mg + Fe) = 0.72.0.77, falls into the alkali group, and is similar in composition to lower grade tourmaline found in corresponding metapelitic rocks in Maine. Charge-balancing calculations and binary diagrams suggest that, like biotite in the region, tourmaline has undergone deprotonation by means of the exchange vectors AlOR -1 (OH) -1 and TiO 2 R -1 (OH) -2 , where R represents Fe + Mg. The restriction of a concordant tourmaline-rich horizon in otherwise tourmaline-free rocks of this granulite-facies region suggests that either: (1) B, released during prograde fluid-absent dehydration reactions of muscovite and biotite, was locally available in a fluid phase or melt for later crystallization near the peak of granulite-facies metamorphism along pathways that provided a conduit for fluid migration; or (2) that this is simply a B-rich compositional horizon (tourmalinite) that survived anatexis and granulite-facies metamorphism and that records the incipient conditions of tourmaline breakdown and subsequent recrystallization near or post-peak metamorphism.

Generally, it has been assumed that signal intensity variations in back-scattered electron (BSE) images of minerals are mainly controlled by chemical heterogeneity. This is especially true for images of single crystals, where effects of different crystal orientations with respect to the incident beam on the observed BSE are excluded. In contrast, we show that local variations of the structural state within single-crystals (i.e., degree of lattice order or lattice imperfectness) may also have dramatic effects on the back-scattering of electrons. As an example, we present BSE images of single-crystals of natural zircon, ZrSiO 4 , whose intensity patterns are predominantly controlled by structural heterogeneity, whereas effects of chemical variations are mostly negligible. In the case of natural zircon, structural heterogeneity affecting the BSE patterns is predominantly due to heterogeneous accumulation of radiation damage. We attempt to explain our observations with lowered penetration and channeling and, thus, enhanced back-scattering of electrons in more radiation-damaged internal zones and microareas. Back-scattered electron contrast of natural zircon is, therefore, considered as a special case of electron channeling contrast. This phenomenon seems to have been generally underappreciated in the discussion of BSE images of radiation-damaged minerals thus far.

Thermal diffusivity (D) data from 12 oriented single crystals and seven polycrystalline samples of olivine group minerals were acquired with the laser-fl ash method at temperatures (T) of up to ~1500 °C. Samples included forsterite, Fe-Mg binary olivines, sinhalite, and chrysoberyl; specimens were characterized using infrared spectroscopy and electron microprobe analysis. Crystal orientation and chemistry both affect D. For our single crystals, D [100] > D [001] > D [010] at all temperatures. Thermal diffusivity decreases with increasing T and becomes virtually constant at high temperatures. At room temperature, D [001] of pure forsterite has the highest observed values, but substitution of a small amount of Co in forsterite (0.3 wt% CoO) lowers D by ~20%. Substitution of ~10% Fe for Mg in forsterite, as in typical mantle olivine, lowers D by ~50%. At room temperature, mantle olivine has D = 3.25, 1.66, and 2.59 mm 2 /s for the [100], [010], and [001] orientations, respectively. The values decrease to 0.93-0.87 mm 2 /s at 790-985 °C for [100], 0.54-51 mm 2 /s at 590-740 °C for [010] and 0.83-0.79 mm 2 /s at 740-890 °C for [001]. Two dunite samples have D of 0.55-0.56 mm 2 /s at 890-1080 °C, showing the effect of preferred orientation of grains dominated by [010]. Thermal diffusivity of polycrystalline samples is controlled by the large amounts of olivine present; minor phases offset the curves for D(T) from the value of the olivine mineral. Our laser-fl ash measurements isolate the phonon component of heat transfer from radiative transfer and show that the phonon contribution becomes nearly constant for the high temperatures expected in the mantle. The other microscopic component (diffusive radiative transfer) depends strongly on temperature and this temperature dependence likely exerts greater control on mantle convection.

To evaluate the stability, potential reactivity, and relaxation mechanisms on different uraninite surfaces, surface energy values were calculated and structural relaxation was determined for the (111), (110), and (100) crystallographic faces of uranium dioxide (UO 2 ) using quantum mechanical (density functional theory) and empirical potential computational methods. Quantum mechanical results are compared with empirical potential results, which use surface slab models with two different geometries, as well as various different empirical force fields. The strengths and weaknesses of the different approaches are evaluated, and surface stabilizing mechanisms such as relaxation, charge redistribution, and electronic stabilization are investigated. Quantum mechanical (q.m.) surface energy results are in agreement with the relative surface energy trends resulting from calculations using three different empirical potential sets for uranium and oxygen (two from Catlow 1977; one from Meis and Gale 1998), and with empirical force-field values published in the literature (Abramowski et al. 1999, 2001). The (111) surface consistently has the lowest surface energy (0.461 J/m 2 from q.m. calculations) and the smallest amount of surface relaxation, followed by the (110) surface (0.846 J/m 2 ; q.m.), and the (100) surface (1.194 J/m2; q.m.) (quantum mechanical surface energy values in parentheses are for surface slabs with a thickness of four UO 2 units). Differences exist, however, in the absolute values of surface energies calculated as a function of potential set used. Quantum mechanical values are consistently lower than values calculated using empirical potential methods. A fourth potential set is presented that is derived from fitting electrostatic and short-range repulsive parameters to experimental bulk properties and surface energies and relaxations from quantum mechanical calculations.

Cu K-edge X-ray absorption near edge structure (XANES) spectra were recorded from individual synthetic brine fluid inclusions as a function of temperature up to 500 °C. The inclusions serve as sample cells for high-temperature spectroscopic studies of aqueous Cu-Cl speciation. Cu + and Cu 2+ can both be identified from characteristic pre-edge features. Mixed oxidation states can be deconvoluted using linear combinations of Cu + and Cu 2+ spectra. This work illustrates how complex Cu XANES spectra can be interpreted successfully. Cu 2+ is the stable oxidation state in solution at room temperature and Cu + at high temperatures. The change in oxidation state with temperature was completely reversible. Cu + was found to occur exclusively as the linear species [CuCl 2 ] - in solutions containing KCl with Cu:Cl ratios up to 1:6. In the absence of K + , there is evidence for higher order coordination of Cu + , in particular the tetrahedral complex [CuCl 4 ] 3- . The importance of such complexes in natural ore-forming fluids is yet to be determined, but may explain the vapor-phase partitioning of Cu as a Cl complex from a Cl-rich brine.

Eight silicate unit vibrational modes were identified in a suite of PbO-SiO 2 glasses using micro-reflectance Fourier Transform infrared (μR-FTIR) spectra that were transformed using the Kramers- Kronig relation. The transformed FTIR spectra, in the 800-1200 cm -1 range, were deconvolved systematically into eight Voigt-shaped bands at centers that were predicted from the second derivative of the spectra. The area of the eight bands varied as a function of SiO 2 content, and these trends were combined with theoretical constraints to identify and assign the bands to seven provisional silicate units: SiO 4 4- (830 and 860 cm -1 ), Si 2 O 7 6- (900 cm -1 ), Si 6 O 18 12- (950 cm -1 ), Si 2 O 6 4- (980 cm -1 ), Si 4 O 11 6- (1010 cm -1 ), Si 2 O 5 2- (1050 cm -1 ), and SiO 2 (1100 cm -1 ). The provisional units were then grouped according to their NBO/T values: NBO/T = 4 (SiO 4 4- ), NBO/T = 3 (Si 2 O 7 6- ), NBO/T = 2 (Si 6 O 18 12- and Si 2 O 6 4- ), NBO/T = 1 (Si 4 O 11 6- and Si 2 O 5 2- ) and NBO/T = 0 (SiO 2 ). The derived quantities of each NBO/T unit compare favorably with nuclear magnetic resonance data for PbO-SiO2 glasses reported in the literature. This new approach for determining glass structure is advantageous because it may be performed on small Fe-bearing samples with minimal preparation, and analyses are rapid and relatively inexpensive.

A series of Ti-doped diopside samples synthesized by fluxgrowth methods under reducing conditions were investigated by microanalytical methods and optical absorption spectroscopy, to assess Ti incorporation mechanisms, valence states, and related electronic transitions. Chemical characterization show that Ti occurs both in the tri-and tetravalent states, with Ti 4+ preferentially ordered to the tetrahedral position whereas Ti 3+ is restricted to the M1 position. Charge-balance is maintained by incorporation of Na and minor B, stemming from the flux compound. Polarized optical absorption spectra reveal three major absorption features. Two relatively narrow bands centered at 18 500 and 15 700 cm -1 , mainly polarized in the crystallographic c-direction, display absorbance values that correlate with the calculated Ti 3+ (M1) concentration, and are assigned to spin-allowed d-d transitions in Ti 3+ . A broader band centered around 24 500 cm -1 displays absorbance values that are well correlated with the product of VI Ti 3+ and IV Ti 4+ . Based on the spectral characteristics of this band, including a strong polarization in the crystallographic b-direction, we assign this band to an M1 Ti 3+ - T Ti 4+ intervalence charge transfer process.

Hugoniot equation-of-state and release adiabat results are presented for magnesite to a pressure of ~140 GPa. A sharp change in the shock velocity and particle velocity relation suggests that a phase transition to a high-pressure phase occurs at 107±10 GPa. Decomposition of magnesite was observed by abrupt volume expansion during the pressure release from a pressure over the phase transition and by investigating post-shock magnesites recovered from hypervelocity impacts of mini-flyers performed using a laser-driven acceleration. Post-shock magnesites above 95 GPa contained MgO crystallites and the amount of MgO increased with increasing shock pressure.

We have used finite element modeling (FEM) to successfully reproduce elastic volume changes of synthetic fluid inclusions in quartz pressurized in a hydrothermal diamond-anvil cell (HDAC) at external pressures up to 250 MPa. At higher pressures, the synthetic inclusions are somewhat stiffer than would be predicted by linear elasticity due to the effect of pressure on the elastic moduli. The finite element models were created to reproduce the inclusion.s approximate shape, and crystallographic orientation within the host, which is elastically anisotropic. The models successfully predict changes in fluid inclusion volume measured using the HDAC, which gives us confidence that FEM may be used to predict the elastic behavior of inclusions in other situations.

Static lattice energy calculations (SLEC), based on empirical pair potentials have been performed for a set of 125 different structures with compositions between pyrope and grossular, and with different states of order of the exchangeable Mg and Ca cations. Total energies of a subset of these configurations have been calculated with a density functional electronic structure method (ab initio). The excess energies derived from ab initio and SLEC results agree well with each other. Excess free energies of the 125 structures have been calculated at 300 and 1000 K and at 0 and 3 GPa and cluster expanded in a basis set of 8 pair-interaction parameters. These ordering parameters have been used to constrain Monte Carlo simulations of temperature-dependent properties in the ranges of 300.1500 K and 0.3 GPa. The free energies of mixing have been calculated using the method of thermodynamic integration. The calculations predict the development of a significant short-range and long-range ordering at the intermediate 50/50 composition. The long-range ordered phase with I41 22 symmetry becomes stable below 600 K. Two miscibility gaps driven by the stability of the intermediate phase develop at both sides of the 50/50 composition. Activity-composition relations in the range of 600.1500 K and 0.3 GPa are described with high-order Redlich-Kister polynomials.

Recent studies of the structure of antigorite by Capitani and Mellini (2004, 2005) and by us (Dódony et al. 2002; Dódony and Buseck 2004a) produced contradictory results. The main point of contention is whether 4- and 8-membered rings of tetrahedra occur at the positions where the tetrahedra in the tetrahedral sheets reverse their orientation. We analyzed electron diffraction patterns and transmission electron microscopy images in the paper of Capitani and Mellini (2005) and found no evidence for 4- and 8-membered rings of tetrahedra. On the contrary, we show that their TEM data confirm our antigorite model (Dódony et al. 2002) for the m = 16 structure. The significance of this debate goes beyond the subtleties of the structure of antigorite and highlights ambiguities in interpretation of HRTEM images as well as problems that can arise during image processing.

The crystal chemistry of a natural topaz [with OH/(OH + F) < 0.5] was reinvestigated by means of laser ablation inductively coupled plasma mass spectroscopy, single-crystal X-ray diffraction (at 298 K) and neutron diffraction (at 298 and 10 K), and polarized infrared spectroscopy to define unambiguously the real symmetry of topaz, the location of the proton and its thermal displacement parameters at room and low temperatures, the hydrogen-bonding and the vibration modes (stretching and bending) of the OH dipole. X-ray and neutron structural refinements allow us to infer that the crystal structure of natural topaz with OH/(OH + F) < 0.5 can be described with the Pbnm space group. Violating reflections, found in the previous investigations and in this study, are likely due to Renninger effect (double diffraction phenomenon). The nuclear density Fourier map shows that the proton is located at Wyckoff 8d position and the refined coordinates are: x = 0.495(2), y = 0.252(1), z = 0.1629(7). The O-H bond lies on the (010)-plane and forms an angle of about 28.9° with the c-axis. Neutron structural refinements at 298 and 10 K show that the displacement ellipsoid of the proton is highly anisotropic. The H-bonding arrangement appears to be complex, with at least four potential H···O/F interactions (distances < 2.38 Å). The topological configuration of the O-H group described by the neutron structural refinements is confirmed by the infrared investigation: the OH stretching mode (at 3640 cm -1 ) has no component of vibration parallel to the b axis (i.e., the O-H direction is perpendicular to [010]). The OH bending mode (at 1161 cm -1 ) shows components along the three crystallographic axes, which appear to be more prominent along the a and b-axes. The possible distribution into the crystal structure of topaz of the minor/trace elements found (Na, Ca, Fe Cr, V, Ti, B), and the implied topological effects, is discussed.

Four colorless tourmalines of the liddicoatite-elbaite series from pegmatites from Anjanabonoina, Madagascar, have been characterized by crystal-structure determination and by chemical analyses. Optimized formulae range from X (Ca 0.57 Na 0.29 □ 0.14 ) Y (Al 1.41 Li 1.33 Mn 2+ 0.07 □ 0.19 ) Z Al 6 T (Si 5.86 B 0.14 )O 18 (BO 3 ) 3 V (OH) 3.00 W [F 0.76 (OH) 0.24 ] [a = 15.8322(3), c = 7.1034(3) Å] to X (Na 0.46 Ca 0.30 □ 0.24 ) Y (Al 1.82 Li 0.89 Fe 2+ 0.01 Mn 2+ 0.01 □ 0.27 ) Z Al 6 T (Si 5.56 B 0.44 )O 18 (BO 3 ) 3 V (OH) 3.00 W [(OH) 0.50 F 0.50 ] [a = 15.8095(9), c = 7.0941(8) Å] (R = 1.3.1.7%). There is a high negative correlation (r 2 = 0.984) between the <T-O> bond-lengths (~1.618.1.614 Å) and the amount of IV B (from the optimized formulae). Similar to the olenites (from Koralpe, Austria) the liddicoatite-elbaite samples show a positive correlation between the Al occupancy at the Y site and IV B (r 2 = 0.988). Short-range order configurations show that the presence of IV B is coupled with the occupancy of (Al 2 Li) and (Al 2 □) at the Y site. The structural formulae of the Al-rich tourmalines from Anjanabonoina, Madagascar, show ~ ⃞ 0.2 (vacancies) on the Y site. We believe that short-range order configurations with Y(Al 2 □) are responsible for these vacancies. Hence, an oft-used calculation of the Li content by difference on the Y site may be problematic for Al-rich tourmalines (olenite, elbaite, rossmanite). Fourier transform infrared (FTIR) spectra were recorded from the most IV B-rich tourmaline sample. The bands around 5195 and 5380 cm -1 can be assigned to H 2 O. Because these bands still could be observed in FTIR spectra at temperatures from -150 to +600 °C, it seems unlikely that they result from H 2 O in fluid inclusions. Interestingly, another FTIR spectrum from a dravite in which the X site is filled completely with Na, does not show bands at ~5200 and ~5400 cm -1 . Although not definitive, the resulting spectra are consistent with small amounts of H2O at the X site of the elbaite. The rare-earth element (REE) pattern of the B-rich elbaite (ΣREE: ~150 ppm) demonstrates that this sample is strongly enriched in LREEs compared to HREEs and exhibits a negative Eu anomaly. This sample shows the strongest enrichment of LREEs and a high La N /Yb N ratio of ~351, which seems to confirm an important role of the fractional crystallization process.

We show that bulk amorphous silica recovered from shock-wave experiments on quartz to 57 GPa is not a true glass but rather keeps a large degree of long-range structural information that can be recovered by static cold recompression to 13 GPa. At this pressure, shock-retrieved silica assumes the structure of crystalline stishovite. A minor amount of material recovers the structure of a recently discovered new silica polymorph.

New experimental data are presented that allows the biotite-magnetite-sanidine equilibrium to be used for estimating water fugacity (f H₂O ) in hydrous phonolitic magmas. It is also demonstrated that the partly ionic model gives the best estimate for the annite activity (a annite ). Crystallization experiments were carried out on a representative sample of peralkaline, phonolitic obsidian of Montaña Blanca (MB) pumice deposit, Tenerife, Canary Islands. Experiments were performed from 720.810 °C and 50.250 MPa. Redox conditions were varied between NNO (nickel + nickel oxide) + 1 (±0.2) and FMQ (fayalite + magnetite + quartz). The majority of the experiments were done under H₂O saturation conditions (P water = P total ). Several experiments were done using a mixed H 2 O-CO 2 fluid phase whereas in other experiments 10 or 20 wt% powdered alkali feldspar was added to the starting material. The pre-eruptive f H₂O of the Montaña Blanca magma is estimated at 676 ± 200 bars. The pre-eruptive f H₂O for the Fish Canyon tuff (753.2978 bars) and Bishop tuff rhyolite (1065.2440 bars) were also calculated, as well as f H₂O for metamorphic biotite from Au Sable Forks (≈ 130 bars). The results of this study suggest that this geohygrometer can be used in any magmatic system in which biotite-magnetite-sanidine is a stable assemblage.

We present the results of a single-crystal X-ray diffraction structural study of chlorite in a diamond anvil cell up to 5.47 GPa. The sample is a clinochlore from Val Malenco, Italy, triclinic polytype IIb-4, S.G. C1̅, with pseudomonoclinic metric and composition (Mg 9.14 Fe 2+ 1.02 Fe 3+ 0.01 Mn 0.01 Ti 0.01 Al 1.76 ) Σ=11.95 (Si 6.32 Al 1.68 ) Σ=8 O 20 (OH) 16 . Structural refinements were performed at several pressures with intensity data collected on a CCD diffractometer. Unit-cell parameters were accurately measured with the point-detector mounted on the same instrument. The bulk modulus of chlorite fitting data to a third-order Birch-Murnaghan equation of state is K 0 . = 88(5) GPa with K’ = 5(3). Results are in fair agreement with data based on powder neutron and synchrotron diffraction methods. The axial compressibility values were β 0a EoS = 3.4(2), β 0b EoS = 3.4(1), and β 0c EoS = 5.4(2) 10 -3 GPa -1 . The metric of the lattice remains triclinic in the investigated pressure range. Axial anisotropy is strongly reduced with respect to the axial compressibilities observed in other phyllosilicates. Comparison of structural refinements at different pressures shows that the main structural deformations affect the interlayer region, where the hydrogen bonds are relevant to the structural properties of the phase. The mean decrease of the OH-O distances is about 10% from ambient pressure to ~5 GPa. Compressibility data may be combined with those on thermal expansion to formulate an equation of state for clinochlore. Taking into account the thermal expansion coefficient reported in literature for a chlorite with a composition quite similar to that of our sample, we can write the equation: V = V 0 (1 - 1.14 10 -2 ΔP + 2.316 10 -5 ΔT), where P is in GPa and T in Celsius. Assuming an average rock density of 2.7 g/cm 3 , this corresponds to an isochoric P-T geothermal gradient of 18 °C/km.

High-pressure phase stabilities of the CaSnO 3 system were investigated using density functional theory static lattice energy calculations. We have identified six phase changes in this system: perovskitepost-perovskite, post-perovskite-B2+cotunnite in CaSiO 3 , B1-B2 in CaO, cassiterite-pyrite, pyriteorthorhombic-I, and orthorhombic-I-cotunnite in SnO 2 . Our results demonstrate a new high-pressure phase sequence of perovskite → post-perovskite → B2 + cotunnite. The post-perovskite transformation occurs at ~12 GPa with ~2.5% volume reduction and then it decomposes into the oxide mixture of B2 CaO and cotunnite SnO 2 at ~70 GPa with 8.1% volume reduction. The results also predict a few percent bulk velocity variations across these phase changes.

Raman spectra of synthetic HfSiO 4 were determined to pressures of 38.2 GPa. Changes in the spectra indicate that HfSiO 4 undergoes a room-temperature phase transition from the hafnon structure (I4 1 /amd space group) to the scheelite structure (I4 1 /a space group) at a pressure of ~19.6 GPa. Upon release of pressure to ambient conditions, the spectra indicate that the sample retains the scheelite structure. Zircon has been classified previously as the least compressible tetrahedrally coordinated silicate known. However, pressure derivatives of the peak positions in hafnon are smaller than those in zircon, and suggest that hafnon is more incompressible than zircon. Furthermore, the pressure derivatives also suggest that the high-pressure, scheelite-structured HfSiO 4 phase is more incompressible than the scheelite-structured ZrSiO 4 (reidite). Thus, the post-hafnon phase appears to be even more incompressible than hafnon, which would make it the least compressible tetrahedrally coordinated silicate known to date.

The second-order elastic constants of brucite were determined by Brillouin scattering to 15 GPa in a diamond anvil cell. The experiments were carried out using a 4:1 methanol-ethanol mixture as pressure medium, and ruby as a pressure standard. Two planes, one perpendicular to the c axis (basal plane) and the other containing the c axis (meridian plane), were measured at room pressure and 10 elevated pressures. Individual elastic stiffnesses, aggregate moduli, and their pressure derivatives were obtained by fitting the data to Eulerian finite strain equations. The inversion yields individual elastic constants of C 11 = 154.0(14) GPa, C 33 = 49.7(7) GPa, C 12 = 42.1(17) GPa, C 13 = 7.8(25) GPa, C 14 = 1.3(10) GPa, C 44 = 21.3(4) GPa, and their pressure derivatives of (∂C 11 /∂P) 0 = 9.0(2), (∂C 33 /∂P) 0 = 14.0(5), (∂C 12 /∂P) 0 = 3.2(2), (∂C 13 /∂P) 0 = 5.0(1), (∂C 14 /∂P) 0 = 0.9(1), (∂C 44 /∂P) 0 = 3.9(1). Aggregate moduli and their pressure derivatives are K S0 = 36.4(9) GPa, G 0 = 31.3(2) GPa, (∂K S /∂P) T0 = 8.9(4), (∂G/∂P) 0 = 4.3(1) for the Reuss bound, and K S0 = 43.8(8) GPa, G 0 = 35.2(3) GPa, (∂K S /∂P) T0 = 6.8(2), (∂G/P) 0 = 3.4(1) for the Voigt-Reuss-Hill average. The ratio of the linear compressibility along the c and a axes decreased from 4.7 to 1.3 over the examined pressure range. The shear anisotropy (C 66 /C 44 ) decreased from 2.6(1) at ambient condition to 1.3(1) with increase of pressure to 12 GPa. Axial compressibilities and a compression curve constructed from our Brillouin data are in good agreement with previous X-ray diffraction data. The increased interlayer interactions and hydrogen repulsion that occurs as brucite is compressed produce a continuous variation of elastic properties rather than any abrupt discontinuities.

Chromium K-edge X-ray absorption near-edge structure (XANES) spectra were recorded at room temperature for 27 CaO-MgO-Al 2 O 3 -SiO 2 (CMAS) glass compositions quenched from melts equilibrated at various oxygen fugacities (f O₂ ) at 1400 °C. Values of Cr 2+ /ΣCr were determined from the intensity of a shoulder on the main absorption edge, attributed to the 1s → 4s transition, which is characteristic of Cr 2+ in these glasses. For each composition, Cr 2+ /ΣCr could be quantified as a function of f O₂ , using a theoretical expression, from as few as three samples (Cr 2+ /ΣCr ≈ 0, 0.5, and 1). This allowed logK’, or the reduction potential of the Cr 3+/2+ half-reaction, and hence the relative change in the ratio γ melt Cr³⁺O1.5 /γ melt Cr²⁺O , to be determined for each composition. At constant f O₂ , log[Cr 2+/Cr3+ ] was found to decrease linearly with increasing optical basicity. The variation in logK’ with composition is controlled by γ melt Cr³⁺O1.5 , corresponding to the capacity of the melt to stabilize both the charge and the preferred solvation site of Cr 3+ . The method was then applied to spectra recorded in situ at 1400 °C for a synthetic mid-ocean ridge basalt (MORB) composition, allowing Cr 2+ /ΣCr to be quantified in a Fe-bearing melt for the first time. Cr 2+ /ΣCr was found to vary from ~0.45 at the nickel-nickel oxide (NNO) f O₂ buffer to ~0.90 at iron-wüstite (IW). This indicates that Cr 2+ is likely to be the dominant oxidation state in terrestrial basaltic melts.

Phosphohedyphane, Ca 2 Pb 3 (PO 4 ) 3 Cl, space group P6 3 /m, a = 9.857(1), c = 7.130(2) Å, V = 599.9(2) Å 3 , Z = 2, is a new mineral from the Capitana mine, Copiapó, Atacama Province, Chile and has been identified from numerous other deposits world-wide. At the Capitana mine, it occurs as transparent, colorless, tapering hexagonal prisms, as individuals up to about 0.5 mm in length and 0.1 mm in diameter, which are commonly doubly terminated. Crystals often occur in subparallel intergrowths and irregular clusters. Phosphohedyphane forms as a secondary mineral in the oxidized zone of the Capitana mine, a Cu-Pb-Ag deposit, where it closely associated with quartz, duftite, and bayldonite. Crystals exhibit core-to-rim chemical zonation and electron analyses of cores/rims yielded CaO 9.24/7.76, PbO 67.60/69.35, P 2 O 5 18.40/17.00, As 2 O 5 2.73/3.68, Cl 3.32/3.22, .O = Cl .0.75/.0.73, total 100.54/100.28 wt%. The name phosphohedyphane is for the relationship of the mineral to hedyphane. The mineral has an apatite structure with ordering of Ca and Pb in the two non-equivalent large cation sites, as in hedyphane. The structure refinement indicates that the Ca2(6h) site is completely occupied by Pb and the Ca1(4f) site is occupied by 92% Ca and 8% Pb. The tetrahedral site refines to 91% P and 9% As. The refinement indicates the 0,0,0 position to be fully occupied by Cl. The ordering of Ca and Pb in phosphohedyphane has important implications with respect to the chlorapatite-pyromorphite solid solution series. An analysis of compositions of natural members of the pyromorphite-mimetite-turneaureite-chlorapatite system suggests the existence of complete solid solution among pyromorphite, mimetite, hedyphane, and phosphohedyphane. No stable solid solutions appear to exist between the joins phosphohedyphane-hedyphane and chlorapatite-turneaureite in natural systems.

Two new quaternary lead oxyhalides, Pb 3 MoO 5 Cl 2 and Pb 3 WO 5 Br 2 , have been prepared. They are isostructural with the mineral pinalite, Pb 3 WO 5 Cl 2 , and its barium analog. The crystal structure of Pb 3 MoO 5 Cl 2 has been refined from powder neutron diffraction data to R p = 0.0564 and R wp = 0.0342. All these structures contain significantly stretched MO 5 (M = Mo, W) square pyramids incorporated into the [Pb 2 O 2 ] sheets. Bond valence sums indicate significant overbonding of one of the metal sites, which increases along with structural distortions when passing from Pb 3 MoO 5 Cl 2 to Pb 3 WO 5 Cl 2 and from Pb 3 WO 5 Cl 2 to Ba 3 WO 5 Cl 2 . Possibilities of preparing isostructural and structurally related compounds are discussed.

We have applied one-dimensional (1D) 1 H MAS NMR, 27 Al → 1 H CP MAS NMR, as well as 2D 27 Al triple-quantum (3Q) MAS NMR, 27 Al → 1 H heteronuclear correlation (HETCOR) and high-resolution 3QMAS/HETCOR NMR techniques to KAlSi 3 O 8 (Or), NaAlSi 3 O 8 (Ab) and NaAlSiO 4 (Ne) glasses containing 0~2 wt% H 2 O to shed light on the dissolution mechanisms of water in aluminosilicate melts (glasses). An Al Q 3 -OH group, characterized by 1 H chemical shifts of 1.3-1.9 ppm, was identified for all hydrous glasses. Its abundance increases with bulk Al/Si ratio. The 27 Al chemical shifts (δ i Al ) of this species are 64-68 ppm, larger than those of Al Q 4 by 3-6 ppm. Despite this difference, it is only through 27 Al → 1 H HETCOR and 3QMAS/HETCOR, but not 27 Al MAS or 3QMAS NMR that the peaks are resolved. This study has demonstrated that depolymerization and formation of AlOH/SiOH is a general water dissolution mechanism for polymerized aluminosilicate melts (glasses), and HETCOR NMR experiments involving 1 H are the key to its revelation.

Electron-microprobe analyses of hydroxylellestadite from the Cioclovina Cave (Romania) gave the composition Ca 10.27 [(SiO 4 ) 2.53 (SO 4 ) 2.17 (PO 4 ) 1.27 ] Σ=5.97 [(OH) 1.66 F 0.21 Cl 0.16 ] Σ=2.03 . The mineral is translucent to transparent, light orange, slightly fluorescent, has a vitreous luster and <1.5 mm in length. A singlecrystal X-ray structure investigation gave the average space-group symmetry P6 3 /m [R1(F) = 0.038 for 783 reflections up to 2θ MoKα = 70° and 42 variables, a = 9.496(2), c = 6.920(2) Å, V = 540.4 Å 3 , and Z = 2]. Some atoms exhibit large anisotropic displacements. Ordering of atoms along with a symmetry reduction is not verified. Fourier-transformed infrared (FT-IR) and micro-Raman spectra exhibit a distinct contribution from (PO 4 ) 3- modes along with the characteristic (SO 4 ) 2- and (SiO 4 ) 4- modes. The occurrence is quite unusual and suggests that an intense thermal process affected a restricted area within the cave. Hydroxylellestadite is associated with berlinite, another high-temperature mineral. It is likely to have formed within highly phosphatized, silicate-rich, carbonate-mudstone sediments heavily compacted and thermally transformed due to in situ bat guano combustion.

Matioliite, ideally NaMgAl 5 (PO 4 ) 4 (OH) 6 ·2H 2 O, occurs as a secondary hydrothermal mineral in the Gentil mine granite pegmatite, Mendes Pimentel Co., Minas Gerais, Brazil. Some crystals are zoned to more Fe-rich compositions with an Fe 2+ :Mg ratio of approximately 1:1, corresponding to an intermediate member of the burangaite-matioliite solid-solution series. Matioliite is intimately associated with fluorapatite, crandallite, and zoned gormanite-souzalite cystals. The mineral forms prismatic to tabular crystals, up to 1 mm long. Matioliite is transparent and displays a vitreous luster; it is blue to colorless with a white streak. It is non-fluorescent. Mohs hardness is about 5. Calculated density is 2.948 g/cm 3 . Matioliite is biaxial negative, η α = 1.597(2), η β = 1.627(2), η γ = 1.632(1) (white light), 2V (meas.) = 43(2)°, 2V (calc.) = 44°, dispersion r > v, orientation X = b, Z ^ c = 6°. Pleochroism is Y > X > Z, X = light blue to colorless, Y = blue, Z = colorless. The empirical formula is (Na 0.94 Ca 0.01 ) Σ0.95 (Mg 0.88 Fe 2+ 0.11 Mn 0.01 ) Σ1.00 (Al 4.84 Fe 3+ 0.13 ) Σ4.97 (PO 4 ) 4.03 (OH) 5.76 ·2H 2 O. The mineral is monoclinic, space group C2/c, a = 25.075(1) Å, b = 5.0470(3) Å, c = 13.4370(7) Å, β = 110.97(3)°, V = 1587.9(4) Å 3 , Z = 4. Crystal-structure determination was carried out and showed it is isostructural with burangaite, dufr énite, and natrodufrénite. Both the description and the name were approved by the CNMMN-IMA (Nomenclature Proposal 2005-011). The .magnesium analog of burangaite. described from the Gold Quarry mine, Carlin-trend, Eureka County, Nevada; “burangaite” from Hochgosch, Millstätter See-Rücken, Kärnten, Austria; and .burangaite. described from Córrego Pomarolli, Linópolis, Divino das Laranjeiras, Minas Gerais, Brazil, are probably matioliite.

Crystal orientations of the sub-micrometer-sized calcite units in oval-shaped coccoliths of a coccolithophore, Pleurochrysis carterae, have been investigated using electron back-scattered diffraction (EBSD). Although the contrast of the acquired EBSD patterns was weak due to the small crystal unit size, the orientations could be uniquely determined from the patterns. The crystal orientations for V- and R-units are close to those reported in a previous work using electron diffraction in a TEM. However, more accurate crystal orientations corresponding to the coccolith morphology were obtained by using EBSD in a SEM. In V-units, the c-axis is declined about 35° from the normal of the coccolith plane and one of the a i -axes is roughly parallel to the coccolith plane. The c-axis in R-units is slightly oblique to the radial direction along the coccolith plane and one of the a i -axes is near vertical to the coccolith plane. The projections of the c-axis of V- and R-units on the coccolith plane deviate considerably from the normal of the coccolith circumference, giving a crystallographic chiral property. The atomic arrangements of calcite contacted with the organic base plate are discussed for both units based on the crystallographic orientations derived from EBSD measurements.

Iron sulfide (FeS) has been examined in a diamond anvil cell to 120 GPa pressure using an in situ angle-dispersive X-ray diffraction technique. The transformation from a monoclinic phase (FeS III) to a newly described orthorhombic phase (FeS VI) was observed at 35-40 GPa and high temperatures. This phase remained stable during the temperature quench. After the decompression, however, the recovered sample was transformed to the troilite structure (FeS I). The relative volume change that accompanies this transformation is ~1%. No further phase transformations were observed at higher pressures up to 120 GPa, even when the sample was laser-heated to ~2000 K. There are four molecules in a single unit cell (Z = 4) of the orthorhombic phase. The isothermal bulk modulus (K 0 ) of the orthorhombic phase is 156(6) GPa, with V 0 = 99.5(7) Å 3 when K 0 ’ is fixed at 4. The a axis of the unit-cell parameter is more compressible than the b and c axes. Our study indicates that the phase transformation from NiAs-type (FeS V) to orthorhombic (FeS VI) phases could occur in the Martian core.