Volume 93, Issue 10

Refractory oxide melts of the binary system MgO-Al 2 O 3 have been studied by molecular dynamics simulation using an advanced ionic interaction model derived from first-principles. The simulations reproduce well experimental densities, structure factors, and transport properties. Anomalous behavior of the latter was observed as a function of melt composition. The minimum in the Al self-diffusion and the respective maximum in the shear viscosity around MgAl 4 O 7 composition are explained by structural changes in the melt.

The oxidation state of Fe in hydrous ultrapotassic (phono-tephritic) melts coexisting with mixed H 2 O-CO 2 fluids was studied experimentally at 1200 and 1250 °C and pressures from 50 to 500 MPa. The oxygen fugacity (f O₂ ) varied from NNO-2.9 to NNO+2.6 in log f O₂ , relative to the Ni-NiO oxygen buffer (NNO), as imposed by external redox conditions in experimental vessels and internal variations in water activity from 0.05 to 1 inside the capsules. The Fe redox state of the quenched melts was determined by colorimetric wet-chemical analysis. This analytical method was optimized to measure the Fe 2+ /ΣFe ratio of milligram-sized samples within ±0.03 (2σ). The accuracy and precision was tested with international reference materials and with standards analyzed by other methods. The Fe 2+ /ΣFe ratio of the experimental glasses covered a range of 0.41 to 0.85. A small negative effect of dissolved water on Fe 2+ /ΣFe at given f O₂ was found, consistent with the thermodynamic model of Moretti (2005). No effect of pressure and temperature on the redox state of Fe was resolvable in the investigated P-T range. Compared to hydrous ferrobasaltic melts that were studied previously under similar conditions, systematically lower Fe 2+ /ΣFe ratios were found for the phono-tephritic melts, in particular at low oxygen fugacities. This effect is attributed to the much higher K 2 O contents of the phono-tephrite (7.5 compared to 0.3 wt%), but the difference in FeO T (7.8 wt% in the phono-tephrite and 12.9 wt% in the ferrobasalt) may have an influence as well. Comparison of the experimentally obtained relationship between log f O₂ and Fe 3+ /Fe 2+ for the studied hydrous ultrapotassic melts with commonly used empirical and thermodynamic models suggest that these models can be successfully applied to phono-tephritc melts, although such compositions were not implemented in the model calibrations. Furthermore, the new data can be used to improve the models with respect to the effects of compositional variables, such as H 2 O or K 2 O, on the redox state of Fe in silicate melts.

In situ, high-temperature Raman spectroscopy was used to study the Q n speciation in binary Nasilicate glasses and melts. Over 300 Raman spectra in the compositional range from 25 to 40 mol% Na 2 O were collected at room and high temperatures between 800 and 1200 K. Quantitative information on the relative abundances of species in melts was obtained from the Raman spectra through a quantification procedure that does not require any a priori assumptions about the line shapes or external calibration of the Raman scattering efficiencies for the various Q n species. The ΔH° associated with the speciation reaction 2Q 3 = Q 4 + Q 2 was found to be 20.3 ± 7.9 kJ/mol. For a given temperature, the speciation is more disordered in sodium than in potassium silicate melts. Because of the smaller temperature dependence of the speciation in the sodium silicate system, the difference in the speciation for the sodium and potassium silicate system decreases with increasing temperature. In addition to the speciation data, the partial Raman spectra for the different species were obtained. The experimentally observed variation of the partial Raman spectra with temperature, and, to a minor extent, with composition, should stimulate future theoretical studies on the vibrational properties of silicate glasses and melts.

An eclogite facies meta-plagiogranite from the Lanzo massif (western Alps, Italy) contains crystals of zircon intimately associated with allanite. Zircon displays different microtextures ranging from pristine, euhedral, and magmatic to fractured, porous varieties with mosaic zoning, and pervasive recrystallization into euhedral microcrystals. Fractures and voids in the recrystallized zircon microcrystals are mainly filled by high-pressure Na-rich pyroxene. Electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic zircon crystals and microcrystals, with less than 2° misorientation among neighboring microdomains. The textural change is coupled with chemical and isotopic modifications: recrystallized zircon domains contain significantly less Th and light- to mid-REE, but are richer in Sr than magmatic zircon crystals. Magmatic zircon preserves the protolith U-Pb age of 163.5 ± 1.7 Ma, whereas zircon microcrystals have a mean age of 55 ± 1 Ma. The coexisting allanite also contains inclusions of Na-rich pyroxene and has chemical features (elevated Sr and Ni contents and lack of Eu anomaly) indicating formation at high pressure. Despite being associated texturally with zircon, allanite yields a younger Th-Pb age of 46.5 ± 3.0 Ma, suggesting that the Lanzo unit remained at relatively high pressure conditions for ~8 m.y. Zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as the agent for a coupled dissolution-reprecipitation process. Recrystallization occurred with minimum transport, in a low-strain environment, and was not significantly enhanced by metamictization. The source of the fluid for zircon recrystallization is most probably related to prograde devolatilization reactions in the surrounding serpentinite.

High-pressure X-ray diffraction was carried out on a single crystal of gypsum compressed in a diamond anvil cell. The sample maintained its crystal structure up to 4.0 ± 0.1 GPa. The fit of pressure dependence of the unit-cell volume to the third-order Birch-Murnaghan equation yielded K T0 = 44(3) GPa and (∂K T /∂P) 0 = 3.3(3), where K T0 and (∂K T /∂P) 0 are the isothermal bulk modulus and its pressure derivative in ambient conditions. The axial compressibility values, fitting data collected up to 3.94 GPa, were β 0a EoS = 6.1(1) and β 0c EoS = 5.6(1) 10 -3 GPa -1 . The value of β 0b EoS was 6.2(8) 10 -3 GPa -1 fitting the data collected up to 2 GPa, due to non-linearity above this pressure; axial compressibility of gypsum is almost isotropic (β 0a :β 0b :β 0c = 1:1:0.9). This behavior is partly unexpected for a layered mineral based on alternate layers of Ca- and S-polyhedral chains separated by interlayers occupied by water molecules. Above 4.0 GPa the compression curve of gypsum shows a discontinuity with a 2.5% contraction in volume. Structural refinements indicate that SO 4 volume and average S-O bond distances remain almost unchanged from room pressure to 3.9 GPa [range 1.637(4)-1.66(9) Å 3 ; 1.4733-1.48 Å]. The SO 4 tetrahedron undergoes distortion: the smaller distance decreases from 1.4731(9) to 1.45(2) Å and the larger increases from 1.4735(9) to 1.51(2) Å. In contrast, the calcium polyhedra show expected high-pressure behavior, becoming more regular and decreasing in volume from 25.84(8) Å 3 at ambient P to 24.7(1) Å 3 at 3.9 GPa. The largest variations were observed in the interlayer region where the water molecules are located. Along the b axis, the two structural layers have very different compressibilities: the polyhedral layer is almost incompressible in the pressure range studied, whereas water layer compressibility is 9.7(3) 10 -3 GPa -1 , about twice that of the other two lattice parameters. At ambient conditions, water molecules form weak hydrogen bonds with the O atoms of Ca and S polyhedra. With increasing pressure, the weakest hydrogen bond becomes the strongest one: from 0.001 to 4 GPa, the distance changes from 2.806(1) to 2.73(2) Å for OW-H1···O2, and from 2.883(2) to 2.69(3) Å for OW-H2···O2. Structure refinements show that water remains in the structure when P increases. The observed distortion of sulfate tetrahedra explains the splitting of the ν1 sulfate stretching mode, and the various measured compressibilities of the two hydrogen bonds and the coalescence of the Raman stretching mode observed at pressures over 5 GPa.

This paper reports a single-crystal FTIR spectroscopic study of leucite, a nominally anhydrous mineral (NAM). Several transparent, inclusion-free samples from different occurrences and localities in the Alban Hills volcanic area (Latium, Italy) were analyzed. The FTIR spectra collected in the 4000-3000 cm -1 H 2 O-stretching region show a broad absorption consisting of overlapping components at 3604, 3500, and 3245 cm -1 . The occurrence of a well-defined combination band at 5225 cm -1 indicates that almost all the examined samples contain hydrous components in the form of structurally bound water molecules. Using the integrated molar absorption coefficient from the working curve of Libowitzky and Rossman (1997), a water content up to >4000 ppm was obtained for the studied specimens. Detailed microspectroscopy mapping shows significant zoning of water in some samples, typically consisting of an anhydrous core mantled by a hydrous rim. The collected data suggest that careful study of the distribution of the volatile content of leucite (and associated volcanic NAMs) may provide a tool to monitor the evolution of the magmatic system where these minerals occur.

The crystal structures of herderite, CaBePO 4 [F 0.75 (OH) 0.25 ], a = 9.7446(4), b = 7.6769(3), c = 4.7633(2) Å, β = 90.667(1)°, V = 356.31(4) Å 3 , P2 1 /a, and Z = 4, from the Mogok Stone Tract, Myanmar, and hydroxyl-herderite, CaBePO 4 [(OH) 0.52 F 0.48 ], a = 9.7615(4), b = 7.6680(3), c = 4.7853(2) Å, β = 90.184(1)°, V = 358.18(4) Å 3 , P2 1 /a, and Z = 4, from the Sauberg mine, Morgenröther Zug, Ehrenfriedersdorf, Germany, have been refined to R 1 indices of 1.7%, in both cases, using MoKα single-crystal X-ray diffraction data. Herderite from Mogok is associated with complex rare-element beryl-type granite pegmatite. Most herderite-series minerals are the product of late-stage processes in granite pegmatite evolution, typically of an exsolved fluid upon crystallization, and significant F-enrichment in these fluids appears to result in the domination of herderite over hydroxyl-herderite. Herderite is now documented from Mogok, Myanmar; Yichung, China; Brazil; and probably Erongo, Namibia. The pegmatites at Ehrenfriedersdorf, Germany have produced samples at the boundary between herderite and hydroxyl-herderite, if not possibly herderite as well.

We present a new technique for the quantification of water in glasses down to the parts per million level, using confocal microRaman spectroscopy with the recently developed “Comparator Technique.” To test this method, we used a suite of glasses and gemstone-quality garnets with varying chemical compositions. Water contents were independently determined with proton-proton (pp) scattering and infrared (IR) spectroscopy. Moreover, water concentrations obtained for the garnets were compared to data from a study by Maldener et al. (2003) using nuclear reaction analysis (NRA). For each sample, we recorded Raman spectra in the frequency range from 3100 to 3750 cm -1 and standardized them using an independently well-characterized glass. In this paper, we demonstrate the usefulness of this technique for quantifying water concentrations in natural and synthetic glass samples and garnets, and verify its adaptability for concentrations from 40 wt ppm up to 40 wt% H 2 O. However, in the case of absorbing material (e.g., Fe-bearing samples), the suggested method needs to be modified to overcome problems due to heating and melting of those phases. Furthermore, we propose an integrated molar absorption coefficient for water in quartz glass, ε itot = 72 000 ± 12 000 Lmol -1 H₂O cm -2 , for quantitative IR spectroscopy that is higher than a previously reported value of Paterson (1982) or that predicted by the general calibration trend determined by Libowitzky and Rossman (1997).

We have conducted a powder neutron diffraction study of the distorted rutile-type hydrous phase δ-AlOO(D 0.74 H 0.26 ) at pressures up to 9.2 GPa. Rietveld refinement reveals that the observed diffraction data are best fitted by a model with an asymmetric hydrogen bond (space group P2 1 nm). The Al-O2 bonds of the AlO 6 octahedron remain longer than the Al-O1 bonds up to 9.2 GPa, reflecting the fact that O2 is the donor and O1 is the acceptor of the hydrogen bond of O2-D···O1. As pressure increases, the Al-O1 bond lengths remain almost constant or increase slightly, whereas the longer Al-O2 bonds are more compressible. The O···O distance of the shared edge of the octahedron is almost constant unlike the other O···O distances that decrease during compression. The bulk modulus of the AlO 6 octahedron is determined to be 197.0(4) GPa, where K' is fixed to 4. The AlO 6 octahedron rotates about 1° during compression to 9.2 GPa. As pressure increases and the O2···O1 distance decreases, the D···O1 bond is shortened, whereas the O2-D bond becomes elongated, with pressure dependencies of -1.81(9) × 10 -2 Å/GPa and 0.57(8) × 10 -2 Å/GPa, respectively, implying the symmetrization of the hydrogen bond at high pressure. The decrease in intensity of the 120 reflection as pressure increases suggests that the transition from the structure with space group P2 1 nm to that with Pnnm would take place in δ-AlOOD at high pressure. This transition could be attributed to the disorder of deuterium or symmetrization of the hydrogen bond.

We have investigated the thermal expansion of several synthetic feldspars, including Li-feldspar, rubicline (Rb-microcline), Rb-sanidine, and buddingtonite (NH 4 -feldspar). When analyzed in conjunction with earlier data on both ordered and disordered Na- and K-feldspars, it is clear that the coefficient of thermal expansion (α) decreases dramatically, and linearly, with increasing room-temperature volume. For “AlSi 3 ” feldspars, then, chemical expansion limits thermal expansion. The relationship between α and room-temperature volume provides a useful predictive tool based simply on the volume of a feldspar at room temperature. This relationship also reveals that volumes of K-Na mixing in naturally occurring alkali feldspars decrease with increasing temperature.

We report results of multiple electron-microprobe analyses of nine grains of alloy minerals, 2-5 mm in size, rich in the platinum-group elements (PGE), from a unique, very old collection (~1890s) of placer material from Trinity Co., California. Osmium, iridium, ruthenium, and rutheniridosmine are the principal alloy species poor in Fe (typically <0.5-1 wt%); they appear to be primary. An Fe-enrichment (up to 4.6 wt%) is observed in lamellae of Ir-Ru-Os alloy exsolved from the Fe-poor Os-Ir-Ru alloy host, and also in a rim-like alteration-induced phase developed along the margin in some of the grains of Ir-Ru-Os alloy. Much greater levels of Fe (up to 19.1 wt%), incorporated via the substitution mechanisms: Fe → Ru, Fe → (Os + Ru), and Fe → Ir, were documented in three grains of Fe-Os-Ru-Ir alloys, which attain Fe-dominant compositions, i.e., hexaferrum. These Fe-Os-Ru-Ir alloys and associated exotic phases enriched in unconventional elements, such as W-(Mo)-bearing rutheniridosmine, (Os,Ir) 5 (W,Mo) and newly recognized (Ir,Os) 5 (W,Mo), appear to be secondary, formed under conditions of low fugacities of O 2 and S 2 as a result of interaction of primary Os-Ir-Ru alloys with a reducing fluid phase. No grains of Pt-Fe alloys were found; these only occur as Pt 3 Fetype (isoferroplatinum or Fe-rich platinum) and Pt 2 Fe-type inclusions (<50 μm), enclosed in a matrix of Ir- or Os-dominant alloys rich in Ru. The Pt 2 Fe alloy appears to be a compositional variant of Fe-rich platinum, possibly reflecting a lower limit of Pt content possible in the mineral platinum. An Au-Ag alloy, ranging up to Au 0.99 , precipitated pseudomorphously by a subsolidus reaction between a residual Au-Ag-rich melt and exsolution-induced inclusions of the Pt-Fe alloy phases. Micro-inclusions of olivine, hosted by a ternary alloy Os 0.33 Ru 0.33 Ir 0.30 , are extremely rich in Mg (Fo 95.1-95.4 ), probably reflecting high-temperature reaction involving chromite or magnesiochromite. The alloy grains from the old collection were likely derived from a mineralized zone of ultramafic rocks, rich in chromitemagnesiochromite and poor in overall S, in the Trinity ophiolite complex of northern California.

The compressional behavior of a synthetic lead fluorapatite [Pb 9.35 (PO 4 ) 6 F 2 ] has been investigated in situ up to about 16.7 GPa at 300 K, using a diamond-anvil cell and synchrotron X-ray diffraction. We find that the compressibility of lead fluorapatite is significantly different from that of fluorapatite [Ca 10 (PO 4 ) 6 F 2 ], chlorapatite [Ca 10 (PO 4 ) 6 Cl 2 ], and hydroxylapatite [Ca 10 (PO 4 ) 6 (OH) 2 ]: lead fluorapatite is much more compressible, and elastically isotropic in the investigated pressure range. The pressurevolume data fitted to the third-order Birch-Murnaghan equation yield an isothermal bulk modulus (K T ) of 54.3(18) GPa and the pressure derivative (K′ T ) of 8.1(6). If K′ T is fixed at 4, the obtained K T is 68.4(16) GPa, which is approximately only two-thirds of the isothermal bulk modulus of the calcium apatites.

The hexagonal Al-rich high-pressure phases are members of a structural family with the same type of framework composed of double-ribbons of edge-sharing octahedra, but variably occupied trigonal and hexagonal channels. This family includes jaffeite, fluoborite, yeremeyevite, painite, and synthetic Ba indates. This kinship broadens the spectrum of elements potentially accommodated by these phases in the mantle.

Abstrac Elastic and structural behavior of a natural tetragonal leucite from the volcanic Lazium district (Italy) were investigated at high pressure by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. A first-order phase transition, never reported in the literature, was observed at P = 2.4 ± 0.2 GPa from tetragonal (I4 1 /a) to triclinic symmetry (analysis of diffraction intensities suggests the space group P1), accompanied by a drastic increase in density of about 4.7%. The transition pressure was bracketed by several measurements in compression and decompression. No further phase-transition has been observed up to 7 GPa. Fitting a second-order Birch-Murnaghan equation of state (BM-EoS) to the pressure-volume data of the tetragonal polymorph, we obtain K 0 = 41.9(6) GPa and K′ = 4 (fixed). In the case of the triclinic polymorph, a second-order BM-EoS gives K 0 = 33.2(5) GPa. The eulerian finite strain (f e ) vs. normalized stress (F e ) curves were calculated for the low- and high-P polymorphs, providing F e (0) = 42(1) and F e (0) = 33.2(4) GPa, respectively. The axial bulk modulus values of the tetragonal polymorph, calculated with a linearized BM-EoS, are K 0 (a) = 34.5(5) and K 0 (c) = 78(1) GPa. For the triclinic polymorph, we obtain K 0 (a) = 35.9(5), K 0 (b) = 34.9(7), and K 0 (c) = 35.5(7) GPa. The elastic behavior of the low-P polymorph appears to be more anisotropic than that of the high-P polymorph. The HP-crystal structure evolution of the tetragonal polymorph of leucite was studied on the basis of six structural refinements at different pressures between 0.0001 and 1.8 GPa. The main deformation mechanisms at high-pressure are due to tetrahedral tilting, giving rise to an increase of the ellipticity of the four- and six-membered rings of the tetrahedral framework. The T-O bond distances are practically invariant within the stability field of the tetragonal polymorph. The complex P-induced twinning, due to the tetragonal → triclinic phase-transition, and the low quality of the diffraction data at pressure above the phase-transition, did not allow the refinement of the crystal structure of the triclinic polymorph.

Hydrofluoric acid solution calorimetric measurements conducted at 50 °C on a series of Al-Si ordered Rb-K feldspars reveal that the enthalpies Rb-K mixing (H ex ) for this series are positive, with maximum values skewed toward Rb-rich compositions. However, the magnitudes of H ex are greatly reduced relative to those for Na-K feldspars, as are the positive volumes of Rb-K mixing observed for this series. Both observations can be explained by the substitution of two ions that are relatively similar in size. The difficulty in synthesizing Rb-rich sanidine via Rb-exchange of K-sanidine, contrasted with the ability to synthesize rubicline from K-microcline by such exchange, together with a vanishingly small ΔV between ordered rubicline and disordered Rb-sanidine suggest that the volume of Rb-feldspar is close to the limit to which the feldspar structure can be expanded at 1 bar.

Demicheleite, ideally BiSBr, is the first natural bismuth sulfohalogenide so far discovered. It is identical with the corresponding and analogous synthetic compound and is one of the very few minerals where bromine is an essential component. It was found in an active high-temperature fumarole at the rim of La Fossa crater, Vulcano Island, Aeolian archipelago, Sicily, Italy. The mineral occurs as prismatic translucent crystals up to 0.5 mm in size in an altered pyroclastic breccia, together with pseudocotunnite, bismoclite, bismuthinite, cotunnite, and challacolloite. The mineral is orthorhombic, space group Pnam, with a = 8.0424(9), b = 9.8511(11), and c = 4.0328(5) Å, V = 319.50(6) Å 3 , Z = 4 (from single crystal); the habit is prismatic with {110} and {210} as prevailing forms, terminated by minor faces of another prism {011}, a pinacoid {010}, and a bipyramid {111}.The color is dark red to black; the streak is red; the luster submetallic. Non-fluorescent. Tenacity: brittle. Cleavage and fracture: not observed. The calculated density is 6.312 g/cm 3 . The chemical analysis obtained by EDS microprobe gave (wt%) Bi 67.6(4), Br 17.4(7), Cl 4.1(4), I 0.6(1), S 10.1(1), total 99.8, corresponding to the empirical formula (based on 3 apfu): Bi 0.99 S 0.97 (Br 0.67 Cl 0.35 I 0.02 ) Σ=1.04 The crystal structure has been refined to a final R index of 0.037 and contains Bi in sevenfold coordination at the center of a monocapped trigonal prism. By sharing the triangular bases, such polyhedra form rows extending along [001]. These rows are connected to symmetry-related rows by sharing S-S edges of the pyramidal caps; these double rows are connected to each other by sharing Br/Cl atoms. The bond lengths are close to those of the synthetic counterparts BiSBr and BiSCl, with Bi-S bonds 2.593(3) and 2.720(2) Å long and Bi-Br/Cl bonds 3.009(1) and 3.488(2) Å long. The strongest 6 lines in the X-ray powder diffraction pattern [d obs (Å) (I) hkl] are: 4.220 (68) (120), 3.740 (62) (210), 2.909 (100) (121), 2.036 (47) (321), 1.865 (63) (022), and 1.774 (88) (411). The mineral is named after Vincenzo de Michele (b. 1936), former curator of the Section of Mineralogy of the Museo di Storia Naturale, Milano, Italy. Both the mineral and the mineral name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification (IMA 2007-022). The type specimen is deposited (no. 2007-1) in the Reference Collection of Dipartimento di Chimica Strutturale e Stereochimica Inorganica of Università degli Studi di Milano.

A recent force field model is used in this paper to analyze, with a nano-scale simulation, the structural and the vibrational properties of a rock system under the application of various high pressures. Our results show that this model reproduces accurately the equilibrium structural properties for calcite, gives comparable results for aragonite with the observed data, and predicts the structural, elastic and vibrational properties of the post-aragonite phase. In addition, we show that the transition from the aragonite to the post-aragonite phase appears at a transition pressure of 35.2 GPa. The longitudinal wave velocity increases rapidly by a factor of 1.71. In contrast the shear wave velocity increases very slowly up to the transition pressure, and thereafter increases rapidly, changing by 45% to 80 GPa. In addition, our calculations for the vibrational properties show that with the new potential model presented here, we observe a softening mode in the Γ-F direction. The phonon frequencies of the post-aragonite phase show more modes than for calcite and aragonite.

Here we present water concentration data for olivine from different host rocks, measured with a nuclear technique using proton-proton scattering. This method, which is used here for the first time on olivine, is very powerful for determining trace amounts of water. The studied olivine specimens differ in their H 2 O contents, ranging from 4 to 51 wt ppm (=10-117 atom ppm H). The lowest concentrations are found in olivine from spinel peridotite xenoliths, the highest concentrations in olivine from alpine-type peridotite; the contents of an ophiolitic and a hydrothermal olivine are intermediate. Infrared spectroscopy was applied to ensure that the measured water contents stem solely from hydroxyl defects in the mineral structure. The infrared spectra differ from sample to sample. Five of six olivine specimens show absorption bands typical of hydroxyl groups associated with Ti defects. These olivines differ in their Ti contents by two orders of magnitude. However, a correlation of water and Ti content was not observed.

To determine which factors affect thermal diffusivity (D) of mineral aggregates, we compared accurate measurements of D for five quartzites, two microcrystalline samples (chert and agate), chalcedony, and two partially crystalline opal specimens (of type opal-CT) with single-crystal quartz. Samples were characterized using infrared (IR) spectroscopy, electron microprobe analysis, optical microscopy, and X-ray powder diffractometry (XRD). Using laser flash analysis, we measured D of the quartzites and microcrystalline samples between room temperature and 1000 °C, and D of the opal specimens and chalcedony up to the temperatures where these samples failed (300 and 650 °C, respectively). Data between 20 and 500 °C can be fit by 1/D = AT + B. Values of A and B for quartzites, microcrystalline samples, opal specimens, and chalcedony are distinct from each other and from those of directionally averaged quartz single-crystals. Lower D at room temperature correlates with inflated B values of polycrystalline samples and results from porosity (all samples), hydration (all samples), grain boundaries (microcrystalline samples and chalcedony), and disorder (opal specimens and chalcedony). Dehydration and pore space reduction alter the temperature derivatives of D; dehydration causes the low A in chalcedony and opal. Above 600 °C, D changes negligibly with temperature and is lower than directionally averaged quartz, suggesting an increase in contact resistance as cracks open during heating. Thermal cracking is greatest in samples with large grain sizes and abundant, fluid-filled pores. The prevalence of cracking in polycrystalline samples suggests that high-temperature laboratory measurements generally underestimate heat transport properties in geologic environments, wherein confining pressures limit thermal expansion.

Significant quantities of silver can be accommodated in galena as a coupled substitution with Ag + Sb and/or Ag + Bi for 2 Pb atoms. The limitations of this solubility are quantified experimentally for the cubic (Fm3̅m) α-galena phase. Above 441.7 °C, the α-galena phase has complete solid solution across the Pb 2 S 2 -AgSbS 2 -AgBiS 2 (galena-miargyrite-matildite) ternary. Dry sinter experiments at 400, 375, and 350 °C extend the known miscibility gap on the Pb 2 S 2 -AgSbS 2 binary into ternary compositions. Phase equilibrium and miscibility gap data and self-consistent thermodynamic data for other high-temperature phases in this system combined with data for other sulfides and sulfosalts are used to constrain an asymmetric regular solution model for the α-galena phase (Pb 2 ,AgSb,AgBi) S 2 . Phase relations in this system suggest that simple retrograde re-equilibration during cooling can produce phase assemblages and textures commonly observed in Ag-bearing ore deposits. Bulk metal ratios of deposits in conjunction with the application of this thermodynamic model may be used to understand district Ag-distribution or as an exploration tool.

An analog of β-FeOOH (akaganéite) containing structurally incorporated molybdate has been synthesized and the conditions for its formation have been studied in the presence of chloride, sulfate, and nitrate anions. The incorporation of molybdate into the β-FeOOH structure has been shown to greatly increase the stability of this phase with respect to its transformation to hematite at elevated temperature. Molybdate incorporation also has been shown to change the particle morphology, with increasing amounts of molybdate causing the rod-shaped nanoparticles to become progressively shorter until spherical nanocrystals are observed. The combined observations of increased stability, morphology changes, and measured levels of incorporated molybdenum suggest that although inclusion of molybdate anions into the β-FeOOH tunnel sites is possible, it is unlikely to be the only mode of molybdate incorporation.

The partitioning of iron among octahedrally coordinated sites in tourmaline, and its stereochemical consequences, were investigated in a Fe-rich dravite in a skarn rock from Utö, Sweden. A multianalytical approach using structure refinement (SREF), electron microprobe analysis (EMPA), and Mössbauer spectroscopy (MS) established the chemical and structural nature of the tourmaline. A structural formula obtained by optimization procedures indicates disordering of Al, Mg, and Fe 2+ over the Y and Z sites, and ordering of Fe 3+ at the Y site. Two Fe-rich tourmalines from the literature, reexamined with the optimizing site assignment procedure, appear to have iron partitioning comparable to that of the Utö tourmaline with Fe 2+ disordered over the octahedral sites. This is best explained by disordered Fe 2+ distributions that minimize the strain state of the Y-O bonds and provide a shielding effect reducing Y-Z repulsion. This is consistent with predictions from bond-valence theory and Pauling’s rules. An indication of Z-site occupancy by Fe 2+ in tourmaline may be signaled by a significant correlation between <Z-O> and the c lattice parameter (r 2 = 0.96). The c value for a very Fe 2+ -rich tourmaline and an ideal end-member schorl, with Fe 2+ and Al ordered at Y and Z (respectively), yielded <Z-O> values larger than 1.907 Å (the likely bond length for < Z Al-O>). These large <Z-O> lengths indicate that Fe 2+ occurs at the Z site. The hypothesis of a dragging effect from <Y-O> to explain lengthening of < Z Al-O> is not supported by experimental evidence.

We have measured Raman spectra of the postperovskite (PPv) phase in CaIrO 3 up to 30 GPa to constrain the Grüneisen parameter (γ). We identified a total of 4 strong modes between 200 and 650 cm -1 , which is in contrast with the Raman spectra of Mn 2 O 3 and MgGeO 3 -PPv where at least nine different modes have been detected. We found no sign of a phase transition in the Raman spectra of PPv CaIrO 3 , which supports the stability of the PPv phase up to 30 GPa and room temperature in CaIrO 3 . The spectroscopic Grüneisen parameter, γ sp,0 = 1.66~1.72, constrained from our Raman data, is in excellent agreement with the thermodynamic Grüneisen parameter, γ th,0 = 1.75 ± 0.05, calculated from recent XRD measurements (Martin et al. 2007) on CaIrO 3 -PPv synthesized at high pressure and temperature similar to our starting material. Our result suggests that γ sp constrained by Raman measurements provides a reasonable estimate on the γ th of the PPv phase in CaIrO 3 .

The thermal behavior of synthetic richterite A Na B (NaCa) C Mg 5 T Si 8 O 22 (OH) 2 , crystallized at 800 °C and 0.35 GPa under hydrothermal conditions, was studied by serial powder diffraction experiments using synchrotron radiation between 123 and 873 K. The a, b, and β cell parameters show a non-linear behavior, whereas the c parameter shows a linear trend; values of the saturation temperature θ s are 410(3), 215(2), and 300(2) K for a, b, and asinβ, respectively. The axial expansion pattern below room temperature is α b > α c > α a , whereas above room T it is α b > α a > α c , the difference being related to the different saturation temperatures of the individual cell parameters. The thermal expansion was modeled following the Debye approximation for the density-of-state of phonons; the refined parameters are V 0 = 908.20 (0.04) Å 3 , k = 1 (4), Q 0 =32 (2) MJ, and the Debye temperature θ D = 586(31) K. The non-linear behavior at low T is well described without systematic differences between the data and the model. The volume thermal expansion coefficient αV changes significantly with temperature, also at temperatures higher than room temperature. It is suggested that this may occur also in other amphiboles and pyroxenes, requiring critical re-examination of the available data. A comparison of the strain tensor for the thermal expansion between amphiboles and pyroxenes shows that in amphiboles the major deformation occurs onto the (010) plane, whereas in pyroxenes it occurs along the b axis. Moreover, the major deformation on (010) in Ca-bearing pyroxenes occurs along the bond of the M2 cation with the furthermost O3 atoms, whereas in compositionally related Ca-amphiboles (i.e., tremolite) it occurs in a direction rotated by 10-20° to a*, i.e., in a direction not corresponding to that of the M4-O5 bonds. It is proposed that the M4 polyhedron contributes less to the thermal deformation of amphibole than the M2 polyhedron contributes to the thermal deformation of pyroxene.

The crystal structure of kelyanite, a rare mercury mineral that was found in oxidized mercuryantimony ores in the Kelyana deposit (Buryatia, Russia), has been determined. The preliminary formula of kelyanite was Hg 34 Sb 3 Cl 3 Br 1 O 28 (assuming the presence of both the Hg 1+ and Hg 2+ ). In contrast to this assumption, kelyanite appears to contain only monovalent Hg and its revised formula is (Hg 2 ) 6 (SbO 6 ) BrCl 2 . Kelyanite is trigonal, space group P3, a = 13.560(4), c = 10.004(6) Å, V = 1593(1) Å 3 , and Z = 3. In the structure, Hg atoms form six crystallographically independent pairs [dumbbells of composition (Hg 2 ) 2+ ] with Hg-Hg distances of 2.482(3)-2.519(2) Å. The Hg and O atoms form O-Hg-Hg-O systems with Hg-O bond lengths of 1.98(3)-2.33(3) Å and HgHgO angles of 140.3(7)-168.3(9)°. Mercury atoms in the (Hg 2 ) 2+ dumbbells have additional coordination to O, Cl, and Br atoms [Hg-O 2.62(2) Å, Hg-Cl 2.68(1)-2.97(1) Å, and Hg-Br 3.00(1)-3.55(1) Å]. Three crystallographically independent Sb atoms are octahedrally coordinated by O atoms with Sb-O distances of 1.96-2.14 Å. The (Hg 2 ) 2+ dumbbells link the (SbO 6 ) octahedra in a 3D structure.

Crystallographic orientations of coccoliths produced by the extant species Emiliania huxleyi and Gephyrocapsa oceanica, and of a fossil cocccolith, Watzunaueria barnesiae, all of which are mainly composed of calcite crystals with horizontally oriented c axes (termed R-units), were investigated using two electron diffraction techniques. According to electron backscattered diffraction (EBSD) analyses, the c-axis inclinations of all R-units were oriented about 10-30° from the sample substrate, which is approximately parallel to the organic base plate of the coccolith. However, the directions are toward the coccolith exterior in living coccoliths but toward the interior in fossil coccoliths. The crystallographic orientations of calcite crystals characterized by sub-vertically oriented c axes (termed V-units) were determined only for W. barnesiae and were similar to those of V-units in another living cococlith, Pleurochrysis carterae. Highly regulated crystallographic orientation and consistent chirality were evidenced for all the coccoliths with only a few degrees of variations by Kikuchi patterns analyses from TEM. To determine the indices of the crystallographic plane and edge direction that constitute distal and proximal shield elements, SEM stereo-photogrammetry in combination with EBSD analyses was applied. In the living coccoliths both shields contain the c-axis and the surface is estimated to be {21̅1̅0} with [481] the longer peripheral edge. On the other hand, the two shield surfaces of W. barnesiae are evaluated {101̅4} and the distal shield surface is surrounded by clear [481] edges. This indicates that the crystallography and morphology of R-units were not fixed for the past 230 million years. Based on these results, the calcite nucleation mechanisms are discussed with respect to the atomic arrangements on the organic template through all the coccoliths.

The effect of iron on the post-perovskite phase transition has been controversial. We have performed direct chemical analyses of co-existing perovskite and post-perovskite that were synthesized from an (Mg 0.91 Fe 0.09 )SiO 3 bulk composition using a laser-heated diamond anvil cell at pressures above 100 GPa and temperatures of 1700-1800 K. Analysis on quenched samples was carried out using the transmission electron microscope (TEM). The results demonstrate that crystalline perovskite grains are enriched in iron compared to adjacent amorphous parts presumably converted from post-perovskite. This indicates that ferrous iron stabilizes perovskite to higher pressures. The ferrous and ferric irons are likely to have competing effects on the post-perovskite phase transition, and therefore the effect of iron may be controlled by aluminum.

High-temperature synchrotron X-ray powder diffraction experiments for the composition of (Ca 0.06 Mg 1.94 )Si 2 O 6 have been carried out in the present study to clarify whether orthopyroxene has a transition between low- and high-temperature phases. Our results show that discontinuous changes of unit-cell dimensions and volume occur at 1170 °C during both heating and cooling processes and that the space group of Pbca does not change during this reversible phase transition. These facts indicate a first-order and isosymmetric phase transition. This high-temperature phase is thermodynamically distinct from the low-temperature phase, i.e., orthoenstatite in the Mg-rich portion of Mg 2 Si 2 O 6 - CaMgSi 2 O 6 phase diagram, although they have the same space group.