Volume 91, Issue 1

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

High-resolution Fourier transform infrared (FTIR) spectroscopy has been used to document the presence of hydrogen, to estimate its concentration, and to document its oxygen speciation in anorthoclase crystals and associated melt inclusions from Mount Erebus, Antarctica. Synchrotron-generated infrared radiation, 100 to 1000 times brighter than globar-generated infrared radiation, permits rapid collection of maps that depict relative intensities of a chosen FTIR band across the mapped area. Spectra and/or compositional maps showing variations in water concentration were collected from anorthoclase megacrysts and melt inclusions in the megacrysts. Studies of anorthoclase megacrysts involved collection of spectra from three mutually perpendicular sections cut from the crystals. FTIR spectra of anorthoclase crystals are characterized by a broad absorption band at approximately 3200 cm -1 in the mid-IR range. The universal mass absorption coeficient for mid-IR range feldspar spectra, established by Johnson and Rossman (2003), was used for quantitative estimates of water concentrations in the feldspar crystals based on integrated area under the 3200 cm -1 band. Water concentration in the anorthoclase sample was approximately 126 ppm, with an overall error of approximately ±30%. FTIR spectra of melt inclusions are characterized by a broad asymmetric absorption band at ~3550 cm -1 that was used to calculate total water concentration. The absence of a band at 1630 cm -1 suggests that water in the melt inclusions occurs as OH. rather than as molecular H 2 O. Absorption coeficients established by Mandeville et al. (2002) for H species in glass were used to calculate water concentrations in the melt inclusions. Melt inclusions in the Mt. Erebus anorthoclase have water concentrations ranging from 0.12 to 0.39 wt%, with an overall error of approximately ±15%. The ratio of water in anorthoclase crystals to water in the melt from which the crystals formed, based on this study, and at these low melt water concentrations, is approximately 1:10. However, water concentration varies significantly from one melt inclusion to another, possibly suggesting initial melt water heterogeneity. Maps of water concentration show that variations in water concentration within melt inclusions are associated with fractures that cut the melt inclusions and in some cases do not extend out into surrounding crystals or into crystal inclusions. Thin (~50 μm thick) zones of elevated water concentrations on the boundaries of the crystals in contact with melt inclusions suggest that water has diffused into the crystals from the melt inclusions.

Mutnovskite, ideally Pb 2 AsS 3 (I,Cl,Br), is a new mineral from the high-temperature fumaroles of the Mutnovsky volcano, Kamchatka Peninsula, Russian Federation. It occurs as microscopic rubycolored short-prismatic crystals up to 100 μm across, closely associated with halogen-sulfosalts of Pb, Bi, and As, Cd-Pb-Bi sulfosalts, pyrite, anhydrite, and cristobalite. Mutnovskite is transparent in thin fragments with a dark-red to blue color. The crystals are soft and fragile. Cleavage and fracture were not observed and the Mohs hardness is approximately 2. In reflected light mutnovskite is silvery lead-grey in color with an iridescent tarnish. Pleochroism and anisotropy are not visible because of the strong orange internal reflections, especially in immersion. Reflectance percentages measured in air in the range 400.700 nm were tabulated. Reflectance percentages (R min and R max ) for the four COM wavelengths are 34.2, 34.6 (470 nm), 33.2, 33.5 (546 nm), 32.5, 32.7 (589 nm), and 31.4, 31.7 (650 nm), respectively. A mean of four electron microprobe analyses gave Pb 62.0(3), As 11.0(4), Bi 0.6(1), S 14.4(2), Se 0.2(3), I 8.9(3), Cl 2.44(9), Br 1.1(7), Cu 0.03(2), Fe 0.01(1), total 100.7 wt%, corresponding, on the basis of a total of 7 atoms, to Pb 1.99 (As 0.98 Bi 0.02 ) Σ1.00 (S 2.98 Se 0.02 ) Σ3.00 (I 0.47 Cl 0.46 Br 0.09 ) Σ1.02 . The nine strongest powder-diffraction lines [d in Å (I/I 0 ) (hkl)] are: 4.69 (32) (002); 4.37 (67) (210); 3.34 (73) (020); 3.19 (100) (212); 2.715 (61) (022); 2.648 (66) (410); 2.539 (31) (213); 2.455 (29) (402); 1.894 (30) (232). Mutnovskite is orthorhombic, space group Pnma, with a = 11.543(1), b = 6.6764(7), and c = 9.359(1) Å, V = 721.3(1) Å 3 , Z = 4. The crystal structure was solved and refined to R = 4.14%. It consists of three independent cation positions: Pb1 and Pb2 have tricapped trigonal prismatic coordinations with S and I atoms (completed with one As atom in the case of Pb2), while As has threefold coordination with S atoms, which form the base of a trigonal pyramid with As at the apex. Pairs of Pb1-Pb2 prisms are connected in columns which extend along c. AsS 3 coordinations are isolated from each other. S atoms and half of the Pb atoms form wavy close-packed layers. Two kinds of channels parallel to b occur between the layers. The smaller channels host As atoms close to the channel walls, with their lone-electron pairs occupying the median part, while the bigger ones accommodate rows of alternating halogen and Pb atoms. The new mineral is named after the type locality, the Mutnovsky volcano, Kamchatka Peninsula, Russian Federation.

The crystal structure of alkali/water-poor beryl (H 2 O + Na 2 O + Cs 2 O < 1.2 wt%) was reinvestigated by means of laser ablation inductively coupled plasma mass spectroscopy, thermogravimetric analysis, neutron diffraction, and polarized infrared spectroscopy to determine the real topological configuration of the extra-framework content in the six-membered ring channels. Analysis of the nuclear density Fourier map suggests that the (water) oxygen is located along the sixfold axis at the 2a site (0,0,1/4), whereas the (water) protons are at -0.028(7), -0.071(3), 0.332(1). The hydrogen atoms are distributed in 6 × 2 equivalent positions, above and below the oxygen site. Geometrical configuration of the water molecule is well defined: the O-H bond distance is 0.949(18) Å and the H-O-H bond angle is 106.9(2.2)°. The H···H vector is oriented at -4° from [001]. This configuration is completely different from that found in alkali-rich beryl, where the H···H vector is perpendicular to [001]. Na is probably located, with the H 2 O oxygen, at the 2a site. According to the chemical analysis, which shows that the amounts of other alkali and earth-alkali cations are negligible (Rb, K, Mg, Mn ≤ 110 ppm, Ca ≤ 225 ppm, Cs ≤ 430 ppm), no effect of other cations on the extra-framework population was observed in the structural refinement. The final agreement index (R 1 ) of the structural refinement was 0.037 for 34 refined parameters and 160 unique reflections with F o > 4σ(F o ). The topological configuration of the H2O molecule into the channel is confirmed by the spectroscopic investigation. Polarized single-crystal IR spectra show that the H 2 O molecule is oriented with the molecular symmetry axis perpendicular to the hexagonal axis and H···H vector parallel (or quasi-parallel) to [001].

The heat capacity of synthetic anhydrous Mg-cordierite (Mg 2 Al 4 Si 5 O 18 ) was measured using adiabatic calorimetry between 6 and 300 K. Mg-cordierite was synthesized from a glass of stoichiometric cordierite composition at 1250 °C for one week. The refined cell dimensions using powder X-ray diffraction give a = 17.0553(2) Å, b = 9.7167(1) Å, and c = 9.3375(1) Å with V = 1547.428(1) Å 3 and a distortion index of Δ = 0.25 indicating complete Al-Si ordering. The absence of channel H 2 O was confirmed by powder IR spectroscopy, which did not show the presence of any bands located around 3600 cm -1 . The C P results values agree with those obtained previously by Weller and Kelley (1963) who measured the C P of Mg-cordierite between 50 K and 298 K. The calculated entropy of Mg-cordierite at standard condition is S 0 298.15 = 404.0 ± 1.6 J/(mol·K) using the CP data from this study. This value is in agreement with the value presented by Weller and Kelley [407.2 ± 3.77 J/(mol·K)], but appears to be very slightly lower than estimates made from various modeling studies incorporating phase-equilibrium data on Mg-cordierite.

Single-pass β ow-through (SPFT) experiments were conducted on a set of non-radioactive Ti-based ceramics at 90 °C and pH = 2 to 12. The specimens contained 27.9 to 35.8 wt% CeO 2 as a surrogate for UO 2 and PuO 2 . Compositions were formulated as TiO 2 -saturated pyrochlore (CeP1) and pyrochlorerich baseline (CePB1) ceramic waste forms. Pyrochlore + Hf-rutile and pyrochlore + perovskite + Hf-rutile constituted the major phases in the CeP1 and CePB1 ceramics, respectively. Results from dissolution experiments between pH = 2 to 12 indicate a shallow pH-dependence with an ill-defined minimum. Element release rates determined from experiments over a range of sample surface areas (S) and β ow rates (q) indicate that dissolution rates become independent of q/S values at 10.8 to 10.7 m/s. Dissolution rates dropped sharply at lower values of q/S, indicating rates that are subject to solution saturation effects as dissolved constituents become concentrated. Forward dissolution rates were 1.3(0.30) x 10 -3 and 5.5(1.3) x 10 -3 g/m 2 ·d for CeP1 and CePB1 ceramics, respectively. Dissolution rates obtained in other laboratories compare well to the findings of this study, once the rate data are placed in the context of solution saturation state. These results make progress toward an evaluation of CeO 2 as a surrogate for UO 2 and PuO 2 as well as establishing a baseline for comparison with radiation- damaged specimens.

An experimental study was carried out to investigate the equilibrium between Fe oxy-component and hydroxy-component in Ti-bearing calcic amphiboles, as described in the dehydrogenation/oxidation reaction Fe 2+ + OH - = Fe 3+ + O 2- + 1/2 H 2 , for which the equilibrium constant (K) can be expressed as where □ = H-vacancy on the O3 anion position, Φ is the activity coeficient term, and Kx represents the thermodynamic mole fraction term (i.e., the K expressed as mole fractions rather than activities). The variation in K x was quantified experimentally by annealing experiments on amphiboles of two different compositions: a mantle-derived kaersutite from Greenland, and a crustal pargasite from the Tschicoma Formation from the Jemez Mountains, New Mexico, volcanic complex. The conditions of the experiments ranged from 700.1000 °C, 1.10 kbar, and f H2 from that of the HM to GM solid buffer assemblages. The results, combined with similar data for a titanian pargasite from Vulcanʼs Throne, Arizona (Popp et al. 1995a), define the variation in log K x as a function of T, P, and amphibole composition as given by the equation: If the T, P, and amphibole composition are known, values of log K x calculated from the equation predict the equilibrium logf H2 of any experiment to within ~0.1 to 0.3 log units. It is assumed that a similar uncertainty in log f H2 would also to apply to the conditions of formation of natural amphiboles in the same composition range. If log f O₂ at the time of equilibration can be estimated independently for natural samples (e.g., mantle-derived amphiboles), the H 2 O activity also can be estimated. An alternate approach for estimating H2O activity from amphibole-bearing mantle rocks is to use a variety of H 2 O-buffering equilibria among end-member components in olivine, two-pyroxenes, amphibole, and other phases: e.g., 2 tr +2 fo = 5 en + 4 di + 2 H 2 O. A self-consistent thermodynamic database (THERMOCALC, Holland and Powell 1990) can be used to determine the a H₂O of such univariant H 2 O-buffering equilibria as a function of P and T. A mantle amphibole assemblage from Dish Hill (sample DH101-E, McGuire et al. 1991) was used to calculate a H₂O using the two different methods. The mean value of log a H₂O determined from seven different dehydration reactions is .1.70, with a 1σ range of ±0.50. That range of water activity is in good agreement with the value of log a H₂O = .1.90 ± 0.3 obtained using the dehydrogenation/oxidation equilibrium, along with an estimate of log f O₂ . The use of xenolith amphiboles to infer values of a H₂O in the mantle requires that the H content of the amphibole does not change significantly during ascent or eruption. Changes in H content have significantly different effects on the dehydration and dehydrogenation equilibria, such that, comparison of the a H₂O estimates from the two different methods may permit quantification of H loss.

Fe 2+ /Fe 3+ charge ordering in the contact layers of lamellar magnetism in the hematite-ilmenite series is not a postulate of the lamellar magnetism hypothesis. Such ordering is possible, however, and a model was suggested earlier in which contact layer Fe 2+ octahedra share faces with ilmenite layer Ti 4+ octahedra, and contact layer Fe 3+ octahedra share faces with hematite layer Fe 3+ octahedra, thus copying the shared-face configurations of ilmenite and hematite, respectively. This model and related charge-balance matters could be explored using bond-valence theory, a simplified picture of complex bonding which takes into account the relationship between bond strength and bond distance, but ignores magnetism. This has now been done, and shows that local oxygen charge satisfaction is strongly favored by a different charge-ordering scheme, in which contact layer Fe 3+ octahedra share faces with ilmenite layer Ti 4+ octahedra, and contact layer Fe 2+ octahedra share faces with hematite layer Fe 3+ octahedra. A new, more sophisticated, Monte Carlo simulation of the interface, including electrostatic and magnetic interaction parameters of cations, reported in detail elsewhere, independently shows the same charge-ordering scheme. For the cation ordered metastable ilmenite 50 composition, bond-valence theory indicates that, unlike the contact layers, the favored charge ordering scheme in Fe layers would have the same shared-face configurations as ilmenite and hematite.

A Late Pan-African calc-alkaline dike swarm (basalt-andesite-dacite-rhyolite) has been investigated in a region of over 2000 km 2 in the Alpine Danubian window, South Carpathians (Romania). Amphibole phenocrysts and microphenocrysts have been investigated by wavelength-dispersive microprobe analysis and BSE imaging. The Ca-amphibole population, represented in all the lithologies, displays a large compositional range, interpreted as the result of two processes: (1) magmatic evolution (kaersutite → Ti-pargasite → pargasite → Ti-magnesiohastingsite → magnesiohastingsite → edenite → tschermakite → magnesiohornblende) linked to magmatic differentiation from andesitic basalt to rhyolite; and (2) deuteritic alteration of the primary amphibole related to late-emplacement hydrothermal activity (yielding numerous varieties comprising those cited above). In all rock types, amphibole phenocrysts equilibrated at a nearly constant pressure of about 0.6 ± 0.1 GPa, but their temperatures of crystallization ranged from 1000.900 °C for basaltic andesites to 700.600 °C for dacites. In rhyolites, edenite to magnesiohornblende crystals reflect a continuous range of P-T conditions from 700 °C/0.6 GPa to 600 °C/0.1 GPa, in agreement with their change of habit from euhedral to subhedral. Complex zonations in pargasite-magnesiohastingsite (including resorption) are interpreted in term of self-organization of oscillatory zoning without significant heating and/or magma mixing.

Iron concentrations in rehomogenized (remelted) melt inclusions hosted in forsterite-rich olivine (Fo88.92) from an alkalic lava have significantly higher Fe contents (FeO* up to 21 wt%) than found in naturally quenched inclusions, matrix glasses, and bulk lava compositions (6.21.6.66 wt% FeO*). The main objectives of this study were to: (1) determine the source of the anomalous Fe concentrations, and (2) evaluate the significance of this signature with respect to rehomogenization of melt inclusions. Heating experiments conducted from 1125 to 1225 °C on crystallized inclusions show that Fe and Mg contents in rehomogenized inclusions increase with homogenization temperature, consistent with dissolution of olivine + magnetite in a 1:1 atomic ratio. The dissolution of magnetite contributes significant excess Fe to the homogenized inclusions, and thus the high FeO* contents of the glasses do not reflect the original composition of the trapped melt. The addition of excess Fe also dilutes the concentrations of other major elements, especially evident in SiO 2 , Al 2 O 3 , and CaO wt%. Although the cause of magnetite formation in the inclusions is unresolved, two models (H+ diffusion and co-entrapment of magnetite) for the formation of significant volumes of magnetite are considered. One of the most significant conclusions for this study is that magnetite formation occurred prior to rehomogenization and that the magnetite did not result from post-entrapment fractional crystallization of the inclusion. For these inclusions, a correction must be made for the dissolution of magnetite (± pyroxene) and olivine to the silicate melt to accurately reconstruct the original Fe content of the melt.

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

Cryolite, Na 3 AlF 6 , is the most abundant aluminofluoride mineral in highly evolved felsic suites and their pegmatites, but its phase transitions and thermodynamic properties at elevated pressures are unknown. We used a simple modification of the TZM pressure vessel to perform differential thermal analysis of cryolite at high pressures. Temperatures of the α-β transition are as follows: 559.30 ± 0.23 °C (1 atm), 562.10 ± 0.28 °C (47 MPa), and 567.33 ± 0.23 °C (101 MPa). Cryolite melting temperatures increase as follows: 1011.4 ± 0.2 °C (1 atm), 1019.2 ± 0.4 °C (50 MPa), and 1028.7 ± 0.4 °C (100 MPa). Both pressure-temperature relationships are linear: (dT/dp) α-β = 78.4 ± 8.4 °C/GPa and (dT/ dp)m = 174 ± 12 °C/GPa. Application of the Clapeyron relationship leads to the following volumetric changes: ΔV α−β = 0.089 ± 0.019 J/(mol·bar) and ΔVm = 1.49 ± 0.12 J/(mol·bar). Despite the significant self-dissociation in the cryolite liquid, melting sensu stricto (without dissociation) dominates the heat and volumetric changes during melting in comparable amounts: 83.3 ± 6.7 % ΔHm and 68 ± 15 % ΔV m and suggests that the degree of dissociation has no significant effect on the (dT/dp)m. Evaluation of previous and current volumetric data for cryolite polymorphs leads to Vβ,1284 = 8.49 ± 0.17 J/(mol·bar); coeficients for the volumetric thermal expansion in the form of the third-order polynomial equation are: V 298 = 7.080 ± 0.012 J/(mol·bar), a 1 = (1.39 ± 0.20)·10 -4 K -1 , a 2 = (.2.15 ± 0.51)·10 -7 K -2 , and a 3 = (2.68 ± 0.34)·10 -10 K -3 . The total (dT/dp)m of cryolite is very similar to that of villiaumite (NaF), whereas ΔV m /V β,1284 of cryolite is smaller than for other alkali halides (NaF, NaCl).

The crystal chemistry of the new mineral dissakisite-(La), the Mg-analogue of allanite-(La) with ideal formula CaLaAl 2 MgSi 3 O 12 (OH), was studied by single-crystal X-ray diffraction and Mössbauer spectroscopy. Diffraction data indicated that it has a partially metamict state with expanded cell parameters a = 8.959, b = 5.7226, and c = 10.232 Å, β = 115.19°, and V = 474.7 Å 3 . The crystal structure was refined to R(I/σ(I) > 4) of 3.45% and wR2 of 8.52% (S.G. P2 1 /m). The displacement parameters are unusually large as consistent with positional disorder. Ca and REE are partitioned between the A1 and the A2 sites, Fe 2+ between the M3 and the A1 sites, and Fe 3+ between the M3 and the M2 sites. This confined to A1. The Mössbauer spectrum implies the presence of five different iron sites, three of which are attributed to Fe 2+ at the M3 site, one to Fe 2+ at the A1 site, and one to Fe 3+ at a rather distorted octahedral site. Annealing at 700 °C caused contraction of cell edges (b and c shrank by 0.5%, a by 0.3%, β only slightly decreased; ~1% in V) and of bond distances, with a consequent overall increase in bond valences. Displacement parameters decreased by about 30%, leading to a significant increase from 1495 to 1627 in the number of reflections with I/σ(I) > 4. Structure rearrangement was achieved by means of flexible points such as the two-coordinated oxygen atoms O8 and O9, and by greater volume decreases in M3, A1, A2, and Si3 polyhedra. Si tetrahedra behaved non-rigidly, at variance with what was observed for isomorphous substitution and thermal expansion. Despite the high Th content and the age of the mineral, the crystal structure is well preserved. The above paradox could be explained by a thermal event after formation of the mineral that reversed previous damage, or by a prolonged period under relatively high-temperature conditions.

Metallic coils (in most cases Cu coils) are often used in high-pressure experiments to protect thermocouple wires. In this paper we show that these coils have important influences on thermocouple EMF and therefore on the temperature measurements. We tested this effect by measuring EMF from Cu coiled single wires of chromel and alumel, and, further, we conducted experiments to compare the EMF from W5Re-W26Re thermocouples with and without Cu coils attached to them. The results show systematic differences in thermocouple readings; the EMFs from W-Re thermocouples with Cu coils give systematically lower values than EMFs from thermocouples without Cu coils. The results were analyzed using a simple model. The difference in thermocouple EMFs between thermocouples with and without protection coils is given by where E i and R i are the EMF and the electrical resistance of metal i in the portion of the Cu coil, and the subscripts 1+, 1-, and 2 indicate positive thermocouple metal, negative thermocouple metal, and coil metal, respectively. The addition of a coil with different metal has a large effect . the ΔE TC will be close to .(E 1+ - E 1- ) < 0 when the resistance of the coil is significantly smaller than that of a thermocouple wire. For a Cu coil and W-Re thermocouple, R 1+, 1- >> R 2 and therefore thermocouple readings with a Cu coil will lead to underestimation of the real temperature. Under common experimental conditions with a multi-anvil apparatus, the error in the temperature estimate caused by Cu protection coils is ~100.150 K for a peak temperature of 1600.2000 K.

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

The phase transformation from ilmenite to perovskite in FeTiO 3 was directly observed using synchrotron-based X-ray diffraction and a large-volume press. The perovskite phase is temperature quenchable at 20 GPa and converts into the LiNbO 3 phase at pressures below 15 GPa at room temperature. The LiNbO 3 phase transforms into the ilmenite phase at 10 GPa and 673 K. However, the back-transformation from the ilmenite to the LiNbO 3 phase was not observed, thus strongly suggesting that the LiNbO 3 phase is not thermodynamically stable but rather a retrogressive phase formed from perovskite during decompression at room temperature. By cycling the pressure up and down at temperatures between 773 and 1023 K, the perovskiteilmenite transformation could be observed in both directions, thus confirming that perovskite is the true high-pressure phase with respect to the ilmenite phase at lower pressures. The phase boundary of the perovskite-ilmenite transformation thus determined in this study is represented by P (GPa) = 16.0 (±1.4) . 0.0012 (±0.0014) T (K), which is inconsistent with P = 25.2 . 0.01 T (K) reported previously (Syono et al. 1980). The discrepancy could be attributed to the different experimental methods (i.e., in situ vs. quench) used in the two studies. The ilmenite-perovskite phase boundary with such a small slope would potentially serve as a useful geobarometer for ilmenite-bearing rocks derived from the deep mantle or for those shocked in meteor craters.

In this paper we explore the structure and physical properties of brucite over a wide range of pressures with density functional theory using the variable cell-shape plane wave pseudopotential method in the local density (LDA) and generalized gradient (GGA) approximations. We probe the energetics underlying the structure and dynamics of the proton sub-lattice by performing a series of constrained and unconstrained static calculations based on an energetically stable √3×√3×1 super-cell wherein proton locations are related to the 6i Wyckoff sites as opposed to the ideal 2d site. The displacement of the hydrogen atom from the threefold axis increases with increasing pressure. This means that even in the absence of thermal energy, the protons are frustrated and would be expected to exhibit long-range disorder akin to a spin glass. To shed light on the dynamic nature of the proton hopping between the 6i-like sites, we determined the activation energy barrier for such jumps. We found that the energy barrier increases with compression, possibly indicating a transition from dynamic proton disorder at lower pressures to static disorder at higher pressure. We have also investigated the possibility of proton jumps across the interlayer, by determining the potential energy well along the O···O vector. We infer that proton jumps across the interlayer are either severely limited or highly cooperative since we did not find any evidence for a double well along the O···O vector. The absence of a double well along the O···O vector, the evolution of O-H···O distances with compression, and the gradual transition to a symmetric O-H···O configuration, all argue for weak hydrogen bonding in brucite.

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

Autunite-group minerals have been frequently identified in contaminated sediments as the long-term controlling phase of U. Under these conditions, the mobility of U in subsurface pore waters is limited by the rate of dissolution of autunite and meta-autunite group minerals, X 3-n (n)+ [(UO 2 )(PO 4 )] 2 ⋅xH 2 O, where X = Ca or Na. Single-pass ß ow-through (SPFT) tests were conducted to quantify the dissolution kinetics of natural Ca meta-autunite, Ca[(UO 2 )(PO 4 )] 2 ⋅3H 2 O, and synthetic Na meta-autunite, Na 2 [(UO 2 )(PO 4 )] 2 ⋅3H 2 O, as a function of pH (7.10) and temperature (5.70 °C) in the presence and absence of aqueous organic material. The data indicate that release of U and P are non-stoichiometric over the range of experimental conditions investigated. In a 0.1 M NH 4 OH buffer solution, acquisition of valid dissolution rate data was limited by uramphite solubility, NH 4 [(UO 2 )(PO 4 )]2·xH 2 O. Dissolution rates obtained in a 0.01 M TRIS [tris (hydroxymethyl) aminomethane] buffered solution increased by a factor of ~100× over the pH interval of 7 to 10 (η = 0.90 ± 0.08), irrespective of temperature. At constant pH the rate data showed a minor increase with temperature. Data from experiments using a more concentrated 0.05 M TRIS buffer exhibited a ~35-fold increase in rates compared to those in a 0.01 M TRIS buffer at constant temperature and pH. The difference in release rate between interlayer cation (Na + or Ca 2+ ) and U is ~10 000 in neutral solutions; however, the difference diminishes to ~10 at higher pH values. The combination of structural dissolution and ion exchange explain these trends in interlayer cation behavior. Data presented here illustrate the significance of pH and dissolved organic material on the dissolution of autunite minerals.

This study aims to contribute to a better understanding of the nature and evolution mechanism of interstratified clay minerals. We examined the <2 μm or <0.2 μm size fraction of interstratified kaolinite- smectite (K-S) formed by hydrothermal and hydrogenic alteration of volcanogenic material from a Tortonian clay deposit (Almería, Spain), a weathered Eocene volcanic ash (Yucatan, Mexico), and a weathered Jurassic bentonite (Northamptonshire, England). The methods used were X-ray diffraction analysis (XRD) of random and oriented preparations, thermogravimetry, chemical analysis, and 29 Si MAS nuclear magnetic resonance. The proportions of kaolinite and smectite in K-S (%K) were determined by fitting the XRD patterns of ethylene-glycol-saturated samples with patterns calculated with the NEWMOD computer program. The obtained range of compositions is 0.85%K. A comparison of the results from the various techniques showed non-linear relationships, indicating that the layers in K-S are complex and hybrid in nature. The smectite-to-kaolinite reaction is a solid-state transformation proceeding through formation of kaolinite-like patches within the smectite layers. The process consists of several non-simultaneous stages: (1) removal of parts of the tetrahedral sheet, resulting in formation of kaolinite-like patches; (2) layer collapse to ~7 Å where the kaolinite-like patches are sufficiently large; (3) Al for Mg substitution in the octahedral sheet, simultaneous or slightly delayed with respect to layer collapse, causing a layer-charge decrease and loss of interlayer cations; (4) Si for Al replacement in the tetrahedral sheet and further loss of interlayer cations. Iron remains in the kaolinite or is lost at the latest stages of the process.

The kinetic behavior of the α-Ni 1-x S → β-NiS transition was investigated via a series of annealquench experiments using Rietveld quantitative phase analysis of powder X-ray diffraction data. Initial compositions of α-Ni 1-x S were found to play an important role in the kinetics of the transition. The activation energy (Ea) for this α- to β-phase transition is 16.0 (±0.5) kJ/mol for NiS in the temperature range 343 to 423 K, and 13.0 (±0.5) kJ/mol in the temperature range 523 to 623 K. For Ni 0.97 S, however, E a decreases from 73.0 (±0.5) to 17.0 (±0.5) kJ/mol over the course of the reaction in the temperature range 573 to 593 K. The relationship between E a and extent of transition (y) for the initial bulk Ni0.97S was derived using the Refined Avrami method. For Ni-deficient compositions, α-Ni 1-x S, the transformation to β-NiS is accompanied by the exsolution of a progressively more Ni-deficient α-Ni 1-x S and Ni3S4, and the reactions become more sluggish for more metal-deficient compositions.

Mt. Vulture trioctahedral micas-1M mainly consist of phlogopite-annite solid solutions with a minor component of brittle micas. However, both Li-free and Li- and F-rich compositions may coexist in the same volcano-stratigraphic level. We report the results of electron microprobe analysis (EMPA), secondary ion mass spectrometry (SIMS), and single-crystal X-ray diffraction (SCXRD) for three crystals (LC7-27Go, LC7-3Go, and LC7-1R) that are representative of both compositions. EMPA and SIMS showed that sample LC7-1R is richer in TiO 2 , Li 2 O, and F and poorer in H 2 O than the other samples. Structure refinements using anisotropic displacement parameters for the three samples, performed in space group C2/m, converged at 3.46 ≤ R ≤ 4.34, 3.89 ≤ R w ≤ 4.39. When considering bond distances, the three samples can be described as homo-octahedral whereas mean atomic numbers suggest that only LC7-1R is meso-octahedral. Significant differences occur among the samples for some distortion parameters commonly used for micas. In particular, LC7-1R displays higher values of BLDM2, shiftM2 and lower values of Δ K-O4 and t K-O4 than those of samples LC7-3Go and LC7-27Go. These differences are ascribed to F- and Ti-substitutions. Cation distributions were obtained by combining EMPA, SIMS, and SCXRD data after analysis of the effect of normalization schemes commonly used in mica formula recalculation. In LC7-3Go and LC7-27Go, Ti is incorporated according to the Ti-Tschermak mechanism, whereas more than one Ti-substitution mechanism occurs in LC7-1R. For the latter sample, octahedral vacancies are present leading to a complex substitution pattern and complex structural distortions related to the special octahedral compositions that were determined.

A shell-like polycrystalline grain (ca. 1 mm) of W-(Mo)-bearing Os-Ir alloy (11.4.18.6 wt% W; up to 1.5% Mo) is present in a very old collection (probably the 1890s) of tiny nuggets from Trinity Co., California. An extensive compositional series [(Os 0.43-0.80 Ir 0.28-0.05 ) W 0.12-0.18 ], and inverse Ir-Os correlation, are observed; the mean composition [Os 0.676 W 0.153 Ir 0.124 Fe 0.021 Mo 0.015 Ru 0.011 ; Σatoms = 1], based on results of 50 electron-microprobe analyses, displays a ratio (Os + Ir):W of 5:1. The observed variations and element correlations suggest that (W + Mo) contents are controlled by Ir, and incorporated via the following substitution scheme: [(W + Mo) + Ir] ↔ Os. The X-ray diffraction data indicate that the W-rich alloy has a hexagonal close-packed structure, related to that of osmium and allargentum, with a = 2.7297(4) Å, c = 4.3377(6) Å, and V = 27.99(1) Å 3 ; the c:a ratio is 1.59. The probable space-group is P6 3 /mmc, and Z = 2; the calculated density is 21.86(1) g/cm3. The W-rich alloy is associated with an Os-Ru-Ir alloy rich in Fe (7.0.9.7 wt%), which exhibits atomic Fe ↔ [Os + Ru] and Ir ↔ [Os + Ru] mechanisms of substitution. We suggest that these W-(Mo)- and Fe-rich alloys formed by metasomatic alteration of a primary Os-Ir-Ru alloy, associated with mineralized ultramafic-mafic rocks of ophiolite afinity. A fluid phase may well have remobilized and transported W, Mo, and Fe. The W-rich alloy likely crystallized from a reducing fluid under conditions of low fugacities of O 2 and S 2 , thus promoting the observed siderophile behavior of W and Mo. These unusual W-(Mo)- and Fe-rich alloy grains were likely derived, as a placer material, from the Trinity ophiolite complex of northern California.

A new occurrence of schreyerite, V 2 Ti 3 O 9 , has recently been found in metamorphic rocks of the Sludyanka complex at the southern shore of Lake Baikal, Russia. In contrast to previously known schreyerite lamellae, which are intergrown with rutile, crystals from the Sludyanka complex occur as isolated single crystals associated with titanite, allowing single-crystal X-ray diffraction experiments. The chemical composition was determined with an electron microprobe giving the composition (V 1.785 Cr 0.157 Fe 0.036 )(Ti 2.536 V 0.468 )O 9 . A peculiarity of this schreyerite sample is the partial substitution of V 4+ for Ti 4+ . The crystal structure was determined by single-crystal X-ray diffraction and was refined in the monoclinic space group C2/c [a = 17.102(2), b = 5.0253(5), c = 7.0579(8) Å, β = 106.636(10)°] to R 1 = 2.84%. The structure is in agreement with the qualitative model of Grey et al. (1973) determined on the basis of electron diffraction and X-ray powder data for synthetic (Fe,Cr) 2 Ti 3 O 9 . Reinvestigation of schreyerite from the type locality indicates that this sample has cell dimensions and symmetry corresponding to the Sludyanka sample. The structure of schreyerite may be considered as a 1:1 polysome composed of slabs of berdesinskiite, V 2 TiO 5 , and Ti 2 O 4 (a high-pressure phase of TiO 2 with α-PbO 2 structure).

Here we report on a newly developed, large-volume, cold-sealed capsule design for hydrothermal synthesis experiments in a piston-cylinder apparatus that should be useful for the production of synthetic fluid inclusions at pressures and temperatures not previously attained in gas- or fluid-pressurized reaction vessels. The design is adapted for large-volume experiments using a 30 mm internal-diameter pressure vessel, but can be scaled down to suit smaller pressure vessels, e.g., 15.9 mm (5/8’’) internal diameter, if required. Calibration experiments show that temperature varies ±5 °C over the length of a 30 mm (length) × 15 mm (diameter) Cu capsule. The design incorporates the thermocouple within the capsule mass to optimize temperature control. Quartz-hosted H 2 O inclusions were synthesized over a range of conditions. Fluid-inclusion densities are consistent with the nominal experimental conditions, suggesting a friction correction is not required. This approach has several advantages over conventional hydrothermal experimental methods: (1) substantially higher pressures are attainable in piston-cylinder than hydrothermal and gas-media apparatus; (2) cold-sealing capsules avoid potential problems associated with welded capsules, such as solution modification; (3) capsule fluids are readily sampled ex situ; (4) the use of relatively thick-walled capsules minimizes H 2 -losses during experiments; (5) synthetic fluid inclusions can be used to derive fluid PVTX properties by combining conventional thermometry with analyses of individual fluid inclusions or independent mineral solubility data.

Syntheses along the Mg 3 Al 2 (SiO 4 ) 3 -Na 3 Al 2 (PO 4 ) 3 join were carried out in the 15.17 GPa range at temperatures between 1200 and 1600 °C. An Na 3 Al 2 (PO 4 ) 3 compound of garnet-like structure, with a cubic-cell parameter of 11.579(2) Å, was synthesized and characterized. Intermediate compositions are also found to crystallize in the garnet structure. Between pyrope and Na 3 Al 2 (PO 4 ) 3 , the cubic unit-cell volume increases by 3.1%. In addition, nominal compositions, Mg 2.5 Na 0.5 Al 2 Si 2.5 P 0.5 O 12 and Mg 2 Na 1 Al 2 Si 2 P 1 O 12 , were run at 3 GPa and 875 °C. They yielded garnet, among other phases, but with no significant phosphorus incorporation as indicated by a pyrope-like unit-cell volume. Phosphorus content in pyrope from the UHP Dora-Maira quartzites appears as a potential pressure indicator where UHP garnet coexists with phosphate minerals. The P-content in upper mantle garnets can be used as a probe of phosphorus activity in the host rock.