Band 83, Heft 12

The speciation and diffusion of sulfur in nominally dry albite melt at 1300-1500 °C has been investigated by analyzing glasses formed in piston-cylinder runs using electron microprobe, micro-Raman, infrared-, UV-, and visible-light-spectroscopy, and other techniques. The sulfate ion is very stable in the albite melt even at low f o₂ and is the dominant species in all glasses. In the presence of graphite the glass acquired a characteristic strong violet color, as a consequence of conversion of some of the sulfate to S - 2 and S - 3 radical anions. The equilibration between S - 2 and S - 3 was rapid and both radicals were found to be very stable at elevated temperatures. Both S - 2 , S - 3 , and sulfide diffuse faster then the sulfate anion, so these species will control the diffusion process when present. In violetcolored albite glass in which some of the sulfur is S - 2 and S - 3 , the diffusion coefficient for bulk sulfur diffusion was found to be: where T is the temperature in K and R the gas constant. This relation defines D values significantly lower than those determined for sulfur in dry andesite, dacite, and rhyolite melt, in which the diffusion is controlled by sulfide. The reason for this difference is the higher degree of melt polymerization, the stabilization of sulfate over sulfide in the albite melt, and the slow diffusion of S - 2 and S - 3 radical anions compared to sulfide.

Solid binary oxides of the platinum-group elements (PGE) decompose on heating in air to metal, oxygen, and often, a mixture of volatile PGE oxides. Compared to most other transition metal oxides, these decomposition temperatures are low, generally below 1100 °C. The modest exception is RuO 2 , which remains stable in air until 1400 °C. Because of its comparatively high oxide stability, Ru, among all the PGE, is the one with the greatest potential for a significant oxidic, i.e., lithophile, geochemistry. A series of 1 atm phase equilibrium experiments were made to establish the limits of solid solution of Ru in hematite (Fe 2 O 3 ) and help evaluate the significance of the oxidic geochemistry of Ru. The solid-solution limit for Ru in hematite may be represented by the following oxidation- reduction reaction: 2RuO 2 + Fe 2 O 3 = 2FeRuO 3 + 1/2O 2 . Experiments equilibrated between 1012 and 1490 °C were buffered with respect to oxygen fugacity by the coexistence of Ru metal and solid RuO 2 , both as essentially pure phases. Solution of Ru in hematite is enhanced at lower oxygen fugacity, but reaches a maximum fixed by the presence of Ru metal. An endothermic enthalpy for the reaction (188.1 ± 0.7 kJ/mol) was retrieved from the redox-controlled solubilities was as a regular solution interaction parameter (18.5 ± 0.5 kJ/mol) that indicated small positive deviations from ideality in the solid-solution region investigated. Estimates based on the new data for solubilities of Ru in a rhombohedral oxide at geologic conditions suggest that natural concentrations might be accommodated at high temperatures without saturation in Ru metal, but that exsolution of Ru metal from oxides would be favored at low temperature.

In many cases the grossular content of garnet and/or the anorthite content of the plagioclase used in thermobarometry are very low, leading to a large pressure uncertainty. The dependence of this uncertainty on mole fractions of grossular and anorthite is evaluated by propagating uncertainties in composition and activity terms into the GASP geobarometer for a series of hypothetical rocks equilibrated at 550 °C and 6 kbar. Results are ± 0.65 kbar (1 standard deviation) at high mole fractions (X grs = 0.15, X an = 0.98), increasing to ± 1.55 kbar at low mole fractions (X grs = 0.03, X an = 0.133. Specific results vary depending on errors chosen, P-T conditions, thermodynamic database, and activity models used, but an overall trend of increasing uncertainty with decreasing mole fraction is robust. These theoretical conclusions are supported by a data set of 42 amphibolite facies metapelitic samples for which pressure and temperature were determined with and without grossular-anorthite-bearing equilibria. If grossular and anorthite mole fractions are large the difference in P determination is low (< 0.5 kbar), but if these mole fractions are < 0.10 and 0.30, respectively, the difference in P determination is variable and can be high (> 3 kbar). A general guideline is that if the product of grossular and anorthite mole fraction is < 0.05, then grossular-anorthite-bearing equilibria should be used only with great caution.

Phase relations on the join CaTiSiO 5 -CaSi 2 O 5 were determined at 1350 °C over the pressure range 3.5-12 GPa by a combination of synthesis and reversal experiments in a piston cylinder and a multi-anvil press. Titanite-like phases were recovered from all experiments in this pressure range. At 3.5 GPa the maximum Si VI content of titanite is 3.0 ± 0.6 mol%, whereas bulk compositions with higher Si content yield a mixture of titanite solid solution plus coesite and walstromite-structured CaSiO 3 . The maximum Si VI content of the titanite increases with pressure to 21 ± 2 mol% at 7 GPa and 46 ± 2 mol% at 7.5 GPa. At pressures of 8.5 to 12 GPa all intermediate compositions yield a single titanite phase. X-ray and TEM analysis show that these have the A2/a symmetry of the titanite aristotype. The variations of the room-pressure unit-cell parameters of the A2/a phases with composition can be described by the equations a [Å] = 7.040(9) - 0.492(15) X(Si VI ); b [Å] = 8.713(7) - 0.316(11) X(Si VI ); c [Å] = 6.564(4) - 0.220(7) X(Si VI ); ß [°] = 113.721(6) - 0.537(12) X 2 (Si VI ); V (cell) [Å 3 ] = 367.8(9) - 47.5(1.6) X(Si VI ). For the CaSi 2 O 5 composition the recovered material has I1̅ symmetry but is known to transform back to A2/a titanite structure at 0.2 GPa at room temperature. Similarly, with increasing pressure the P2 1 /a CaTiSiO 5 transforms to A2/a symmetry at 3.6 GPa at room temperature. The conclusion is that at 1350 °C and pressures in excess of 8.5 GPa there is complete solid solution between CaTiSiO 5 and CaSi 2 O 5 based upon the isovalent exchange of Si for Ti in the octahedral site of the A2/a structure. Rietveld structure analysis of intermediate compositions reveals no evidence for ordering or intermediate phases. Preliminary experiments at pressures between 13.5 GPa and 16 GPa yielded mixtures of titanite solid solution plus perovskite and stishovite. From these data and information on the phase relations for the CaSiO 3 -CaTiO 3 join the topology of the phase relations between ~ 3 and ~ 13 GPa in the central part of the CaO-TiO 2 -SiO 2 ternary have been deduced.

The high-pressure behavior of the clinopyroxenes kanoite Mn 0.9 Mg 1.1 Si 2 O 6 , MnSiO 3 , and CrMgSi 2 O 6 (all space group P2 1 /c) was studied by single-crystal X-ray diffraction in a diamond-anvil cell at room temperature. Phase transitions from P2 1 /c to C2/c clinopyroxene were found and reversed at 5.06 ± 0.14 GPa for kanoite, 2.905 ± 0.005 GPa in MnSiO 3 , and 3.60 ± 0.03 GPa in CrMgSi 2 O 6 . The phase transitions are first-order in character and are accompanied by a volume decrease of approximately 2.3%. The structure of high-pressure C2/c kanoite was determined from single-crystal X-ray intensity data collected at 5.20 GPa, and is very similar to that of high-pressure (HP) C2/c-ferrosilite. Although the space group C2/c is the same as for the high-temperature (HT) kanoite form, the two phases have significantly different structures. The silicate chains are extremely kinked in HP kanoite while they are almost straight in HT kanoite. Compared with the transition pressure of clinoenstatite-clinoferrosilite, the new data suggest that the effective ionic radii of M1 and M2 cations do not exclusively control the transition pressure and that the HP C2/c clinopyroxenes with Cr 2+ and Fe 2+ gain additional stabilization energy from crystal field effects.

Intercalation of an ordered kaolinite with potassium acetate (KCH 3 COO) under a pressure of 20 bars and 220 °C, induced new Raman bands at 3590, 3603, and 3609 cm -1 in addition to the normal kaolinite bands. These bands are attributed to the inner surface hydroxyls hydrogen bonded to the acetate. It is proposed that the intercalation under 20 bars pressure at 220 °C caused the differentiation of the inner surface hydroxyl groups, resulting in the appearance of these additional bands. Diffuse reflectance infrared spectra of the potassium acetate intercalated kaolinite that was formed at 20 bars and at 220 °C showed new bands at 3595 and 3605 cm -1 . Upon formation of the intercalate at 2 bars and at 120 °C additional infrared (IR) bands were found at 3592, 3600, and 3606 cm -1 . These IR bands correspond well with the observed Raman spectra. It is proposed that the effect of intercalation of the highly ordered kaolinite under pressure caused the kaolinite to become disordered and this disordering was dependent on the temperature of intercalation. It is suggested that when pressure is applied to the kaolinite crystal in the presence of an intercalating agent, the hydrogen bonds between adjacent layers are broken to create space for the intercalating agent between the layers. A direct result is that the order of the kaolinite crystals shows a decrease resulting in more defect structures. This is evidenced by the additional spectroscopic bands in both the Raman and IR spectra.

Interstratified illite-smectites (I/S) and illite-smectite-vermiculites (ISV) representing both hydrothermal and diagenetic transformations and having different degrees of structural order were investigated for cis-trans occupancy in the octahedral sheet by X-ray diffraction (XRD) and by differential thermal analysis (DTA) in combination with evolved water analysis (EWA) using an infrared detector. By XRD, the amounts of cis (w cv ) and trans (w tv ) vacant 2:1 layers were determined for the three-dimensionally ordered samples using both the WILDFIRE simulation program and calculations based on positions of the 11l and 11l̅ reflections. Based on the EWA curves, the I/S and ISV could be divided into three groups having (1) one strong and one or more weak EWA peaks; (2) two well-resolved peaks; and (3) a complex EWA curve. The amounts of cis- and trans-vacant sites were determined by peak fitting of the total dehydroxylation curve. The complex EWA curves were, however, in addition split into separate dehydroxylation processes during a stepheating technique. If the EWA peaks below and above 600 °C were attributed to trans vacant (tv) and cis vacant (cv) octahedra, respectively, the w cv values determined by XRD and by EWA were in agreement. For the three-dimensionally ordered minerals, both XRD and EWA should be used, whereas the EWA method can be applied to the structurally disordered samples having no diagnostic 11l reflections. Accordingly, a combination of XRD and EWA for the determination of w cv and w tv supports an evaluation of the mechanism of illitization in various geological environments. Thus, significant changes in w cv and w tv during illitization are likely due to a dissolution-precipation, whereas almost constant values indicate a solid-state transformation.

Solid products from hydrothermal experiments conducted at 250 °C and P v,soln were characterized by powder X-ray diffraction (XRD) and ATEM/HRTEM. Experiments were conducted with muscovite, kaolinite, and quartz or amorphous silica in 2M KCl solutions for 43 to 176 d. Post-experiment solution compositions lie either within the illite(0.88 K) stability field or on the illite(0.88 K)-kaolinite or illite(0.88 K)-diaspore univariant boundaries in log (a K + /a H + ) vs. log a H₄SiO₄ activity space. Transmission electron microscopy (TEM) observations of muscovite grain edges reveal the neoformation of illite crystals with a range of compositions (ATEM) from 0.31 to 0.89 K/O 10 (OH) 2 . The range of K-contents appears to narrow toward 0.88 K/O 10 (OH) 2 with increased experiment duration. HRTEM suggests the presence of 2 to 11 layer fundamental particles composed of illitic layers with 10 Å periodicity. Fundamental particle thicknesses increase toward an average of 8 layers/particle with increased experiment duration. In the longer duration experiments, fundamental particle thicknesses were normally distributed about thicknesses of 4 and 8 layers, whereas fundamental particles with thicknesses < 4 layers were common in a shorter duration experiment. The compositions and structure of the illites are consistent with the multiphase model, which states that the smectite-to-illite transition occurs through the step-wise formation of solubility-controlling phases consisting of fundamental particles with thicknesses of 1, 2, 4, and ≥ 8 layers. The increase in K-content and fundamental particle thickness with the extent of reaction suggests that the illite crystals underwent a prograde reaction culminating in the formation of end-member illite [0.88 K/O 10 (OH) 2 ]. This reaction, in conjunction with the previously observed, retrograde reaction from muscovite to end-member illite, demonstrates the stability of end-member illite in the system K 2 O-Al 2 O 3 -SiO 2 -H 2 O at 250 °C.

To examine the relationship between product secondary minerals and dissolution of anorthite (An 95 Ab 5 from Fugoppe, Hokkaido, Japan), anorthite batch dissolution experiments were carried out. The dissolution experiments were done at 90, 150, and 210 °C for 3 to 355 days at pH 4.56 measured at 25 °C, which corresponds to 4.69, 4.97, and 5.40 at the respective experimental temperatures. A sequence of secondary minerals including boehmite, ‘‘modified boehmite’’, and kaolinite formed with increasing time. Modified boehmite, probably a metastable phase, is basically similar to boehmite in structure, but their stacking orders of the Al octahedral layers as well as morphologies and chemistries are different. Modified boehmite shows laminations normal to the b* axis and contains 3 to 30 mol% Si. Silicon may be present between the Al octahedral layers of modified boehmite. The anorthite dissolution is incongruent under the above conditions and approximated by a two-stage process. The first is characterized by the formation of boehmite, and the second by formation of modified boehmite. The dissolution rate in the second stage is slower than the first by approximately one order of magnitude because of the saturation state with respect to anorthite. To estimate the effect of the formation of secondary minerals on the anorthite dissolution, Gibbs free energies of anorthite dissolution (ΔG) were calculated, assuming conditions without the formation of secondary minerals. The calculations reveal that the formation of secondary minerals decreases the ΔG values significantly, and thus we can predict that the dissolution rates of anorthite increase due to the influence of the secondary minerals on ΔG. Modified boehmite functions as a sink for Si, and thus accelerates the dissolution rate of anorthite. The results indicate that the overall dissolution rate near equilibrium is affected by both the saturation with respect to a primary mineral and the formation of secondary minerals, but in the opposite sense.

Molecular dynamics (MD) simulations were carried out on molten CaAl 2 Si 2 O 8 at nine pressures from 0.2 to 76 GPa at 4000 K, well above the computed glass transition temperature. A simple effective pair-potential with both Coulombic interaction and Born-Mayer- Huggins short-range repulsion was used. Simulations of duration 30 ps long were performed in the microcanonical ensemble (NEV) utilizing 1300 particles (O+Ca+Si+Al). As pressure increases, profound changes occur in the relative abundance of [4] T, [5] T, [6] T, [7] T, and [8] T (T = Si,Al) as well as in the coordination of Ca by O. At low pressure, most T are in fourfold coordination with oxygen, oxygen is twofold coordinated to T, and Ca has six to seven nearest O atom neighbors. At 5 GPa, where [5] T maximizes, more than half of the T atoms are pentahedrally coordinated ( [5] T) in distorted trigonal bipyramids. Both [4] T (25%) and [6] T (18%) are present at 5 GPa and change rapidly as pressure increases ( [4] T decreases and [6] T increases). At 5 GPa, the average coordination number (CN) of oxygen about Ca is 8. At 20 GPa, [6] T constitutes more than 50% of the TO n subunit population and the CN of oxygen around Ca is 10. At medium range (3 to 10 Å ), the most significant change in melt structure as pressure increases is replacement of corner-sharing ring-former TO n polyhedra with edge-sharing polyhedra pairs such as TO 5 -TO 6 and TO 6 -TO 6 . The destruction of the ring structure correlates with the maximization of Ar solubility as pressure increases. Ring structure collapse is geometrically captured by variation in the distribution of T-O-T angles with increasing pressure. The low-pressure maximum at 140° with large variance agrees with X-ray data and follows from the distribution of N-member rings with N ranging from 4 to 8 (or larger). At higher pressure, the T-O-T distribution narrows and shifts toward smaller angles around 105° consistent with the dominating presence of edge sharing between TO 6 -TO 6 and TO 5 -TO 6 pairs. Tracer diffusivities for O, Ca, Al, and Si increase at low pressure as pressure increases. The diffusivity of oxygen correlates with ring collapse and attains a maximum at 5 GPa (D ox = 6.6 × 10 -9 m 2 /s); the ratio of diffusivities for Ca, Al, and Si relative to oxygen are 0.67, 0.62, and 0.51, respectively. These data suggest that pentahedrally coordinated T acts as an ‘‘activated complex’’ for oxygen diffusion. Below 5 GPa, activation volumes (V*) for all species are large in absolute value and lie in the range - 25 to - 35 cm 3 /mol; these are similar in absolute value to the partial molar volume of Ar in molten CaAl 2 Si 2 O 8 of 22 cm 3 /mol. The large absolute values for V a at low pressure correlate with the contribution to the isothermal compressibility made by configurational effects inferred from Brillouin scattering experiments. At higher pressure, the anomalous diffusion disappears. V a decreases for all species in absolute value and changes sign; at 20 GPa, V a is about + 3 cm 3 /mol, much smaller than the partial molar volume of Ar in molten anorthite.

Bornite (Cu 5 FeS 4 ) exists in three polymorphic forms related by superstructuring, with structural transitions at 200 and 265 °C. The phase transitions and structural behavior in two natural bornite samples were investigated as a function of temperature using differential scanning calorimetry (DSC) to characterize the thermal anomalies associated with each transition and in-situ high-resolution neutron powder diffraction to determine the variation in superlattice intensity and lattice parameters. These two methods, carried out over the temperature range 50-350 °C, provided insight into the short- and long-range interactions, respectively, and a comparison of measurements taken up-T and down-T enabled thermal hysteresis effects to be quantified. The high to intermediate transition at 265 °C involves long-range cation ordering and vacancy clustering, resulting in a doubling of the high-temperature cubic unit cell (m3m). The square of both the spontaneous strain and the 2a superlattice intensity varied linearly with temperature, indicating that the transition is tricritical in character. Loss of long- and short-range order occurred simultaneously during heating, whereas during cooling, the reappearance of 2a superlattice reflections was depressed some 50 8C below the short-range transition recorded by DSC due to the formation of antiphase domains. The presence of antiphase domains also caused the discontinuity in the strain associated with the transition to occur at a different temperature to the appearance of superlattice intensity. The intermediate-low transition associated with the development of a 2a4a2a orthorhombic superlattice (Pbca) is strongly first-order and exhibits a large intrinsic hysteresis (38 °C). The down-T transition temperature of the intermediate- low transition is shown to be dependent on the degree of order attained in the intermediate phase.

X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), and open circuit potentials were used to compare the size, chemical state, and distribution of adsorbed and reduced gold from Au 3+ chloride solution on pyrite and arsenopyrite. Many small Au 0 particles grow on the arsenopyrite surface, whereas few, much larger, gold particles appear on pyrite. These results mimic the differences in distribution of gold in some coexisting natural pyrites and arsenopyrites. The rate-limiting step in deposition of gold from Au 3+ chloride solutions is the reduction of Au 3+ to Au + , whereas the open-circuit potential for deposition is determined by the reduction of Au + to Au 0 . The open-circuit potential of pyrite or arsenopyrite is a corrosion potential. Presence of Au 3+ shifts the corrosion potential to a value that depends on the relative rates of the reduction of Au 3+ and the oxidation of the mineral. Open-circuit potential measurements indicate that the rate of deposition of gold on pyrite is controlled almost entirely by the rate of reduction of Au 3+ . By contrast, the rate of reduction of gold on arsenopyrite is controlled significantly by the rate of oxidation of arsenopyrite.

The interaction of a natural face of FeS 2 (100), cleaned in ultra-high vacuum (UHV), with H 2 O and H 2 S has been investigated with X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and the photoemission of adsorbed xenon (PAX). PAX is sensitive to the short-range order of the surface and allows the effects of defects on the surface reactivity of FeS 2 (100) to be studied. PAX results suggest that both H 2 S and H 2 O bind most strongly to defect sites that we propose are, at least in part, sulfur anion vacancy sites. Whereas the majority of H 2 O adsorbate desorbs from these sites in the temperature interval of 200-300 K, H 2 S dissociates upon heating to 500 K into adsorbed surface hydrogen, S, and SH. This dissociation occurs on defect sites that then release part of the dissociation fragment, which is thought to be surface hydrogen, onto other regions of the pyrite surface that are proposed to be stoichiometric FeS 2 . Heating to 600 K causes further reaction of S containing dissociation fragments with sulfur-deficient sites to form new surface sites that resemble FeS 2 . The results also suggest that surface hydrogen dissolves into the pyrite bulk upon heating to 600 K.

Millerite, NiS, fractured under high vacuum and reacted with air and water has been analyzed by X-ray photoelectron spectroscopy (XPS). The pristine millerite surface gives rise to photoelectron peaks at binding energies of 853.1 eV (Ni 2p 3/2 ) and 161.7 eV (S 2p), thus resolving ambiguities concerning binding energies quoted in the literature. Airreacted samples show the presence of NiSO 4 and Ni(OH) 2 species. There is evidence for polysulfide species (S 2- n, where 2 ≤ n ≤ 8) on air-oxidized surfaces. These may occur in n a sub-surface layer or may be intermixed with the Ni(OH) 2 in the oxidized layer. The NiSO 4 species at the millerite surface occur as discrete crystallites whereas the Ni(OH) 2 forms a thin veneer covering the entire millerite surface. The NiSO 4 crystallites form on the surface of millerite but not on surfaces of adjacent minerals. Surface diffusion of Ni 2+ and SO 2- 4 across the millerite surface is thought to be responsible for the transport and 4 subsequent growth of NiSO 4 crystallites developed on millerite surfaces. Although it is clear that Ni and SO 2- 4 does not diffuse onto surfaces of adjacent minerals in sufficient 4 quantity to form crystallites, the explanation is uncertain. XPS results for water-reacted surfaces show little difference from the vacuum fractured surfaces with the exception that minor amounts of polysulfide and hydroxy nickel species are present. Similar reaction products to those formed in air [NiSO 4 and Ni(OH) 2 ] are believed to be produced, but these are removed from the millerite surface by dissolution, leaving behind a sulfur-enriched surface (polysulfide) and hydroxyl groups chemisorbed to nickel ions at the millerite surface.

Mullite is an aluminosilicate of the composition Al 2 (Al 2+2x Si 2-2x )O 10-x with x generally ranging between 0.2 and 0.5. XRD and 29 Si and 27 Al nuclear magnetic resonance (NMR) have been used to investigate the structure of various mullite compositions (x = 0.26, 0.36, 0.42, and 0.69) as well as sillimanite (x = 0) and γ-Al 2 O 3 (x = 1). 27 Al magic angle spinning (MAS) NMR of the central and satellite transitions have been used to determine the chemical shifts and quadrupole interaction parameters for the various AlO 4 and AlO 6 units. The isotropic chemical shifts of the various units are 5.9 ppm for AlO 6 , 69.1 ppm for AlO 4 (T), 55.0 ppm for AlO 4 (T′), and 45.7 ppm for AlO 4 (T*) where AlO 4 (T) denotes the aluminum tetrahedra in the double chains, and AlO4(T′) and AlO4(T*) those next to the oxygen Oc*. Quantitative numbers of the aluminum occupancy of these sites have been determined for the various powder samples. These results are in good agreement with the average structure model of mullites. NMR and XRD proved the presence of impurities of kyanite in natural sillimanite and of γ-Al 2 O 3 in the mullites with x = 0.42 and 0.69.

Premelting effects are common in silicates and have been mostly observed as significant heat capacity anomalies beginning 100 to 200 °C below congruent melting points. To assess the role of cation dynamics in this phenomenon, we collected nuclear magnetic resonance (NMR) spectra and relaxation time data to within 20 to 50 °C of the melting points of sodium metasilicate (Na 2 SiO 3 ), which displays a large premelting effect, and isostructural lithium metasilicate (Li 2 SiO 3 ), which displays little premelting. From 7 Li NMR, Li + site hopping is clearly observed in Li 2 SiO 3 by a partial averaging of the 7 Li quadrupolar peak shape, requiring exchange among a few, ordered orientations of LiO 4 tetrahedra. From 23 Na NMR, Na + site hopping in Na 2 SiO 3 appears to involve a more liquid-like behavior, implying exchange among many sites with different orientations in a disordered fashion. For this phase, 29 Si spectra indicate that in an oxidizing environment, no liquid phase is present at 20 °C below the melting point, well within the calorimetric premelting regime. However, partial averaging of the low-temperature, biaxial chemical shift powder pattern (typical of Si sites in chain silicates) occurs, suggesting some kind of extensive, librational motion of SiO 4 tetrahedra that is possibly linked to rapid Na + diffusion near the melting point. In contrast to the simple Li + diffusion in Li 2 SiO 3 , this process may require considerable nonvibrational energy and may thus be related to the heat capacity anomaly just prior to melting.

Quantitative 29 Si NMR spectra of single-crystal a-quartz, shock compressed to 12-38 GPa and recovered, provide new information about the complex response of quartz to shock loading. Spectra from samples recovered from shock pressures of 12-20 GPa show a broadening of the 29 Si NMR peak and the development of asymmetry toward lower NMR frequency (indicating an increase in the mean Si-O-Si intertetrahedral bond angle). NMR spectra of samples shock compressed above ~ 25 GPa show increasing amounts of a separate amorphous phase of SiO 2 with a mean Si-O-Si bond angle roughly 5° narrower, and 10-15% denser, than fused SiO 2 . Small amounts of crystalline material remain with a mean Si-O-Si bond angle up to 3° larger than unshocked a-quartz. The recovery of dense glass indicates that post-shock temperatures were sufficiently low to also preserve stishovite, had any been created in our experiments. The paucity of stishovite or [6] Si in an amorphous phase in our recovered samples suggests that the formation of stable, highcoordinated Si is kinetically hindered in shock compression experiments up to about 35- 40 GPa, except in regions of high temperature, such as planar deformation features (PDFs), microfaults (pseudotachylites), or voids.

ABstract Forty garnets are anisotropic among 48 in a suite spanning the quaternary system Mg 3 Al 2 Si 3 O 12 -Fe 3 Al 2 Si 3 O 12 -Ca 3 Al 2 Si 3 O 12 -Mn 3 Al 2 Si 3 O 12 (Py-Al-Gr-Sp). The eight isotropic specimens are too thin (< 0.4 mm) for detection of weak anisotropy. Birefringence (δ) in the remaining 40 garnets is low (δ = 0.0001 to 0.0006) and undulatory in appearance, suggesting that most optical anomalies in quaternary garnets, including the pyrope-almandine- spessartine ternary, originate through residual strain. Multiple or alternate origins are not precluded for the few samples with nearly uniform retardation or unusual sector twinning. An inverse correlation exists between degree of birefringence and stresses encountered during tectonic deformation. That is, mantle garnets from kimberlites (e.g., Py 37 Al 36 Gr 27 ) have the highest δ values. As geologic setting is difficult to separate from composition, the inverse trend suggests that birefringence arises partially from internal factors. We propose that the mismatch in size between Ca 2+ and Mg 2+ exacerbates retention of residual strain. Short-range ordering is not entirely ruled out, but clustering has only been inferred for synthetics in the middle of the Py-Gr binary, a composition range unknown for natural samples, and domains are precluded in the natural samples by the recent X-ray studies of samples from similar localities.

Xenotime, from a geochemically heterogeneous series of mildly to strongly peraluminous granites of the Erzgebirge, Germany, displays extended compositional variability with respect to abundances of HREE, Y, U, and Th. With few exceptions, the maximum and minimum concentrations for the lanthanides and actinides exceed those noted for this mineral from other geologic environments. Xenotime chemistry is dominated by the theoretical end-members HREE-PO 4 and YPO 4 (> 90 mol% in total). Typical xenotime grains from the Erzgebirge granites and from other granites worldwide contain 70-80 mol% YPO 4 , and 16-25 mol% HREE-PO 4 . This study documents the occurrence of xenotime, abnormally rich in HREE in place of Y, with up to 45 mol% HREE-PO 4 . Substitutions of thorite-coffinite, (Th,U,Pb)SiO 4 , brabantite, (Ca,Th,U)(PO 4 ) 2 , and monazite, (La-Sm)PO 4 , are of minor importance, reaching maximum levels of 5-6 mol% each with a total contribution typically « 10 mol%. Typically, the incorporation of actinides in xenotime from these and other granites, as well as from other rocks, is dominated by thorite-coffinite substitutions. However, in some xenotime grains, the U and Th concentrations are largely accounted for by the substitution mechanism 2 (REE,Y) 3+ ↔ (Th,U) 4+ + Ca 2+ . The total lanthanide and actinide contents in xenotime and host granite are not strongly correlated. However, formation of xenotime unusually rich in HREE occurs only in the A-type Limica granites that are strongly enriched in these elements. The shapes of HREE patterns of xenotime and host rock are similar which, combined with mass-balance calculations, indicate the importance of this mineral in affecting the HREE as well as Y evolution of late-stage granitic melts. Late xenotime may show strong Y/Ho fractionation, thus recording the decoupling of Y and the HREE, which can occur in the latest stages of crystallization of evolved granites and which may be associated with deuteric alteration.

The Mössbauer spectra of arrojadite, (K,Ba)(Na,Ca) 5 (Fe 2+ ,Mn,Mg) 14 Al(PO 4 ) 12 (OH,F), at 298 and 95 K were investigated for the first time. The spectra at both temperatures were analyzed in terms of their Fe 2+ quadrupole splitting distributions (QSDs). The overall QSDs at both temperatures can be interpreted in terms of five octahedral siteFe 2+ QSD contributions. The quadratic elongation, 〈λ〉, and the variation of bond angles, σ 2 , for the different sites were calculated on the basis of the structural data obtained by Moore et al. (1981). The five QSD contributions are tentatively assigned to Fe 2+ in the M3, M4, M5, M6, and M7 sites, based on the structural determination and the relation of the quadrupole splitting to the distortion of the octahedra, respectively. The Fe 2+ ions are randomly distributed over the M3, M4, M5, M6, and M7 sites. In addition, Mössbauer data from arrojadite and related phosphate minerals indicate that the mean value of the isomer shift of Fe 2+ in the octahedral sites in phosphate minerals is ~ 0.07 mm/s larger than that in silicate minerals. This difference is explained in terms of electron affinity.

We describe a new titanate mineral, haggertyite, from the Prairie Creek lamproite, Arkansas, U.S.A. The mineral was found exclusively within the reaction zones surrounding small mafic xenoliths in the lamproite. Haggertyite occurs as isolated platelets, typically 30-70 μm maximum dimension, which often show hexagonal morphology. Associated minerals are diopside, olivine, phlogopite, Ti-K-richterite, chrome spinel, ilmenite, priderite, and jeppeite. Haggertyite has a magnetoplumbite-type structure. Typical microprobe analyses give (as wt% oxides) TiO 2 = 36.5-41.6, FeO = 39.4-42.9, MgO = 0.7-3.6, BaO = 9.5-10.5, K 2 O = 1.3-1.5, Cr 2 O 2 = 0.0-5.6, MnO = 0.6-1.1, and NiO = 0.1-0.4. The average composition (based on 19 O atoms and an Fe 2+ /Fe 3+ ratio to give 12 small cations) is Ba 0.68 K 0.31 Ti 5.05 Fe 2+ 3.91 Fe 3+ 2.01 Mg 0.69 (Cr,Mn,Ni) 0.34 O 19 . Haggertyite is hexagonal, space group P6 3 /mmc, Z = 2, a = 5.9369(1), c = 23.3445(6) Å , calculated density of 4.74 g/cm 3 . The structure was refined to R 1 = 0.034 for 339 unique reflections with F o > 4σ Fo using single-crystal data. Strongest reflections are [d calc (Å), I calc , hkl] 2.641 100% (114), 2.795 90% (017), 1.634 47% (02,11), 2.437 46% (023), and 2.963 44% (110). In reflected light, haggertyite is pale gray, opaque, and without internal reflections. It is not discernably pleochroic or bireflectant, but it is weakly anisotropic in shades of dark brown. Quantitative specular reflectance measurements for Ro and Re′ n air and in oil immersion, respectively, are: 470 nm, 17.3, 16.9, and 5.37, 5.13%; for 546 nm 16.8, 16.35, and 5.19, 4.90%; for 589 nm 16.9, 16.3, and 5.29, 4.92%; and for 650 nm 17.1, 16.4, and 5.42, 5.00%. VHN 50 = 500, with a range of five indentations =460-540. The mineral is named for Stephen E. Haggerty in honor of his contributions to the mineralogy and crystal chemistry of uppermantle titanate minerals.

Scandiobabingtonite, ideally Ca 2 (Fe 2+ ,Mn)ScSi 5 O 14 (OH) is the scandium analogue of babingtonite; it was found in a pegmatitic cavity of the Baveno granite associated with orthoclase, albite, muscovite, stilbite, and fluorite. Its optics are biaxial (+) with 2V = 64(2)°, a = 1.686(2), b = 1.694(3), g = 1.709(2). D meas = 3.24(5) g/cm 3 , D calc = 3.24 g/cm 3 , and Z = 2. Scandiobabingtonite is colorless or pale gray-green, transparent, with vitreous luster. It occurs as submillimeter sized, short, tabular crystals, slightly elongated on [001], and characterized by the association of forms {010}, {001}, {110}, {1 1̅0}, and {101}. It occurs also as a thin rim encrusting small crystals of babingtonite. The strongest lines in the X-ray powder pattern are at 2.969 (S), 2.895 (S), 3.14 (mS), and 2.755 (mS) Å. The mineral is triclinic, space group P1̅, with a = 7.536(2), b = 11.734(2), c = 6.748(2) Å, α = 91.70(2), β = 93.86(2), γ = 104.53(2)°. Scandiobabingtonite is isostructural with babingtonite, with Sc replacing Fe 3+ in sixfold coordination, but no substitution of Fe 2+ by Sc takes place. Due to the lack of a suitably large crystal of the new species, such a replacement has been confirmed by refining the crystal structure of a Sc-rich babingtonite (final R = 0.047) using single-crystal X-ray diffraction (XRD) data.

The rubidium analogue of microcline, rubicline, (Rb,K)AlSi 3 O 8 , ideally RbAlSi 3 O 8 , was found in a pollucite-bearing rare-element pegmatite at San Piero in Campo, Elba, Italy. Rubicline is the first mineral with rubidium as an essential constituent. It occurs as abundant but small (≤ 50 μm) rounded grains in 1-2 cm wide veins of rubidian microcline (± albite, muscovite, quartz, and apatite) that crosscut pollucite. Rubicline is brittle, transparent, and colorless. Refractive indices are slightly higher than those of the host microcline. The birefringence is low (1st order gray interference colors), and crystals are apparently untwinned. In thin and polished sections, cleavage passes through both the host microcline and grains of rubicline; by analogy with microcline, the cleavage is {001} perfect and {010} good. Determination of additional physical properties is hindered by an average grain size of 20 mm, heterogeneous composition, and structural coherency of rubicline with the enveloping microcline. Rubicline is triclinic, probable space group P1̅, with a = 8.81(3), b = 13.01(3) c = 7.18(4) Å, α = 90.3(1), β = 115.7(3), γ = 88.2(1)°, V = 741 Å 3 , Z = 4, and axial ratios a:b:c of 0.677:1:0.577 (calculated from electron-diffraction data). Chemical analysis by electron microprobe gave 58.68 SiO 2 , 16.48 Al 2 O 3 , 6.23 K 2 O, 17.47 Rb 2 O, 0.92 Cs 2 O, 0.12 Fe 2 O 3 , sum 99.90 wt% and the formula (Rb 0.574 K 0.407 Cs 0.020 ) Σ1.001 (Al 0.993 Fe 0.005 )Si 3.001 O 8 . Rubicline is, in many cases, structurally coherent with the host microcline; it formed by exsolution from a (K,Na,Rb)-enriched precursor, followed possibly by fluid-induced modification.

In the Bílina Mine (Czech Republic), a very chemically pure material corresponding to hartite was found as a product of distillation into the pelosiderite nodule during coalification of a lignite seam in the Miocene or later. The amount of the material available allowed characterizing this natural mineral equivalent of the hydrocarbon (1)-phyllocladane [16α(H)-phyllocladane] thoroughly. Elementary analysis of hartite from Bílina gave C = 87.45 wt% and H = 12.55 wt%, which corresponds to the formula C 20 H 34 . The IR spectrum demonstrated the presence of gem-dimethyl groups (doublet δ s CH 3 at 1369 and 1383 cm -1 ). Other bands indicated the presence of methyl and methylene groups. NMR studies showed that the hydrocarbon (hartite) molecule contains four CH 3 groups, nine CH 2 groups, four CH groups, and three quaternary carbon atoms. The sample exhibits a well-defined melting point between 71 and 71.5 °C. Measurements of optical rotation gave [α] D = 124.28 (CHCl 3 , c = 0.7). Lattice parameters from single-crystal study measured at 293(2) K were a = 11.407 (1), b = 20.952 (2), c = 7.4060 (8) Å, α = 93.941 (9)8, β = 100.750 (8)°, and γ = 80.499 (9)°. The crystal system is triclinic with space group Pl, Z = 4, V = 1713.8 (3) Å 3 . Densities are D m = 1.04 g/cm 3 and D x = 1.064 g/cm 3 , respectively. Isotopic composition of C in hartite from Bı´lina gave δ 13 C (PDB) = - 24.4(1) ‰, a common value for coal and organic sedimentary material derived from higher plants.

The interaction of atomically clean (100) and (111) crystallographic planes of FeS 2 with H 2 O vapor, O 2 , and a H 2 O/O 2 mixture was investigated. A combined high pressure/ultra-high vacuum (UHV) apparatus allowed the surfaces to be reacted at environmentally relevant pressures and studied with X-ray photoelectron spectroscopy (XPS) without exposure to the atmosphere. Neither surface exhibited significant reaction in pure O 2 . Exposure of FeS 2 (111) to H 2 O vapor resulted in significant oxidation, but under these same experimental conditions FeS 2 (100) exhibited a much smaller amount of oxidation. It is suspected that on FeS 2 (100), H 2 O only reacted on nonstoichiometric regions (i.e., defects). Both surfaces showed substantial reaction in H 2 O/O 2 . The amount of FeS 2 (100) and FeS 2 (111) oxidation in the H 2 O/O 2 mixture was more than simply the sum of the reaction observed individually in pure O 2 and H 2 O. This result suggests that there is a synergy between H 2 O and O 2 in oxidizing pyrite. In all cases, the amount of oxidation that occurred on FeS 2 (111) was greater than on FeS 2 (100). We believe that this experimental observation is due to a higher concentration of under-coordinated Fe in the outermost surface of FeS 2 (111), relative to FeS 2 (100).

The compressional sound wave velocity, v p , of synthetic MgO in the (100) direction and the unitcell parameter have been measured up to a maximum pressure of 6.1 GPa using a new type of diamond-anvil cell. The main feature of the cell is the transverse access of the X-ray beam into the sample chamber. This allows us to undertake single crystal X-ray measurements while the ultrasonic attachment is mounted on the diamond-anvil cell. The sound velocity and the elastic parameter c 11 have been determined from these measurements; the variation with pressure can be described by მc 11 /მp = 9.35(13), in good agreement with previous studies.

Synchrotron micro-XANES spectroscopy (SmX) is used to examine the amount and distribution of Fe 2+ and Fe 3+ in five samples of fayalite previously studied by Mössbauer spectroscopy. Rockport fayalite is homogeneous and the Fe in it is completely reduced. Olivine from Qianan is almost completely oxidized, and probably contains finely intergrown silica, laihunite, and hematite with hematite predominating. Pantelleria olivine has an oxidized rim around a reduced core, suggesting either a post-crystallization reaction with fayalite and oxygen going to laihunite and hematite or a change in prevailing oxidation during growth. The texture of olivine from the Mourne Mountains suggests exsolution from a fayalite-laihunite solid solution, based on the substitution of three Fe 3+ atoms for two Fe 2+ and one vacancy, that was stable at high temperature. Laihunite from the type locality (Lai-He village) shows 1-3 mm mottling between light and dark areas in backscatter images, but these areas cannot correspond to pure fayalite and laihunite exsolution because the bulk Fe 3+ /ΣFe of such a combination would not correspond to the SmX value of 67% Fe 3+ . Rather, the texture observed in backscatter is interpreted to represent alternating areas of magnetite-rich and laihunite-rich compositions with a similar (i.e., 67% Fe 3+ /ΣFe) extent of oxidation.