Band 88, Heft 1

The structural features of albite (atomic coordinates and distances, thermal displacements) at 25 and 1040 °C, obtained by Molecular Dynamic simulations following the Car-Parrinello approach, were favorably compared with those obtained by single-crystal diffraction experiments. Starting from this basis, it was shown that the marked anisotropy of electron density distribution about the positions of the sodium atoms is due to a time average of highly anisotropic thermal vibrations, and not to a space average of multiple positions occupied by Na. Although the large displacement of Na from its center of gravity results in great variations over time of the individual Na-O distances of the sodium coordination polyhedron, the average distance remains approximately constant, reaching its minimum variation when the 9 nearest O atoms are considered, thus supporting a true 9-coordination of sodium.

Electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopies have been used to study Brazilian and Central African carbonados. We report the first observation of the N 2 V + and N 2 + nitrogen defect centers in carbonados. The fact that Brazilian carbonados contain the N 3 V (previously reported), N 2 V + , and N 2 + defects, the formation of which requires complicated aggregation of nitrogen, indicates histories at elevated temperature and pressure. The observation of both the radiation-induced A1 defect, which has an annealing temperature of 700 K, and radiation defects containing two and/or five vacancies, suggest a continuous process of the formation and transformation of radiation defects in the carbonado under variable pressure and temperature conditions while in the upper mantle. The temperature at which these defects are formed varies from 300 to 1500 K. In addition we report a hydrogen-containing defect labeled H1, previously only observed in synthetic diamonds prepared by chemical vapor deposition. Together with the previously reported light carbon isotope ratio, the results of this study confirm the hypothesis that carbonados are derived from hydrocarbon source material.

In situ atomic force microscopy (AFM) was used to follow the far-from-equilibrium dissolution of the {001} cleavage surface of natural gibbsite in nitric acid. The main dissolution mechanism was the retreat of straight monolayer steps, the edges of which are parallel to the <110>, <010>, and <100> directions. The stability of these steps can be expressed as <110> > <010> > <100>. The results are explained in terms of the positions of the terminal O atoms and their associated Al atoms at the steps. New steps were formed at etch pits that opened where screw dislocations emerged on the surface. The dissolution rates were calculated from the change in size of pits and islands. The values obtained were 9.5 × 10 -9 - 2.3 × 10 -8 mol/m 2 ·s, normalized to the total surface area, and 1.8 - 3.6 × 10 -7 mol/m 2 ·s, normalized to the surface area of the step fronts. The rates of dissolution calculated using only the surface area of the step fronts are similar to literature values obtained by other methods.

The structure of the members of the antigorite polysomatic series is based on corrugated octahedral layers with tetrahedral layers attached at the convex side. The half-waves in the basic antigorite structure are characterized by constant medium aperture angles of 20.3° that are independent of the wavelength. Higher-order antigorite structures based on half-waves with aperture angles that are a multiple of 20.3° are possible. Antigorite aggregates found in ophicarbonates from different localities in the Alps, replacing chlorite (chl) and tremolite (tr), show a systematic orientation relationship characterized by an angle of 20.3° or multiples thereof between the basal planes of adjacent grains. This angular relationship is explained by the presence of a wave with an aperture angle >20.3° connecting the neighboring grains. The first antigorite crystals with twice the basic aperture angle and with a wavelength of 10.0 nm were found in oceanic serpentinites from Hess Deep (Pacific Ocean). The average experimental amplitude of 0.7 ± 0.05 nm is close to the calculated amplitude of 0.732 nm for a second class antigorite with the observed wavelength. Antigorite that nucleates and/ or replaces type-II chlorites in the studied samples shows a doubling of the c-axis length. The contrast in HRTEM images is compatible with a two-layer structure based on unit-cell twinning, the antigorite thereby inheriting the orientation of the octahedral layers of the primary chlorite.

The molecular structures of the basic copper phosphate minerals pseudomalachite, libethenite, and cornetite were studied using a combination of infrared emission spectroscopy, infrared absorption, and Raman spectroscopy. Infrared emission spectra of these minerals were obtained over the temperature range 100 to 1000°C. The infrared spectra of the three minerals are different, in line with differences in crystal structure and composition. The absorption spectra are similar, particularly in the OH stretching region, but characteristic differences in the bending regions are observed. Differences are also observed in the phosphate stretching and bending regions. The IR emission of the basic copper phosphates studied shows that the minerals are completely dehydroxylated by 550°C.

The influence of F content on the formation and stability of Al-rich phlogopite has been investigated. Samples with varying OH/F-ratios and nominal gel compositions of K(Mg 3-x Al x )(Al 1+x Si 3-x O 10 )(OH) y (F) 2-y (0.0 < x < 0.8 and 0.5 < y < 1.8) were studied using 29 Si, 1 H, and 19 F MAS NMR spec­troscopies, powder X-ray diffraction, electron-probe microanalysis, and scanning electron mi­croscopy. The synthetic phlogopites were synthesized from sol-gels in cold-seal pressure vessels at 1073 K, 2 kbar. The main phase (phlogopite) and three other impurity phases [corundum (α-Al 2 O 3 ), kalsilite (KAlSiO 4 ), and potassium aluminum hexafluoride (K 3 AlF 6 -0.5H 2 0)] were clearly identi­fied by powder X-ray diffraction and electron-probe microanalysis. For phlogopite, the unit-cell parameters a 0 and b 0 decrease whereas C 0 increases with increasing Al-content (x). The average crystal size of phlogopite is about 1-2 pm. The 29 Si MAS NMR spectra show up to four resonances at approximately -91, -87, -83, and -80 ppm, which can be assigned as Q 3 (n Al) signals with n = 0 - 3. The ideal Si/ iv Al ratio calculated from the initial composition is always lower than that derived experimentally. Hydroxyl-rich compositions indicate an increased Al-content in the tetrahedral sheets, suggesting a stabilizing effect on the formation of Al-rich phlogopite. These conclusions are sup­ported by 1 H and 19 F MAS NMR spectra. The 1 H MAS NMR spectra show a water signal at 4.7 ppm, a signal due to Mg 2 AlOH at 1.8 ppm, and a signal due to Mg 3 OH at 0.7 ppm. The 19 F MAS NMR spectra exhibit a signal for Mg 2 AlF at -150 ppm, one due to AlF 6 (K 3 AlF 6 -0.5H 2 0) at -157 ppm, and one for Mg 3 F at -174 ppm. Comparison of the 1 H and 19 F MAS NMR spectra for different Al- and F- contents reveals a non-statistical distribution of F- and OH-groups at the crystallographic sites in the octahedral sheets where F prefers sites coordinated by three Mg, and OH sites with Al in the next- nearest coordination sphere.

Five sodium aluminosilicate glasses in the series Na 2 O: xAl 2 O 3 ,:(3-x) SiO 2 and Na 2 0:Al 2 O 3 :ySiO 2 were prepared and subjected to leaching at pH 2 under ambient conditions for up to 1000 h. Solid- state nuclear magnetic resonance (NMR) spectroscopy revealed the identity of aluminate and silicate environments in the leached surface layers of these glasses as well as a sample of albite crystal subjected to the same aqueous leaching. While 29 Si and 27 Al magic-angle spinning (MAS) NMR experiments report on the bulk structures of the samples, cross-polarization from hydrogen atoms that are present only in the surface layers provides structural information from the regions of the sample transformed during treatment. Aluminum in octahedral coordination by O atoms (Al VI ) is confirmed for the first time on the near-surface region of an albite crystal under the treatment conditions of this study. A quantification of the change in the amount of six-coordinate aluminum as a function of bulk Al/Si ratio is made possible by comparing the relative amounts of 1 H → 27 Al CPMAS signals from AF and Al VI obtained from different samples under reproducible experimental conditions. The relative contribution of Al VI to total Al in the hydrated layers in­creases with the Al/Si ratio of the glasses studied. The leached albite crystal sample has an anoma­lously high concentration of Al VI given its Al/Si ratio. This anomaly is probably related to the relatively low thickness of the leached layer developed on this phase: little hydrogen penetrates the crystal surface and almost all of the Al in the thin leached layer is octahedrally coordinated, similar to Al in solution. These data suggest that hydrolysis of bridging O atoms around Al atoms in the glass or crystal hydrated layer is accompanied by a change in the coordination number of the Al atom. Aging of surfaces documents no formation of Al VI during storage after leaching. The MAS data, coupled with 27 Al → 29 Si CPMAS experiments, describe the bulk network structure and provide further insight into the surface structures, including documentation of repolymerization of the silicon network in the surface layer of a nepheline glass via formation of condensed Q 4 units. Further triple-resonance experiments correlate 1 H, 29 Si, and 27 Al environments in the glasses, and indicate that the repolymerized structures in nepheline glass are not phase-separated from alumi­num-containing network structures. These data for acid dissolution under ambient conditions yield the first picture of the complicated series of reactions relating connectivity and coordination number of Al and Si at the altered surfaces of geologically interesting aluminosilicates.

The low-temperature heat capacity of hercynite (FeAl 2 O 4 ) was measured between 3 and 400 K, and thermochemical functions were derived from the results. The measured heat-capacity curve shows a small lambda-shaped anomaly peaking at around 13 K. From our data, we suggest a standard entropy for hercynite at 298.15 K of 113.9 ± 0.2 J/(mol·K), which is some 7.6 J/(mol·K) higher than reported previously by a calorimetric study that missed the entropy contributions of the low-temperature anomaly.

The aqueous phase interaction of divalent Sr with silicate-containing solutions was studied over a range of pH values, dissolved silica, and Sr concentrations, and extending to basic solution (0.2 M NaOH) at room temperature (22-23°C). Formation of an amorphous Sr silicate phase of variable composition was observed, beginning at approximately pH 10. The precipitate became quasi-crystalline at higher base concentrations and with aging time. The phase that formed at high base concentration (beginning at pH 11.5-12.0) was identified by total chemical analysis, X-ray diffraction, and thermogravimetric analysis to have a chemical formula of Sr 5 Si 6 O 16 (OH) 2 ·5H 2 O, with a tobermorite-like structure. The Sr 5 Si 6 O 16 (OH) 2 ·5H 2 O phase has not been previously reported. Scanning electron microscopy (SEM) analysis of alcohol-washed precipitates showed a fine granular structure similar to Ca silicate hydrates. De-ionized water washing of the precipitates resulted in dissolution and growth of surface phases with the classical “sheaf of wheat” nucleation structure previously identified only for Ca silicate hydrates. Solubility studies of the precipitates, which extended to 286 days of equilibration, yielded an estimate of the solubility product of quasicrystalline, tobermorite-like Sr silicate of Log K = -38.0 ± 0.7 for the overall reaction, 5H 2 O + Sr 5 Si 6 O 16 (OH) 2 ·5H 2 O = 5Sr 2+ + 6H 3 SiO 4 - + 4OH - . The first estimate of the association constant for the species SrH 2 SiO 4 (aq) of Log K = 2.86 was also made.

In-situ synchrotron X-ray diffraction experiments were conducted using the MAX-III multianvil press of KEK on an omphacite (Di 63 Jd37), for which Di = Ca(Mg,Fe)Si 2 O 6 and Jd = NaAlSi 2 O 6 . Pressure-volume-temperature data were collected at up to 10 GPa and 1000 K. A fit to the hightemperature Birch-Murnaghan equation of state yielded an isothermal bulk modulus K T0 = 126(1) GPa, an assumed pressure derivative of the bulk modulus K' T = 4.0, a temperature derivative of the bulk modulus (∂K T /∂T) P = -0.015(4) GPa/K, and a volumetric thermal expansivity α = 2.2(1) × 10 -5 K -1 , when the equation of state of NaCl by Brown (1999) is adopted for the pressure scale. The derived K T0 value is consistent with the linear interpolations from K T0 values for diopside and jadeite in the literature.

Oxygen and C isotope compositions of CO 2 gas released by thermal decomposition of siderite, calcite, and dolomite were measured using a new “real-time” continuous-flow technique to determine whether fractionation associated with simple thermal decarbonation could explain the large isotopic variations and mineralogy such as those found in the ALH84001 meteorite. Oxygen and C isotope fractionation between calcite or dolomite and evolved CO 2 gas during thermal decarbonation in a 3 bar He pressure environment is very small. The δ 13 C and δ 18 O values of evolved CO 2 gas are nearly identical to those of the carbonate, very different from the calculated equilibrium 18 O calcite-CO2 value of -4 to -5‰ at 800-900°C or from previous experimental results of decarbonation in vacuum. The kinetic Δ 18 O siderite-CO2 values are ~-2‰, whereas Δ 13 C siderite-CO2 values increase logarithmically with time, from ~1‰ for the earliest stages of decarbonation to >5‰ in the final stages. Incomplete siderite decomposition produces both magnetite (δ 18 O = 3.5‰ SMOW) and minor graphite. CO and O 2 were detected during the decarbonation process. The data can be explained by simultaneous oxidation and reduction by the reaction: 6FeCO 3 → 2Fe 3 O 4 + 2x CO + 4y CO 2 +(6 - 2x - 4y) C + (5 - x - 4y) O 2 , where x and y are between 0 and 1. Siderite decomposition in the presence of H 2 gas produces wüstite and Fe metal in place of oxidized Fe minerals. The experiments in this study are not a perfect analog for possible decarbonation conditions that might have occurred to the carbonates in ALH84001. Nevertheless, the large δ 13 C and δ 18 O variations observed in ALH84001 (>10‰ for O) are significantly larger than those expected by thermal decarbonation, suggesting instead a low-temperature mechanism for their formation.

The equation of state and the structural behavior of synthetic MgAl 2 O 4 have been investigated using synchrotron X-ray powder diffraction data collected to 30 GPa at room temperature. The Birch-Murnaghan, Vinet, and Poirier-Tarantola models have been fitted to the observed P-V data. The Birch-Murnaghan equation of state, with V0 fixed at its experimental value, yields K 0 = 190.8(±1.2) GPa, K' 0 = 6.77(±0.15) and K″ 0 = -0.075 GPa -1 (implied value). The compression of spinel occurs with a negligible change of the fractional coordinate of oxygen. Therefore the structural shrinking is a function of cell edge shortening alone. The results presented here are compared with those from the literature.

The high-pressure stability of antigorite in the systems MSH and MASH was investigated using two structurally and chemically well-constrained natural samples. Careful sample selection and characterization ensured that the only significant difference between the samples was Al content, one sample being essentially Al free, and the other containing 3.06(2) wt% Al 2 O 3 . In the system MSH, the reaction antigorite = forsterite + clinoenstatite + water was bracketed, under water-saturated conditions, between 630 and 650 °C at 1.6 GPa, between 620 and 660 °C at 2.5 GPa, between 620 and 660 °C at 3.9 GPa, and between 4.5 and 5.0 GPa at 520 °C. In the system MASH, the reaction antigorite = forsterite + clinoenstatite + chlorite + water was bracketed, under water-saturated conditions, between 660 and 700 °C at 2.0 GPa, between 660 and 680 °C at 2.9 GPa, and between 5.0 and 5.5 GPa at 600 °C. At pressures above 5.8 GPa, intersection of this reaction with the reaction chlorite + clinoenstatite = pyrope + forsterite + water leads to an additional reaction whereby the Al component of the antigorite is transferred to pyrope upon antigorite breakdown. The addition of a few weight percent Al 2 O 3 into antigorite is shown to stabilize the antigorite structure to significantly higher temperatures and pressures, possibly by minimizing structural misfit among the component octahedral and tetrahedral sheets. This effect partially explains the considerable discrepancies noted between previous studies on the stability of antigorite at high pressure. In addition, antigorite breakdown in the system MASH transfers a significant volume of water to chlorite-bearing assemblages, thereby greatly increasing the range of temperatures over which water is tied up in hydrous phases relative to the system MSH.

The 1bar, thermodynamic properties of crystalline and liquid NaAlSi3O8 are used to calculate the fusion curve of albite to 10 kbar. The calculated temperatures (±2σ) of the melting reaction are 1152 (±7) °C at 3 kbar and 1247 (±24) °C at 10 kbar. The location of the calculated fusion curve to 10 kbar is in excellent agreement with phase-equilibrium constraints on the maximum and minimum temperatures of the fusion curve at 3 and 15 kbar, respectively. Calculation of the melting reaction at pressures >10 kbar requires that the pressure dependence of the liquid compressibility (K 0 ' = dKT,0 / dP, where K T,0 = 1/β T,0 ) be known. On the basis of five half-reversal, crystallization experiments in the literature, which collectively provide minimum temperatures of the fusion curve between 12 and 32 kbar, K 0 ' (derived from the Birch-Murnaghan relation) is constrained to be ≥10 for liquid NaAlSi 3 O 8 . A comparison with other silicate liquids shows that there is a strong, positive correlation between the compressibility at one bar (β T,0 ) and K 0 '. In addition, data on the water-saturated fusion curve of albite are used to quantify the effect of small amounts of H 2 O (≤1 wt%) on lowering the melting temperature of albite (≤68 degrees).

A correction procedure is proposed to account for the effect of secondary X-ray fluorescence near grain boundaries in electron microprobe analysis. The procedure is based on the Monte Carlo simulation method, which is used to calculate the X-ray spectrum emitted by the mineral couple (i.e., the mineral of interest with the neighboring mineral). The contribution of secondary fluorescence from the neighboring mineral, which appears in the simulated spectrum in a natural way, is then subtracted from the measured k-ratio and thereafter conventional matrix corrections are applied. The Monte Carlo simulation algorithm used is largely based on the general-purpose simulation package PENELOPE. In order to assess the reliability of this code, simulated “apparent” element profiles are compared with electron microprobe measurements found in the literature, in which the effect of secondary fluorescence was characterized and the reliability of the different assumptions underlying the proposed procedure is discussed. Finally, the procedure is used to assess data from the olivine-clinopyroxene thermobarometer in a spinel lherzolite xenolith. The application of the secondary fluorescence correction leads to: (1) higher systematic pressure estimations than those obtained from uncorrected data, with lower uncertainties; and (2) a better agreement between the olivine-clinopyroxene temperature estimations and those estimated using the two-pyroxene thermometer. Estimated P-T conditions indicate a decompressional path with a slight decrease in temperature from core to crystal rim. However, if the effect of secondary fluorescence is not taken into account, an apparent heating event is observed.

The central portions of garnet porphyroblasts from Harpswell Neck, Maine, exhibit small discrete regions of high Mn content, dispersed in three dimensions. Orientation contrast imaging (OCI) and quantitative crystallographic orientation measurements from electron backscatter diffraction (EBSD) patterns show that most garnet porphyroblasts contain no variations in crystallographic orientation. It is difficult to reconcile these data with a model in which each high-Mn region represents a discrete garnet nucleus. An alternative hypothesis invokes a model of garnet growth in which precursor phases rich in Mn are overgrown, and their Mn is incorporated locally into the garnet structure. Although this model requires length scales for equilibration of Mn during the early stages of garnet growth that are shorter than any previously documented, that inference is consistent with the very low temperatures of garnet nucleation in these rocks, and with compositional evidence for progressively larger scales of equilibrium as reaction progressed. Very short length scales for equilibration of Mn, identified in this study for the lowest-grade portion of the history of the rocks, call into question the common assumption of rock-wide equilibrium for Mn during garnet growth at low temperature. The absence of rock-wide equilibrium for Mn at low grade has potential negative implications for all common interpretive methods based in phase equilibria, including thermobarometric studies and thermodynamic analysis of the origin and meaning of zoning in low-grade garnet.

Real-time, temperature-resolved synchrotron powder X-ray diffraction data and Rietveld refinements were used to investigate the behavior of the todorokite structure from 100 to 820 K. At 298 K, the lower valence cations, such as Mn 3+ , occupy large octahedral sites at the edges of the triple chains, and four water/cation sites are in the tunnels. In our heating experiments in vacuum, todorokite began to break down at ~450 K and fully decomposed by 620 K, at which point hausmannite started to form. Our results suggest that the loss of O2 accompanied the reduction of Mn 4+ to Mn 3+ and Mn 2+ , and these reactions impelled a breakdown of the octahedral framework and the subsequent release of the tunnel water molecules. Rietveld refinements revealed a gradual thermal expansion of the todorokite structure to ~450 K. At higher temperatures, the unit-cell volume gradually decreased, primarily as a result of a decrease in c, and the decline in β with increasing temperature was accelerated. A mechanism for forming the inverse spinel hausmannite structure from todorokite is presented.

Evidence is presented for retrograde reaction of silicate minerals with saline brine inside fluid inclusions during post-entrapment cooling. Ferropyrosmalite [(Fe,Mn) 8 Si 6 O 15 (OH,Cl) 10 , where Fe>>Mn] has previously been interpreted as a daughter mineral in saline inclusions in magmatic quartz from altered granodiorite associated with the Vyhne-Klokoč Fe-skarn deposit of Slovakia. Based on combined Raman spectroscopic, microthermometric, and SEM-EDX techniques this phase is shown to react, on heating above 450 °C with a Ca-enriched saline inclusion fluid, to form clinopyroxene. This suggests that clinopyroxene was originally present in the inclusions at high temperature, and then underwent retrograde reaction with the saline fluid, on cooling, to form ferropyrosmalite. In its simplest form, the equilibrium reaction for this transformation is: Fe 8 Si 6 O 15 [(OH) 6 Cl 4 ] + 3Ca 2+ (aq) = 3CaFeSi 2 O 6 + 5Fe 2+ (aq) + 4Cl - (aq) + 3H 2 O (ferropyrosmalite) (hedenbergite) In practice, the reaction is complicated by the presence of Mn and Mg. The resulting “daughter” mineral assemblage observed at room temperature is actually a low-temperature equilibrium assemblage very different to that originally present at high temperature. Although silicate daughter minerals such as clinopyroxene are rarely, if ever, described in fluid inclusions, they may have originally been present but underwent similar retrograde reactions. Ferropyrosmalite itself is likely to be frequently overlooked, as in the absence of Raman spectroscopic analyses it may be mistaken for one of a range of possible hydrated iron-chloride minerals. Improved Raman spectra for ferropyrosmalite are presented which will make future identification easier. Failure to recognize intra-inclusion retrograde reactions such as this may lead to misinterpretation of fluid inclusion chemistry, including metal and silica solubilities.

The susceptibility of uranophane, a uranyl sheet silicate, ideally Ca(UO 2 ) 2 (SiO 3 OH) 2 (H 2 O) 5 . to ionizing irradiation has been evaluated by systematic irradiations with 200 keV elections over the temperature range 94 to 573 K. High-resolution transmission electron microscopy revealed that amorphous domains formed locally, concurrently with a gradual disordering of the entire structure. Amorphization doses at room temperature were 1.1 × 1O 10 Gy for uranophane. 1.3 × 1O 10 Gy for Sr- substituted uranophane. and 1.9 x 1O 10 Gy for Eu-substituted uranophane: thus, there was an in­crease in amorphization dose with increasing average atomic mass. At 573 K. the amorphization dose of uranophane was 2.0 × 1O 11 Gy. The temperature dependence of the amorphization dose of uranophane has two stages: ≤413 K and >413 K. Based on a defect accumulation model, the effec­tive activation energies for amorphization at each stage are 0.0440 eV and 0.869 eV, respectively. This suggests that the presence of H 2 O (and OH - ) reduce the energy deposition required to cause amorphization. Above 413 K. the amorphization dose increased due to the absence of H 2 O and OH - and the absence of radiolytic decomposition of H 2 O and OH - .

Water-vapor sorption isotherms, relative humidity (RH) controlled powder X-ray diffraction (XRD) data, and new and previously published multi-nuclear NMR spectroscopic data for a wide range of layered double hydroxides (LDHs) provide greatly increased understanding of the effects of hydration state on the structure and dynamical behavior of interlayer and surface anions and the factors controlling the expansion behavior of this group of minerals. Li,Al and Mg,Al LDH phases containing SO 4 2- , SeO 4 2- , PO 4 3- , HPO 4 2- , MoO 4 2- , ClO 4 - , SeO 3 2- , CO 3 2- , F - , Cl - , Br - , I - , OH - , and NO 3 - were examined. The phases studied can be grouped into three types based on basal spacing expansion, water sorption, and interlayer anion dynamics: Type 1, significantly expandable (1.5-3.0 Å); Type 2, slightly expandable (expansion <0.5 Å) and with significant interlayer water exchange; and Type 3, essentially non-expandable (0-0.2 Å) and with little interlayer water exchange. For Type 1, the fully expanded phases have a two-water layer structure, and the phase transition from one layer to two layers as determined by XRD consistently correlates with a significant step in the water sorption isotherm and with changes in the interlayer structure and dynamics as observed by NMR spectroscopy. For Type-2 phases, only one-water layer structures form, and the interlayer anions may undergo dynamical disordering with increasing RH, as observed by NMR. For both Types 1 and 2, the first water layer does not cause significant basal spacing expansion due to occupancy of vacant interstitial sites between the anions by the water molecules. For Type-3 phases, there is little interlayer water sorption because the interlayers are essentially closed due to the small size or planar shape of the anions and their strong electrostatic and hydrogen bonding interaction with the hydroxyl layers. RH has no effect on the structural environments and dynamics of the interlayer anions in this group.

Study of mineral inclusions in garnet in orogenic peridotites provides important information on their metamorphic history. Similar study of mineral inclusions in pyroxene is also encouraging. Edenite is found as relict inclusions in phlogopite-clinopyroxene intergrowths forming the cores of clinopyroxene porphyroblasts in a garnet lherzolite at Zhimafang in the Chinese Su-Lu ultrahighpressure metamorphic terrane. Texture shows that the phlogopite and clinopyroxene were products of the breakdown of edenite. The composition of the edenite is represented by K-Ed 6.0 Ed 70.7 Fe- Ed 6.6 Ts 8.4 Fe-Ts 1.8 Tr 1.2 Cm 5.4 . Those of the clinopyroxene and phlogopite are, Ur 4.5 Jd 3.7 Ae 6.5 CaTs 2.2 Di 81.1 En 1.9 and Phl 86.0 Na-Phl 13.3 Na-Ann 0.7 , respectively. The edenite breakdown is accompanied by gains of Ca, Si, and Cr, and losses of H, Mg, Al, Fe, Na, and K. The hypothetical composition of clinopyroxene resulting from isochemical breakdown of edenite is higher in Jd, CaTs, and En but lower in Di components than the cores of clinopyroxene porphyroblasts. The calculated mode of phlogopite is much higher than measured. The gain of Si and loss of H can be explained by involvement of SiO 2 -enriched fluids during formation of the Cpx+Phl intergrowth, whereas the other gains and losses suggest the existence of an older generation clinopyroxene slightly more enriched in Ca and Cr and depleted in Na, Mg, and Al, which homogenized with the clinopyroxene produced by the breakdown of edenite. Interaction with fluids also increased the Ca/Mg ratio of the system. It is suggested that, after having been exhumed to a shallow level for the first time, the peridotite was infiltrated by crust-derived fluids resulting in the replacement of clinopyroxene by edenite. During subduction, the edenite was replaced by clinopyroxene + phlogopite. Relics of edenite in clinopyroxene were able to survive ultrahigh-pressure metamorphism.

Norrishite, K(Mn 2 3+ Li)Si 4 O 10 (O) 2 , an OH-free mica, is found in a silicate-rich sample from the Wessels mine, Kalahari manganese field, South Africa. The mineral forms centimeter-sized black aggregates in dark-purple sugilite, and is associated with kornite and Mn-rich pectolite. Single-crystal X-ray data, collected on norrishite of end-member composition, yielded a monoclinic C2/m lattice with a = 5.291(2), b = 8.904(2), c = 10.049(4) Å, β = 98.15(5)°, V = 920.6 Å 3 , and Z = 2, characteristic of the 1M polytype. 40 Ar- 39 Ar analyses on a gem-quality sugilite yielded an age of 1048.1 ± 5.9 Ma interpreted as a mineral crystallization age that dates the main hydrothermal event that led to enrichment of the Mn ore. The norrishite age data are more complex and suggest a crystallization age ≈ 1010 Ma and a subsequent alteration at ca. 850 Ma. The age results obtained suggest that the hydrothermal activity is related to the collision and tectonism affecting the edges of the Kalahari craton, and show that sugilite, and probably also unaltered norrishite, are suitable for dating metamorphism.

The amblygonite-montebrasite series of minerals, common constituents of granitic pegmatites and topaz-bearing granites, show complete solid solution with ideal composition LiAlPO 4 (F, OH). These compounds are ideal for studying F ↔ OH solid solution in minerals because natural members of the series generally show little deviation from the ideal composition. In this study, we used powder and single-crystal neutron diffraction and solid-state 6 Li MAS, CP MAS, and REDOR NMR techniques to study the effect of F ↔ OH substitution on the series. Lattice parameters refined from single-crystal neutron diffraction data show increasing b and decreasing a, c, and V with increasing F/(F + OH). The volume is highest for the OH end-member because of the presence of an additional atom (H). The a and c parameters decrease with increasing F/(F + OH) because the O-H vector is close to the a-c plane and the Al-OH/F vectors are approximately parallel to c. Lattice parameters refined from neutron powder diffraction patterns collected at lower T show that thermal contraction increases with F/(F + OH), presumably because the F anion takes up less space than the OH molecule. The results show that the OH/F position is always fully occupied. The H displacement ellipsoid shows little change with occupancy, which obviously corresponds negatively with increasing F/(F + OH). However, the Li displacement ellipsoid becomes extremely large and anisotropic with increasing F fraction. Most of the distortion is associated with the U3 eigenvalue, which lies between the c and c* directions. U eq values corresponding to the Li atom show a greater reduction with decreasing temperature than the other atoms. The temperature dependence of Li is the same regardless of F content. Even when extrapolated to absolute zero the Li displacement ellipsoid is very large, which implies a large static disorder. At the montebrasite end of the series, there are five short Li-j (j = O, OH, F) distances and one very long Li-O4 bond. With increasing F content, the Li-O 4 distance decreases and the Li-O3f distance increases, such that at the amblygonite end, the coordination is 4 + 2. The disorder in the Li site is obviously caused by the substitution of F for OH. The driving force is the loss of the hydrogen bond to O4, which causes the Li-O4 bond to strengthen and improves the bond valence to O4. The results show that the H atom position is imbedded within the distorted octahedral oxygen coordination of the Li atom. To represent the disorder better, we used a split Li site model. The results show that Li1 occupancy increases and the Li2 occupancy decreases with increasing F content, and that the Li1-Li2 distance is longer for intermediate compositions than for the end-members. The 6 Li MAS NMR experiments provide important structural information complementary to the neutron diffraction results. The spectra of samples in the amblygonite-montebrasite series show two well-resolved peaks, confirming the presence of Li disorder over two distinct sites, and highly resolved 6 Li MAS NMR spectra are obtained at the very high magnetic field strength of 18.8 T. The peaks at -0.3 ppm and -0.9 ppm were unambiguously assigned to the Li2(OH) and Li1(F) sites found in the neutron diffraction structures. The isotropic chemical shifts are consistent with the coordinations of these Li sites found in the neutron diffraction structures. The relative intensities of the two peaks across the series of samples reflect the varying F/(F + OH). In addition to confirming the assignments of the peaks, it is possible to measure H-Li2 and F-Li1 internuclear distances by 6 Li{ 1 H} and 6 Li{ 19 F} CP and REDOR NMR that are consistent with the corresponding distances from the neutron diffraction structures. The 6 Li{ 1 H} and 6Li{ 19 F} CP and REDOR results indicate that the Li disorder is random throughout the crystals rather than over large domains, a conclusion that cannot be made from diffraction experiments. Variable temperature 6 Li MAS NMR spectra confirm that the disorder is static and there is no dynamic exchange involving F, OH, or Li. Each Li ion has access to only one of the two observed sites as determined by the presence of either OH or F in its immediate environment and there is no possibility of a dynamic exchange.

Several compounds of the NaMn(Fe 1-x In x ) 2 (PO 4 ) 3 solid solution were synthesized by solid state reaction in air; pure alluaudite-like compounds were obtained for x = 0.00 to 1.00. X-ray Rietveld refinements indicate the presence of Na + at the A1 and A2’ sites, Mn 2+ at the M1 site, and Fe 2+ , Fe 3+ , and In 3+ at the M2 site. The presence of small amounts of In 3+ at the M1 site, and Mn 2+ at the M2 site, indicates a partially disordered distribution between these cations. A good correlation was also established between the M1-M2 bond distance and the β angle of the alluaudite-like compounds. The disordered distribution of Fe 2+ , Fe 3+ , and In 3+ at the M2 site is confirmed by the broadness of the infrared absorption bands. The Mossbauer spectra, measured between 90 and 295 K, were analyzed in terms of a model that takes into account the next-nearest neighbor inter­actions around the M2 crystallographic site. In all cases these spectra reveal the unexpected pres­ence of small amounts of Fe 2+ at the M2 site, an amount that decreases as the In 3+ content increases. The Fe 2+ and Fe 3+ isomer shifts are typical of the alluaudite structure and vary with temperature, as expected from a second-order Doppler shift. The derived iron vibrating masses and Mossbauer lattice temperatures are within the expected range of values for iron cations in an octahedral environment. The Fe 2+ and Fe 3+ quadrupole splittings are also typical of the alluaudite structure and the temperature dependence of the Fe 2+ quadrupole splitting was fit with the model of Ingalls (1964), which yielded a ground state orbital splitting of ca. 380 to 570 cm -1 for the Fe 2+ sites.

Gatelite-(Ce), ideally (Ca 1 REE 3 ) Σ=4 [Al2(Al,Mg)(Mg,Fe,Al)] Σ=4 [Si 2 O 7 ][SiO 4 ] 3 (O,F) (OH,O) 2 , is a newly identified mineral from the Trimouns talc deposit, Luzenac, Ariège, French Pyrenees. The mineral occurs as striated colorless crystals finely intergrown with minute lamellae of törnebohmite- (Ce). Associated minerals include pyrite, aeschynite-(Y), dolomite, törnebohmite-(Ce), dissakisite- (Ce), talc, and quartz. Gatelite-(Ce) is insoluble in HCl, relatively hard (Mohs hardness 6-7), brittle with irregular fracture, and exhibits good {100} and imperfect {001} cleavages. Gatelite-(Ce) is monoclinic, space group P2 1 /a, with the following unit-cell parameters: a = 17.770(4), b = 5.651(1), c =17.458(4) Å, b =116.18(2)°, V = 1573.3(6) Å 3 , and Z = 4. The strongest five powder-diffraction lines [d in Å (I/Io) (hkl)] are 15.67 (87) (001); 3.49 (50) (4̅12); 2.97 (100) (2̅15); 2.83 (44) (020); and 2.61 (56) (6̅12). Electron-microprobe analysis supported by single-crystal structure determination yielded the following empirical formula: (Ca 1.09 La 0.54 Ce 1.36 Pr 0.14 Nd 0.75 Sm 0.11 Dy 0.01 Y 0.04 ) Σ4.04 (Al 3.06 Mg 0.51 Fe 2+ 0.32 Nb 0.01 ) Σ3.90 Si 5.06 O 20.26 (OH) 1.60 F 0.14 . The calculated density (from the empirical formula) is 4.51 g/ cm 3 . The structure was solved by direct methods and refined to R obs = 4.65%. It consists of edgesharing octahedral chains running along the b axis, cross-linked by SiO 4 and Si 2 O 7 groups. The remaining large cavities are occupied by Ca and REE. The structure of gatelite-(Ce) can be easily described as a regular alternance of slabs of epidote-type structure (E) and slabs of törnebohmitetype structure (T) parallel to the (001) plane. Gatelite-(Ce) can be regarded as a ET polisome within a polysomatic series having epidote and törnebohmite as end-members.

Clinopyroxenes in eclogites from the deep subducted crust in the Kokchetav, Kazakhstan, ultrahigh- pressure metamorphic terrane were found by infrared spectroscopy to contain up to 3020 ppm OH. These rocks were recrystallized at pressures over 60 kbar and temperatures over 1000 °C, where no hydrous minerals are stable. Infrared spectra of the clinopyroxenes show three hydroxyl absorption bands in the regions 3440-3460 cm -1 , 3500-3530 cm -1 , and 3600-3625 cm -1 . The hydroxyl absorbance increases with recrystallization pressure of the eclogites and with vacancy concentration in the pyroxene structure (Ca-Eskola component). Clinopyroxenes represent about 40-50 vol% of eclogites, such that the bulk eclogites contain approximately 1300 ppm hydroxyl at depths greater than 150 km. Thus, subducted oceanic crust can transport trace amounts of H 2 O into the upper mantle, even in the absence of nominally hydrous minerals.

[The crystal structure of kingite, Al 3 (PO 4 ) 2 (F 5 OH) 2· B(H 2 O, OH), a secondary mineral from a Cambrian-Precambrian phosphate deposit at Tom’s Quarry, near Kapunda, South Australia, has been determined from a powder sample using synchrotron X-ray diffraction data. The structure was determined ab initio by direct methods and refined to R bragg = 0.022 and R wp = 0.039 using the Rietveld method. The triclinic structure was solved and refined in the space group P1̅, a = 9.377(1), b = 10.113(1), c = 7.138(1) A, σ = 97.60(1), β= 100.88(1), γ = 96.01(1)°, V= 653.0(1) A 3 , Z= 2. The structure of kingite contains finite strings of three corner sharing Al Φ6 octahedra (where Φ represents O, OH - , F - , or H 2 O). These strings are cross-linked via PO 4 tetrahedra to produce layers that are perpendicular to [100]. The layers are linked via hydrogen bonding through H 2 O located in the interlayer space. Kingite is shown to have a different stoichiometry to that reported earlier. The relationship of kingite to the structures of wavellite, Al 3 (P0 4 ) 2 (OH) 3· 5H 2 O, and mitryaevaite, A1 5 (PO 4 ) 2 [(P,S)O 3 (OH,O)] 2 F 2 (OH) 2 (H 2O ) 8 -6.48H 2 O, are briefly discussed.

Autunite, Ca[(UO 2 )(PO 4 )] 2 (H 2 O) 11 , is amongst the most abundant and widely distributed of the uranyl phosphate minerals, yet because of its pseudo-tetragonal symmetry and rapid dehydration in air, the details of its symmetry, stoichiometry, and structure were previously uncertain. The crystal structure of synthetic autunite was solved by direct methods and refined by full-matrix least-squares techniques to agreement indices R1 = 0.041, calculated for the 1497 unique observed reflections (|F 0 | ≥ 4σF), and wR 2 = 0.119 for all data. Autunite is orthorhombic, space group Pnma, Z = 4, a = 14.0135(6), b = 20.7121(8), c = 6.9959(3) Å, V = 2030.55(15) Å 3 . The structure contains the wellknown autunite type sheet with composition [(UO 2 )(PO-)] - , resulting from the sharing of equatorial vertices of the uranyl square bipyramids with the phosphate tetrahedra. The calcium atom in the interlayer is coordinated by seven H 2 O groups and two longer distances to uranyl apical O atoms. Two symmetrically independent H 2 O groups are held in the structure only by hydrogen bonding. Bond-length-constrained refinement provided a crystal-chemically reasonable description of the hydrogen bonding.