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The origin of sulfite ([S.sup.4+]) species in silicate glasses was evaluated using XANES at the S K-edge. Systematic investigations show that the presence of [S.sup.4+] species in silicate glasses is an analytical artifact related to changes in the sulfur species caused by irradiation with an electron beam during EMPA or by irradiation with an intense focused X-ray beam during synchrotron analysis. The data shown here indicate that [S.sup.2-] and [S.sup.6+] are the only significant sulfur species occurring in silicate glasses synthesized under geologically relevant conditions. Keywords: XAS (XAFS, XANES), sulfur K-edge, glass properties, sulfur oxidation state, beam damage, sulfur speciation

Two series of leucite group materials, with K, Rb, and Cs as extra-framework cations, have been synthesized by ion exchange from a natural well-ordered analcite and a natural disordered leucite. 29Si and 27Al MAS NMR data for the analcite-derived series provide complementary information on tetrahedral cation ordering. The ordering in terms of the number of Al next-nearest neighbors, Q4(nAl) (short-range order), does not change significantly during ion exchange, indicating that Al and Si remain essentially fixed in their original positions. In contrast, the ordering of Al over T1, T2, and T3 (long-range order) for the analcite-derived series changes dramatically with changing alkali cation; the Al occupancies for the three analcite-derived samples expressed as T1:T2:T3 are approximately 0.25:0.50:0.25 for KAl[Si.sub.2][O.sub.6], 0.40:0.20:0.40 for RbAl[Si.sub.2][O.sub.6], and 0.15:0.70:0.15 for CsAl[Si.sub.2][O.sub.6]. During the ion exchange, at temperatures above the cubic-tetragonal phase transitions, only one symmetrically distinct T site is present. It is proposed that on cooling through the cubic-tetragonal phase transition the structure collapses around the non-framework cations to give the lowest energy Si-Al distribution over the three T sites irrespective of the original T-site ordering in the starting material. Our data suggest that the identity of the cation in the W site affects the orientation of the framework distortions associated with the cubic-tetragonal phase transition and leads to the possibility that a particular tetrahedral cation site can take on the characteristics of a T1, T2, or T3 site. The data and their interpretation have important implications for the mechanism of this type of structural phase transition.

The structural phase transitions of monoclinic tridymite that occur at elevated temperatures, as well as the structure of the high-temperature modifications, have been studied by 29Si MAS NMR. The sample, originating from the surface of a refractory silica brick, displays sharp discontinuities in the first moment ([M.sub.1]), second moment ([M.sub.2]), or both, of the 29Si MAS NMR spectrum at the following phase transitions: 108 [+ or -] 1 [degrees] C for the monoclinic (MC) to orthorhombic (OP) transition, 165 [+ or -] 5 [degrees] C for the transition from OP to a second orthorhombic phase (OS), and 206 [+ or -] 4 [degrees] C for the transition from OS to a third orthorhombic phase (OC). [M.sub.1] for the OC phase becomes more negative with increasing temperature up to [approximately]323 [degrees] C, at which there is also a small discontinuity in [M.sub.2], indicating a transition from OC to a hexagonal phase (LHP). The spectrum of the MC phase consists of 12 narrow lines of equal area, consistent with the known crystal structure. The spectrum of the OP phase is broad and asymmetric, indicating a distribution of local environments at the Si sites. It can be simulated assuming an incommensurate structural modulation with a maximum amplitude of [approximately]5 x [10.sup.-2] [Angstrom], but it is also consistent with a recent domain-formation model, provided that most disorder occurs at the Si6 site. The spectrum of the OS phase is asymmetric, indicating a nonlinear incommensurate modulation with a maximum amplitude of [approximately]2 x [10.sup.-2] [Angstrom]. The spectra of higher temperature phases are consistent with the single Si site of the known crystal structures, but [M.sub.1] values imply significantly smaller mean Si-O-Si angles e.g., = 152.6 [degrees] at 220 [degrees] C from NMR, compared with 168.0 [degrees] from X-ray diffraction data. This difference is consistent with disorder of O atoms around Si-Si axes.

A synthetic analog of leucite was analyzed by magic angle spinning spinning nuclear magnetic resonance (MAS NMR) for silicon (Si) and aluminum (Al) ordering and was compared with MAS NMR observations of natural leucite. The synthetic sample was made from ion exchange treatment of analcite. The synthetic sample had 45-50% of the total Al on T2 crystal sites and the rest divided equally on the T1 and T3 sites. Models based on this finding were then applied to the natural sample. Al in the natural sample is probably ordered on the T2 crystal site.

Eight glasses with molar Mg/2Al [approximately equals] 1 in the system MgO-[Al.sub.2][O.sub.3]-Si[O.sub.2] have been studied by magic angle spinning (MAS) NMR spectroscopy. Using triple quantum (3Q) NMR techniques we find evidence for significant concentrations of Al coordinated to five O atoms in all glasses, the proportion increasing with decreasing Mg/Al and decreasing silica content. In glasses with Mg/2Al = 1, up to 6% of the Al is estimated to be coordinated to five rather than four O atoms. Calculations of the polymerization state of these liquids made assuming that all aluminum is in tetrahedral coordination charge balanced by magnesium are thus seriously in error. Such errors may be of even greater importance at the high temperatures and pressures relevant to the Earth and materials sciences.

Experimental and theoretical evidence for the coupled substitution of B and H in synthetic forsterite and a natural olivine is presented. The intensities of OH bands at 3704 cm-1 (//z), 3598 cm-1 (//x,y), and 3525 cm-1 (//x) in a heterogeneous B-doped synthetic forsterite crystal matches the zoning of B concentration measured by ion probe. The two anti-symmetric stretching vibrations of BO3 groups occur at 1301 cm-1 (//x) and 1207 cm-1 (//z) for the 10B and at 1256 and 1168 cm-1 for the 11B isotope. A microscopic model of the mixed (B,H) defect that accounts for experimental observations is obtained from quantum mechanical calculations. The BO3 group lies on the (O3-O1-O3) face of the vacant Si site and the H atom is bonded to the O2 atom at the remaining apex. The occurrence of the same OH bands associated with v3 BO3 vibrations in a natural olivine sample from Pakistan confirms the occurrence of this defect in nature. The three diagnostic OH bands can be used as a signature of H associated with boron substitution in olivine and forsterite, leading to a quantitative analysis of their contribution to H-defects. [ABSTRACT FROM AUTHOR]

Pressure-temperature melting curves in two carbonate-bearing systems of relevance to Earth's mantle have been determined using the laser-heated diamond-anvil cell (LH-DAC). The solidus along the MgSiO3-MgCO3 join in the MgO-SiO2-CO2. system (MS-CO2) was defined from 15 to 80 GPa using in situ melting criteria, reaching a maximum temperature of ~2340 K at 80 GPa. The occurrence of melting has been confirmed with ex-situ textural and chemical analysis of recovered samples. The melting curve has a negative dT/dP slope at pressures between ~15 and 23 GPa resulting from the subsolidus phase transition of ilmenite- to perovskite-structured MgSiO3. The shallow slope of the melting curve at pressures higher than this transition indicate that for plausible mantle geotherms carbonate-bearing silicate lithologies will melt throughout the lower mantle. The solidus of a mixture along the MgCO3-CaCO3 join was determined as a proxy for alkali-free carbonate lithologies. Melting temperatures increase from 1860 K at 16 GPa to ~2100 K above 35 GPa, where the melting curve flattens. The melting reaction magnesite + post-aragonite (high-pressure CaCO3) = melt was confirmed using an in situ experiment. We conclude that crystalline Mg and Ca carbonate mixtures are unstable with respect to molten carbonate at conditions of the convective lower mantle. The flat melting curves at high pressures in both systems suggests that subducted carbonates will undergo melting at lower mantle conditions, a process that may be important for superdeep diamond formation and carbon storage in the deep mantle. [ABSTRACT FROM AUTHOR]

The partitioning of phosphorus between olivine and melt was measured by conducting partitioning experiments within the system Mg2SiO4-Ca2Al2Si2O9-NaAlSi3O8-Mg3(PO4)2, using olivines crystallized with cooling rates in the range 1 to 10 °C/h. Partition coefficients, DPol/melt, vary from 0.05 to 1.41 and are strongly influenced by melt composition, with high partition coefficients observed for more polymerized melts. Correlations between elemental abundances in the olivines demonstrate that phosphorus substitutes on a near one for one basis for Si, charge-balanced by Mg vacancies; no correlation between P and Al concentration in the olivine was observed. A complete understanding of the controls on DPol/melt is required to model quantitatively the zoning of P in olivine that has recently been shown to be a common feature of igneous olivine crystals. The strong melt compositional dependence observed in this study implies that both changing concentrations of P in the melt during fractionation, and changing values of DPol/melt can contribute to core-to-rim variations in P when the surface composition of a growing olivine crystal is in equilibrium with the melt. The common observation of zones in natural olivines where high P is correlated with high Al and Cr must be explained by processes other than equilibrium growth and, as suggested previously, such zones are probably related to solute trapping during episodes of rapid growth. [ABSTRACT FROM AUTHOR]

The partitioning of phosphorus between olivine and melt was measured by conducting partitioning experiments within the system Mg2SiO4-Ca2Al2Si2O9-NaAlSi3O8-Mg3(PO4)2, using olivines crystallized with cooling rates in the range 1 to 10 °C/h. Partition coefficients, DPol/melt, vary from 0.05 to 1.41 and are strongly influenced by melt composition, with high partition coefficients observed for more polymerized melts. Correlations between elemental abundances in the olivines demonstrate that phosphorus substitutes on a near one for one basis for Si, charge-balanced by Mg vacancies; no correlation between P and Al concentration in the olivine was observed. A complete understanding of the controls on DPol/melt is required to model quantitatively the zoning of P in olivine that has recently been shown to be a common feature of igneous olivine crystals. The strong melt compositional dependence observed in this study implies that both changing concentrations of P in the melt during fractionation, and changing values of DPol/melt can contribute to core-to-rim variations in P when the surface composition of a growing olivine crystal is in equilibrium with the melt. The common observation of zones in natural olivines where high P is correlated with high Al and Cr must be explained by processes other than equilibrium growth and, as suggested previously, such zones are probably related to solute trapping during episodes of rapid growth. [ABSTRACT FROM AUTHOR]

The structure of the synthetic high-pressure sheet-disilicate Phase-X (PhX), a possible host of H2O and K in the mantle, has been determined for a crystal synthesized at 16 GPa/1300 °C/23 h. The composition of the sample is close to K1.5Mg2Si2O2H0.5, which is 50% PhX/50% Anhydrous-PhX and has 25% of interlayer K sites vacant. The structures of four crystals were determined by single-crystal X-ray diffraction and had very similar diffraction characteristics and structural results; the structure of one of the larger crystals is reported here. Reflection intensity statistics strongly indicate that PhX is centrosymmetric, space group P63/mcm, in contrast to other studies that have reported non-centrosymmetric space group P63cm. While it was possible to obtain good agreement indices for refinements in P63cm, there were strong correlations between atoms that are equivalent in P63/mcm, suggesting that the correct structure is centrosymmetric. Full anisotropic refinement in space group P63/mcm gave R1 = 0.036, wR2 = 0.079, GoF = 1.467. As with all previous studies of PhX, the H atom was not located. Difference-Fourier maps of the residual electron density indicated that the K atom is displaced from the 4c site lying on the sixfold axis on to three split 12j sites 0.2 Å away, each having 1/4 occupancy, giving a total of 3 K atoms per unit cell and corresponding to 1.5 K apfu, in good agreement with the content derived from electron microprobe analysis. Diffraction patterns of all four crystals examined, reconstructed from the full-intensity data collection, consistently show the presence of a large hexagonal superstructure with dimensions 8asub x 8asub x csub, having Z = 128, compared with Z = 2 for the two-layer subcell. Complex arrays of superlattice reflections occur in layers with l = 2n, but are absent from l = 2n + 1 layers. Unpolarized infrared spectra of single crystals of PhX were obtained that are similar to those reported previously in the literature. Spectra in the OH-stretching region consist of a major absorption band at 3595 cm-1 and three much weaker bands at 3690, 3560, and 3405 cm-1. Bond-valence analysis of PhX indicates that O1 is very over-bonded, whereas O2 is slightly under-bonded and a possible site for protonation. We present geometrical and crystal-chemical arguments that exclude O1 as a candidate for protonation, whereas a much better case can be made for O2. In PhX structures, H must be located at a partially occupied site with a multiplicity 4 ≤ m ≤ 24 in P63/mcm or 4 ≤ m ≤ 12 in P63cm. Such low occupancies for H sites are the likely reason for their invisibility to diffraction. We outline a model for the incorporation of H into PhX of composition K1.5Mg2Si2O7H0.5 that suggests a mechanism for ordering based upon avoidance of H and K, coupled with K-site vacancies. Such behavior may also be the origin of the superstructure. The P62/mcm structure and the presence of an underlying superstructure may well be characteristic of ordered intermediate compositions at or near PhX50/Anhydrous-PhX50. Identification of a new space group and recognition of a previously unobserved superstructure point to new possibilities for PhX and its derivatives that may bear significantly upon their stability at mantle conditions. [ABSTRACT FROM AUTHOR]

The origin of sulfite (S4+) species in silicate glasses was evaluated using XANES at the S K-edge. Systematic investigations show that the presence of S4+ species in silicate glasses is an analytical artifact related to changes in the sulfur species caused by irradiation with an electron beam during EMPA or by irradiation with an intense focused X-ray beam during synchrotron analysis. The data shown here indicate that S2- and S6+ are the only significant sulfur species occurring in silicate glasses synthesized under geologically relevant conditions. [ABSTRACT FROM AUTHOR]

Measures the dissolved water (H2O) concentrations of experimentally produced, iron-free, nominally anhydrous minerals (NAM) using hydrogen (1H) magic-angle-spinning nuclear magnetic resonance spectroscopy. Peak broadening; Cluster incorporation into a silicate structure; Broad component due to submicroscopic hydrous mineral or glass impurities or static H2O molecules present at grain boundaries.

The structure of the synthetic high-pressure sheet-disilicate Phase-X (PhX), a possible host of H2O and K in the mantle, has been determined for a crystal synthesized at 16 GPa/1300 °C/23 h. The composition of the sample is close to K1.5Mg2Si2O2H0.5, which is 50% PhX/50% Anhydrous-PhX and has 25% of interlayer K sites vacant. The structures of four crystals were determined by single-crystal X-ray diffraction and had very similar diffraction characteristics and structural results; the structure of one of the larger crystals is reported here. Reflection intensity statistics strongly indicate that PhX is centrosymmetric, space group P63/mcm, in contrast to other studies that have reported non-centrosymmetric space group P63cm. While it was possible to obtain good agreement indices for refinements in P63cm, there were strong correlations between atoms that are equivalent in P63/mcm, suggesting that the correct structure is centrosymmetric. Full anisotropic refinement in space group P63/mcm gave R1 = 0.036, wR2 = 0.079, GoF = 1.467. As with all previous studies of PhX, the H atom was not located. Difference-Fourier maps of the residual electron density indicated that the K atom is displaced from the 4c site lying on the sixfold axis on to three split 12j sites 0.2 Å away, each having 1/4 occupancy, giving a total of 3 K atoms per unit cell and corresponding to 1.5 K apfu, in good agreement with the content derived from electron microprobe analysis. Diffraction patterns of all four crystals examined, reconstructed from the full-intensity data collection, consistently show the presence of a large hexagonal superstructure with dimensions 8asub x 8asub x csub, having Z = 128, compared with Z = 2 for the two-layer subcell. Complex arrays of superlattice reflections occur in layers with l = 2n, but are absent from l = 2n + 1 layers. Unpolarized infrared spectra of single crystals of PhX were obtained that are similar to those reported previously in the literature. Spectra in the OH-stretching region consist of a major absorption band at 3595 cm-1 and three much weaker bands at 3690, 3560, and 3405 cm-1. Bond-valence analysis of PhX indicates that O1 is very over-bonded, whereas O2 is slightly under-bonded and a possible site for protonation. We present geometrical and crystal-chemical arguments that exclude O1 as a candidate for protonation, whereas a much better case can be made for O2. In PhX structures, H must be located at a partially occupied site with a multiplicity 4 ≤ m ≤ 24 in P63/mcm or 4 ≤ m ≤ 12 in P63cm. Such low occupancies for H sites are the likely reason for their invisibility to diffraction. We outline a model for the incorporation of H into PhX of composition K1.5Mg2Si2O7H0.5 that suggests a mechanism for ordering based upon avoidance of H and K, coupled with K-site vacancies. Such behavior may also be the origin of the superstructure. The P62/mcm structure and the presence of an underlying superstructure may well be characteristic of ordered intermediate compositions at or near PhX50/Anhydrous-PhX50. Identification of a new space group and recognition of a previously unobserved superstructure point to new possibilities for PhX and its derivatives that may bear significantly upon their stability at mantle conditions. [ABSTRACT FROM AUTHOR]