The electronic spin state of iron in lower mantle perovskite is one of the fundamental parameters that governs the physics and chemistry of the most voluminous and massive shell in the Earth. We present experimental evidence for spin-pairing transition in aluminum-bearing silicate perovskite (Mg,Fe)(Si,Al)O-3 under the lower mantle pressures. Our results demonstrate that as pressure increases, iron in perovskite transforms gradually from the initial high-spin state toward the final low-spin state. At 100 GPa, both aluminum-free and aluminum-bearing samples exhibit a mixed spin state. The residual magnetic moment in the aluminum-bearing perovskite is significantly higher than that in its aluminum-free counterpart. The observed spin evolution with pressure can be explained by the presence of multiple iron species and the occurrence of partial spin-paring transitions in the perovskite. Pressure-induced spin-pairing transitions in the perovskite would have important bearing on the magnetic, thermoelastic, and transport properties of the lower mantle, and on the distribution of iron in the Earth's interior.

The structures of sodium silicate and aluminosilicate glasses quenched from melts at high pressure (6-10 GPa) with varying degrees of polymerization (fractions of nonbridging oxygen) were explored using solid-state NMR [O-17 and Al-27 triple-quantum magic-angle spinning (3QMAS) NMR]. The bond connectivity in melts among four and highly coordinated network polyhedra, such as Al-[4], Al-[5,Al-6], Si-[4], and Si-[5,Si-6], at high pressure is shown to be significantly different from that at ambient pressure. In particular, in the silicate and aluminosilicate melts, the proportion of nonbridging oxygen (NBO) generally decreases with increasing pressure, leading to the formation of new oxygen clusters that include 5- and 6-coordinated Si and Al in addition to 4-coordinated Al and Si, such as Si-[4]-O-Si-[5,Si-6], Si-[4]-O-Al-[5,Al-6] and Na-O-Si-[5,Si-6]. While the fractions of Al-[5,Al-6] increase with pressure, the magnitude of this increase diminishes with increasing degrees of ambient-pressure polymerization under isobaric conditions. Incorporating the above structural information into models of melt properties reproduces the anomalous pressure-dependence of O2- diffusivity and viscosity often observed in silicate melts. Copyright (C) 2004 Elsevier Ltd.

X-ray Absorption Near Edge Structure (XANES) experiments made between 600 and 700 T at the Fe K-edge have been used to study the kinetics of iron oxidation in a supercooled melt of Fe-bearing pyroxene composition. To provide a firmer basis to redox determinations, the redox state of a series of samples was first determined from wet chemical, Mossbauer spectroscopy and electron microprobe analyses. The XANES experiments show that variations in relative abundances of ferric and ferrous iron can be determined in situ, even just above the glass transition, and that some information can also be obtained on the structural environment around iron cations. The kinetics of iron oxidation do not vary much with temperature down to the glass transition. This observation suggests that the rate-limiting factor in this process is not oxygen diffusion, which is coupled to relaxation of the silicate network, but, as described by Cooper and coworkers, diffusion of network modifying cations along with a counter flux of electrons. (C) 2004 Elsevier B.V. All rights reserved.

The applicability of a speciation model to quantify the thermodynamic properties of silicate liquids was evaluated in the Na2O-SiO2 system. Based on spectroscopic data, four sodium-silicate species with various numbers of bridging oxygen atoms were considered. We used a thermodynamic model that assumed ideal mixing between the species and temperature-independent, additive heat capacities of the species.

Mineral/melt trace element partition coefficients, D-i(mineral/melt) (=concentration ratio, mineral/melt), can vary by up to two orders of magnitude in the melt composition range of natural magmatic liquids. As most geochemically interesting minor and trace elements are network-modifiers in silicate melts, one would expect, as observed, positive correlation between mineral/melt partition coefficients, D-i(mineral/melt), and the degree of polymerization of the silicate melt, NBO/T. Linear relationships between D-i(mineral/melt) and NBO/T sometimes exist over restricted NBO/T-ranges where the type of Q"-species in the melt does not change and their abundance does not vary greatly. Other experimental D-i(mineral/melt) vs. NBO/T data suggest log-linear or log-log relations in some cases, whereas in other studies, there are no simple relationships between D-i(mineral/melt) and NBO/T of the melt. When relating mineral/melt partition coefficients to degree of melt polymerization, NBO/T, it is assumed that a principal melt structural control on D-i(mineral/melt) is the activity of nonbridging oxygen in the melt, a(NBO). The a(NBO) is related to NBO/T. The NBO/T is not, however, a quantitative measure of a(NBO) because the nonbridging oxygens in coexisting Q-species in melts are energetically non-equivalent. This is evident in relationships between activity coefficient ratios of melt network-modifying cations, y(i)/y(j), where the ionization potentials of i and j differ, and NBO/T of the melt. Such relationships are parabolic with minima or maxima at NBO/T near 1 and resemble the relations of abundance ratio, X-Q3/X-Q2, vs. NBO/T of the melt. Interestingly, this NBO/T-value is near that of natural basalt melt. These observations suggest that i-NBO(Q(3)) and j-NBO(Q(2)) bond characteristics differ from one another, govern trace element solution behavior in silicate melts and, therefore, control the effect of melt composition on mineral/melt partitioning in silicate systems. (C) 2004 Elsevier B.V. All rights reserved.

We report spectroscopic evidence for the pressure-induced structural changes in B2O3 glass quenched from melts at pressures up to 6 GPa using solid-state NMR. While all borons are tri-coordinated at 1 atm, the fraction of tetra-coordinated boron increases with pressure, being about 5% and 27% in the B2O3 glass quenched from melts at 2 and 6 GPa, respectively. The fraction of boroxol ring species increases with pressure up to 2 GPa and apparently decreases with further compression up to 6 GPa. Two densification mechanisms are proposed to explain the variation of boron species with pressure.

A suite of six hydrous (7 wt.% H2O) sodium silicate glasses spanning sodium octasilicate to sodium disilicate in composition were analyzed using Si-29 single pulse (SP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, H-1-Si-29 cross polarization (CP) MAS NMR, and fast MAS H-1-NMR. From the Si-29 SPMAS data it is observed that at low sodium compositions dissolved water significantly depolymerizes the silicate network. At higher sodium contents, however, dissolved H2O does not affect a significant increase in depolymerization over that predicted based on the Na/Si ratio alone. The fast MAS H-1-NMR data reveal considerable complexity in proton environments in each of the glasses studied. The fast MAS 1H-NMR spectra of the highest sodium concentration glasses do not exhibit evidence of signficantly greater fractions of dissolved water as molecular H2O than the lower sodium concentration glasses requiring that the decrease in polymerization at high sodium contents involves a change in sodium solution mechanism. Variable contact time H-1-Si-29 cross polarization (CP) MAS NMR data reveal an increase in the rotating frame spin lattice relaxation rate constant (T-1 rho*) for various Q(n) species with increasing sodium content that correlates with a reduction in the average H-1-Si-29 coupling strength. At the highest sodium concentration, however, T-1 rho* drops significantly, consistent with a change in the Na2O solution mechanism. Copyright (c) 2005 Elsevier Ltd.

Partitioning of Ca, Mn, Mg, and Fe2+ between olivine and melt has been used to examine the influence of energetically nonequivalent nonbridging oxygen in silicate melts. Partitioning experiments were conducted at ambient pressure in air and 1400 degrees C with melts in equilibrium with forsterite-rich olivine (Fo > 95 mol%). The main compositional variables of the melts were NBO/T and Na/(Na+Ca). In all melts, the main structural units were of Q(4), Q(3), and Q(2) type with nonbridging oxygen, therefore, in the Q(3) and Q(2) units.

The structure of multi-component silicate melts and glasses (e.g., Ca-Mg and Ca-Na aluminosilicates) can provide insight into the properties of natural silicate melts and has implications for relevant magmatic processes. In spite of its importance, the atomic and molecular structure of most multicomponents (e.g., quaternary) melts and glasses has not been fully described, primarily because of insufficient resolution obtained with conventional spectroscopic and scattering methods; the information obtained by these methods is compromised by severe inhomogeneous peak broadening due to structural complexity. Here we report the first O-17 and Al-27 3QMAS NMR spectra for quaternary, Ca-Mg and Ca-Na peralkaline aluminosilicate glasses (i.e., M/Al > 1, M is one monovalent or one-half a divalent cation). These data reveal new details into the molecular structure of multi-component aluminosilicate melts, which include the presence of a substantial fraction of Al-V in the Ca-Mg aluminosilicate glasses and Al-IV-O-Al-IV in both glasses at 1 atm. Traditional models of glass structure do not support the presence of such species given these high-silica, peralkaline compositions. These results suggest that Al avoidance is violated in the multi-component peralkaline aluminosilicate glasses, and that the presence of Mg2+ in the melts increases the extent of disorder in the melts (compared with Call and Na+). These factors lead to an increase in configurational entropy and the activity coefficients of the oxides, and may provide an explanation for the decrease in viscosity of these complex melts.

Solubility mechanisms of water in depolymerized silicate melts quenched from high temperature (1000 degrees-1300 degrees C) at high pressure (0.8-2.0 GPa) have been examined in peralkaline melts in the system Na2O-SiO2-H2O with Raman and NMR spectroscopy. The Na/Si ratio of the melts ranged from 0.25 to 1. Water contents were varied from similar to 3 mol% and similar to 40 mol% (based on O = 1). Solution of water results in melt depolymerization where the rate of depolymerization with water content, partial derivative(NBO/Si)/partial derivative X-H2O, decreases with increasing total water content. At low water contents, the influence of H2O on the melt structure resembles that of adding alkali oxide. In water-rich melts, alkali oxides are more efficient melt depolymerizers than water. In highly polymerized melts, Si-OH bonds are formed by water reacting with bridging oxygen in Q(4)-species to form Q(3) and Q(2) species. In less polymerized melts, Si-OH bonds are formed when bridging oxygen in Q(3)-species react with water to form Q(2)-species. In addition, the presence of Na-OH complexes is inferred. Their importance appears to increase with Na/Si. This apparent increase in importance of Na-OH complexes with increasing Na/Si (which causes increasing degree of depolymerization of the anhydrous silicate melt) suggests that water is a less efficient depolymerizer of silicate melts, the more depolymerized the melt. This conclusion is consistent with recently published H-1 and Si-29 MAS NMR and H-1-Si-29 cross polarization NMR data. Copyright (c) 2005 Elsevier Ltd.