The structural behavior of Al3+ in peralkaline glasses and melts along the Na2Si3O7-Na-2(NaAl)(3)O-7 join has been examined to 1200degreesC at ambient pressure with Si-29 MAS NMR and Raman spectroscopy. The distribution of Al3+ among coexisting Q(4), Q(3), and Q(2) structural units in the glasses and melts was determined as a function of bulk Al/(Al+Si) and temperature. The Al3+ resides principally in Q(4) structural units, which contain more than 70% of the total amount of Al3+. The Q(2) units contain the smallest amount of Al3+ among the Q(4), Q(3), and Q(2) structural units. There is no evidence for temperature-dependent distribution of Al3+ among the coexisting structural units at least to 1100-1200degreesC.

Cation ordering in covalent oxide glasses and melts profoundly affects the macroscopic properties, such as viscosity, diffusivity, and thermodynamic potentials. It is commonly assumed that in glasses and melts nonframework cations such as Na+, Ca2+, and Ba2+ distribute randomly around nonbridging oxygen (NBO). Several macroscopic studies on the melting of silicates and thermodynamic data have suggested that a possible nonrandomness may exist among cations around NBO in mixed-cation silicate glasses. Here, we report unambiguous experimental evidence of chemical ordering of nonframework cations and demonstrate a clear preference for certain types of cation-NBO complexes in mixed-cation silicate glasses using O-17 magic angle spinning (MAS) and multiple quantum MAS NMR. Particularly, complete bonding preference and cation ordering occurs in Ba-Mg silicate glasses (BaMgSi2O6) glass in such a way that nonbridging oxygen either only has Ba2+ as a nearest neighbor (Ba-NBO) or exists as a complex containing one Ba+ and two Mg2+ as nearest neighbors while no detectable fraction of Mg-NBO is observed. Ba-Na silicate glasses, on the other hand, show a wide distribution of configurations for two types of cations around NBO, forming Ba- and Na-NBO as well as substantial intensity of mixed NBO peaks, which indicates a prevalence of dissimilar pairs around NBO or a stronger preference to Ba-O-Si-[4] over Na-O-Si-[4]. The present results, combined with the previous results on Na-Ca silicate glasses, highlight the tendency for chemical ordering upon cation mixing in oxide glasses and may provide an atomistic explanation for diffusivity anomalies as well as activity-composition relationship of silicate melts.

Structural interaction between dissolved fluorine and silicate glass (25degreesC) and melt (to 1400degreesC) has been examined with F-19 and Si-29 MAS NMR and with Raman spectroscopy in the system Na2O-Al2O3-SiO2 as a function of Al2O3 content. Approximately 3 mol.% F calculated as NaF dissolved in these glasses and melts. From F-19 NMR spectroscopy, four different fluoride complexes were identified. These are (1) Na-F complexes (NF), (2) Na-Al-F complexes with Al in 4-fold coordination (NAF), (3) Na-Al-F complexes with Al in 6-fold coordination with F (CF), and (4) Al-F complexes with Al in 6-fold, and possibly also 4-fold coordination (TF). The latter three types of complexes may be linked to the aluminosilicate network via Al-O-Si bridges.

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.