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Coordination and local geometry around Si cations in silicate liquids are of primary importance in controlling the chemical and physical properties of magmas. Pressure-induced changes from fourfold to sixfold coordination of Si in silicate glass samples quenched from liquids has been detected with (29)Si magic-angle spinning nuclear magnetic resonance spectrometry. Samples of Na(2)Si(2)O(5) glass quenched from 8 gigapascals and 1500 degrees C contained about 1.5 percent octahedral Si, which was demonstrably part of a homogeneous, amorphous phase. The dominant tetrahedral Si speciation in these glasses became disproportionated to a more random distribution of bridging and nonbridging oxygens with increasing pressure.

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Ab initio molecular orbital calculations have been carried out for silicate, aluminosilicate, and aluminate clusters in order to study the NMR characteristics of oxygen sites that are possibly present in the structure of aluminosilicate glasses and melts. Of particular interest are the different types of bridging oxygens (oxygens bonded to two tetrahedrally coordinated Si and/or Al) and tricluster oxygens (oxygens linked to three tetrahedrally coordinated Si and/or Al). The calculated values for the 17O quadrupolar coupling constants (QCC) of Si−O−Si, Si−O−Al, and Al−O−Al bridging oxygens agree reasonably well with experimental NMR data for similar oxygen sites in crystalline silicates, aluminosilicates, or aluminates, and do not show significant dependence on the basis set and theory (Hartree−Fock vs density functional theory) of calculations. The calculated values for the 17O isotropic chemical shift (δi O), on the other hand, show large dependence on the basis set and method of calculations, although the relative differences in δi O for the clusters of interest do not vary as much. Our calculations indicate that bridging oxygens give progressively larger 17O QCC and larger, but overlapping, δi O from Al−O−Al to Si−O−Al to Si−O−Si. Coordination of Si−O−Al or Al−O−Al bridging oxygens to Ca2+, a network-modifying cation, tends to decrease their 17O QCC and increase their δi O, consistent with experimental NMR data. Tricluster oxygens give progressively larger 17O QCC and larger δi O values from O(Al3), to O(SiAl2), to O(Si2Al), to O(Si3). The O(Al3) and O(SiAl2) tricluster oxygens have 17O NMR characteristics overlapping with those of Si−O−Al bridging oxygens, and the O(Si2Al) tricluster oxygen overlapping with those of Si−O−Si bridging oxygens. Our calculations suggest that it may be difficult to distinguish O(Al3), O(SiAl2), or O(Si2Al) tricluster oxygens from bridging oxygens in aluminosilicate glasses on the basis of 17O NMR data alone, although it should be possible to distinguish O(Al3) tricluster oxygen from Al−O−Al bridging oxygen in aluminates using 17O NMR. The O(Si3) tricluster have larger 17O QCC and larger δi O values than all types of bridging oxygens and thus may be unambiguously identified on the basis of 17O NMR data, if present in the structure of aluminosilicates or silicates.

In order to shed light on the proton distributions and order/disorder in high-pressure delta-Al(OH)3 and delta-AlOOH phases, two-dimensional, high-resolution 1H CRAMPS (FSLG)-MAS NMR and 27Al 3QMAS NMR spectra have been obtained. For delta-Al(OH)3, the 1H CRAMPS-MAS NMR revealed two peaks with an intensity ratio close to 2:1. The 27Al MAS and 3QMAS NMR suggest a single Al site with a well-defined local structure. For delta-AlOOH, the 1H and 27Al NMR indicate the presence of a single H and Al site each. These results are consistent with crystal structures refined from X-ray diffraction. For comparison, 1H MAS and CRAMPS-MAS NMR spectra were also obtained for several other hydroxides/oxyhydroxides, including In(OH)3 and InOOH that have similar structures to delta-Al(OH)3 and delta-AlOOH, respectively. These data not only provide additional insights into the proton distributions in these important crystal structure classes but also together provide a better defined quantitative correlation between 1H chemical shift and hydrogen-bonding O...O distance.