Hanna Nekvasil

In support of a depolymerization model for water in sodium aluminosilicate glasses:: Information from NMR spectroscopy

A variety of 29Si, 27Al, 23Na, 17O and 1H NMR data are consistent with the formation of Q3 AlOH and SiOH groups upon dissolution of H2O into aluminosilicate glasses. 29Si single-pulse and 1H-29Si CP/MAS data present more direct evidence for the formation of Q3 Si species. A model that involves rupture of Si-O-Si, Al-O-Si, and Al-O-Al linkages, and possibly cation exchange between H+ and Na+ is most consistent with the variety of data available. In this model the water dissolution process involves several reactions that are likely to proceed concurrently throughout the glass compositional space defined by Al/Si ratio and water content. Because the types of reactions do not change with Al/Si ratio, this model predicts smooth variations in hydroxyl and molecular water species abundances throughout compositional space within the system NaAlSiO4-SiO2-H2O.

Si-F bonding in aluminosilicate glasses: Inferences from ab initio NMR calculations

Abstract Although it has been well accepted that F-Al linkages dominate in F-bearing alkali aluminosilicate glasses, the possibility of F-Si linkages remains unresolved. Recent 19F NMR results have been interpreted as indicative of not only the presence of the Al-F-Na(n) linkages but also of the presence of Si-F-Na(n) linkages. High level ab initio NMR calculations were performed to investigate the nature of possible species contributing to the 19F spectra. The B3LYP/6-31G* level was used to optimize structures and a scaling technique applied to the calculation of NMR properties using both HF and B3LYP methods at the 6-311+G(2df,p) level. The applicability of this scaling method was checked by comparing calculated results against experimental data on several crystalline materials; the results are improved over HF or DFT methods alone. The calculation results show that none of the F-Al species investigated can produce the -147 ppm peak of Schaller et al. (1992). However, several species in which F is bonded to fourfold-, fivefold-, and sixfold-coordinated Si produce a 19F chemical shift of approximately -147 ppm. These results verify the assignments of this peak by Zeng and Stebbins (2000) to tetrahedral Si-F and NaSiF6-like species, and rule out many other possible species. Several additional species such as a fivefold coordinated Si-F species may further contribute to the -147 ppm 19F peak.

Feldspar crystallisation in felsic magmas: a review

Understanding the controls on the evolution of natural feldspars is greatly assisted by coupling experimental determinations of feldspar/melt equilibria with thermo- dynamic modelling and the calculation of crystallisation paths. Such a combined approach permits the evaluation of the influence of intensive and extensive variables on feldspar compositions. Feldspar compositional paths are influenced only to a minor degree by pressure. The presence of H 2 O or other melt component incompatible in feldspar has a more major effect, not only in increasing the temperature interval over which feldspar crystallises, but also in decreasing the amount of Ab enrichment of the feldspar which occurs during crystallisation

Ab initio studies of possible fluorine-bearing four- and fivefold coordinated Al species in aluminosilicate glasses

Abstract Ab initio NMR gauge-including atomic orbital (GIAO) calculations were used to constrain assignments of resonances in 27Al NMR spectra of F-bearing alkali aluminosilicate glasses. The effect of bond angles within the range 126-150° on the chemical shift was investigated using cluster models of next-nearest atoms that are charge balanced by hydrogen atoms. GIAO calculations used geometries obtained through optimization at fixed Al-O-Si bond angles. The calculated peak positions for all of the 4-fold coordinated Al species yielded calculated 27Al NMR peak positions in good agreement with the experimental data, suggesting that any or all of the species AlF4-, AlF3O(SiH3)-, AlF2O2(SiH3)2-, and AlFO3(SiH3)3- may be present. Three of the investigated 5-fold coordinated species AlF52-, AlF3O2(SiH3)22-, and AlF2O3(SiH3)32- fit the experimental requirements well, whereas the remaining 5-fold coordinated species that were tested [AlF4O(SiH3)2-, and AlFO4(SiH3)42-] did not.