A multinuclear magnetic resonance study of the structure of hydrous albite glasses

Abstract

The structures of a series of hydrous albite glasses quenched from melts at high pressures and temperatures have been studied using 29Si, 23Na, 27Al, and 1H nuclear magnetic resonance. Changes in the isotropic chemical shift, the chemical shift dispersion, and the mean nuclear quadrupole coupling constant for 23Na as a function of dissolved water concentration were deduced from spectra obtained at two different magnetic fields. Major changes in the sodium environment occur, but the spectra for 29Si and 27Al, and hence their structural environments, are similar throughout the range of water concentrations studied (0–67 mol%). No previous model is consistent with the results of this study. The data suggest the existence of the following structural features: i) exchange of H + for Na + as a charge-balancing cation; ii) formation of Na(OH) complexes; iii) incorporation of molecular water; iv) no octahedrally coordinated aluminium; v) no Al-OH or Si-OH. These features can be summarised in terms of the equilibrium NaAlSi 3 O 8 + H 2 O ⇋ HAlSi 3 O 8 + Na( OH). In contrast to all previous interpretations, we see no evidence for depolymerisation of the aluminosilicate framework, although an increase in the symmetry of the aluminium environments and decrease in the chemical shift dispersion of the sodium environments suggests a more ‘ordered’ structure than in the dry glass. If the structures of hydrous albite melts are the same as those of the glasses studied here the current understanding of the effect of dissolved water on the physical properties of felsic melts must be reassessed.