A MODEL FOR H2O SOLUBILITY MECHANISMS IN ALBITE MELTS FROM INFRARED-SPECTROSCOPY AND MOLECULAR-ORBITAL CALCULATIONS

Infrared spectra of H2O- and D2O-NaAlSi3O8 (albite) glasses were measured and contain two major differences from the anhydrous glass spectra. The first is the presence of a number of bands above 3000 cm-1 arising from O-H stretching modes. The second change in hydrous glass spectra is the appearance of a shoulder at approximately 900 cm-1. No frequency shift of the 900 cm-1 shoulder was detected with H-D substitution. We conclude, based on our infrared spectra and molecular orbital calculations as well as previous NMR (KOHN et al., 1989) and Raman (MYSEN and VIRGO, 1986a) spectra, that the 900 cm-1 band in the vibrational spectra of H2O-albite glass arises from an Al-(OH) stretching vibration in an Al Q3 site. The model proposed in this paper is that below 30 mol% [H2O]tot, molecular water interacts with the network Al3+ to produce Al-(OH) and a minor concentration of Si-(OH) bonds. Above 30 mol% [H2O]tot, the dominant species is molecular H2O, and H+ exchanges with Na+ at the charge-balancing site to produce molecular NaOH or hydrated Na+(H2O)n complexes in the melt.