Water in Albitic Glasses

Abstract

Infrared spectroscopy has been calibrated to provide a precise and accurate method for determining the concentrations of molecular water and hydroxyl groups in hydrous albitic glasses. At total water contents less than 4 wt.%, most of the water is dissolved as hydroxyl groups; at higher total water contents, molecular water becomes the dominant species. For total water contents above 4 wt.%, the amount of water dissolved as hydroxyl groups is nearly constant at about 2 wt.% and additional water is incorporated as molecular water. These trends in the concentrations of the H-bearing species are similar to those observed in other silicate glasses using infrared and NMR spectroscopies. The ratio of molecular water to hydroxyl groups at a given total water content is independent of the pressure and only weakly dependent on the temperature of equilibration. Molecular water and hydroxyl group concentrations in glasses provide constraints on the dissolution mechanisms of water in silicate liquids. Several mixing models involving homogeneous equilibria of the form H_2O+O^(2−) = 2OH^− among melt species in albitic melts have been developed. These models can account for the measured species concentrations if the effects of non-ideal behavior or mixing of polymerized units are included, or by allowing for several different anhydrous species. We used two thermodynamic models of hydrous albitic melts to calculate phase equilibria. The first assumes that Henry's law holds for molecular water in albitic liquids; i.e. that the activity of molecular water in the melt is proportional to its mole fraction as determined by infrared spectroscopy. The second describes the speciation and thermodynamics of hydrous albitic melts using the formalism of a strictly regular solution. These models can account reasonably well for the position of the vapor-saturated solidus of high albite and the pressure and temperature dependence of the solubility of water in albitic melts. To the extent that these models are successful, our approach provides a direct link between measured species concentrations in hydrous albitic glasses and the macroscopic thermodynamic properties of the NaAlSi_3O_8-H_2O system.