Sodium Environments in Dry and Hydrous Albite Glasses: Improved 23Na Solid State NMR Data and Their Implications for Water Dissolution Mechanisms

Abstract

The sodium environments in albite glasses with water concentrations ranging from 0 to 60 mol% were studied using 23Na off-resonance quadrupole nutation and magic angle spinning (MAS) NMR spectroscopy. Crystalline albite was used as a model compound to demonstrate that off-resonance nutation is a suitable method for determination of the quadrupole coupling constant (C q) for 23Na. Off-resonance nutation experiments gave a mean C q = 1.75 ± 0.2 MHz for all the albite glasses studied here. MAS NMR experiments were performed at three magnetic fields, 7.05 T, 9.4 T, and 14.1 T in order to deduce the mean isotropic chemical shift, δ iso, and to provide an independent measurement of the values of C q. The mean isotropic chemical shift is a strong function of dissolved water concentration, but the mean C q is essentially constant at 2.1-2.2 ± 0.2 MHz over the water concentration range studied. The distributions of both chemical shift and quadrupolar interactions decreases markedly with increasing water concentration, consistent with earlier suggestions that the hydrous glasses have a much more ordered structure. These new data using off-resonance nutation and faster MAS combined with higher applied magnetic fields supersede the 23Na NMR data of Kohn et al. 1989a and should be used in preference in devising or testing models for water dissolution mechanisms in albite melts and glasses. Our revised data provide no evidence for a change in water dissolution mechanism at 30 mol% H 2O, but the other conclusions of Kohn et al. 1989a and the principal features of the dissolution mechanism developed by Kohn et al. 1989a, Kohn et al. 1992, Kohn et al. 1994 are essentially unchanged.