SODIUM ENVIRONMENTS IN DRY AND HYDROUS ALBITE GLASSES - IMPROVED NA-23 SOLID-STATE NMR DATA AND THEIR IMPLICATIONS FOR WATER DISSOLUTION MECHANISMS

The sodium environments in albite glasses with water concentrations ra nging from 0 to 60 mol% were studied using Na-23 off-resonance quadrup ole nutation and magic angle spinning (MAS) NMR spectroscopy. Crystall ine albite was used as a model compound to demonstrate that off-resona nce nutation is a suitable method for determination of the quadrupole coupling constant (C-q) for Na-23. 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, 7.0 5 T, 9.4 T, and 14.1 T in order to deduce the mean isotropic chemical shift, delta(iso), and to provide an independent measurement of the va lues of C-q. The mean isotropic chemical shift is a strong function of dissolved water concentration, but the mean C-q is essentially consta nt 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 wit h earlier suggestions that the hydrous glasses have a much more ordere d structure. These new data using off-resonance nutation and faster MA S combined with higher applied magnetic fields supersede the Na-23 NMR data of Kohn et al. (1989a) and should be used in preference in devis ing or testing models for water dissolution mechanisms in albite melts and glasses. Our revised data provide no evidence for a change in wat er dissolution mechanism at 30 mol% H2O, but the other conclusions of Kohn et al. (1989a) and the principal features of the dissolution mech anism developed by Kohn et al. (1989a, 1992, 1994) are essentially unc hanged. Copyright (C) 1998 Elsevier Science Ltd.