Structural implications of water dissolution in haplogranitic glasses from NMR spectroscopy: Influence of total water content and mixed alkali effect

To study the effects of total water content and alkali substitution on the structure of aluminosilicate glasses, two series of glasses belonging to the ternary system Quartz (Qz)-Albite (Ab)-Orthoclase (Or) were synthesized and investigated with nuclear magnetic resonance (NMR) spectroscopy. Series I consisted of seven glasses with normative composition Ab(39)Or(32)Qz(29) (AOQ) and water contents ranging from 0 to 6 wt%. Series II consisted of dry and hydrous glasses (similar to2.0 wt% H2O) with five compositions along the join Qz(37)Ab(63)-Qz(34)Or(66) (AQ-OQ) varying the alkali content (Na/K) at constant Si/Al ratio. All glasses were investigated with H-1, Na-23, Al-27 and Si-29 magic angle spinning (MAS) NMR. Si-29 MAS spectra of AOQ glasses showed no change upon hydration, suggesting little variation of the Si environments although the large linewidth of the Si-29 signal may hide the presence of some Si Q(3)-OH. The isotropic chemical shift (delta (iso)) of Al-27 showed no change upon hydration, regardless of the amount of dissolved water. The Al-27 mean quadrupolar coupling constant (C-q) decreased with increasing water content, indicating a general increase of symmetry of the charge distribution around Al, which suggests the absence of significant amounts of Al Q(3)-OH. Nonetheless, the evolution of C-q upon hydration suggests a correlation with OH concentration in the quenched glass. The evolution of Na-23 Na isotropic chemical shifts upon hydration appears to be correlated with total water content or with the concentration of dissolved H2O molecules. In general, the NMR data are consistent with the water solubility model of Kohn et al. (1989), involving the exchange of charge balancing cations by protons. However, in addition to the presence of molecular water, H-1-NMR results showed at least two types of OH groups of which one may be related to Al-OH. Although the small intensity of this signal indicates that only a minor fraction of OH groups is present in this species, it demonstrates that water dissolution in aluminosilicate glasses might involve several mechanisms of hydroxyl formation. NMR data for Series II showed only a significant mixed alkali effect (nonlinear behaviour) on NMR parameters for Na-23 but not for Si-29 or Al-27. Therefore, these data suggest that the mixed alkali effect is related to the charge balancing cation rather than a modified aluminosilicate network. Copyright (C) 2001 Elsevier Science Ltd.