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

Abstract

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 Ab39Or32Qz29 (AOQ) and water contents ranging from 0 to 6 wt%. Series II consisted of dry and hydrous glasses (∼2.0 wt% H2O) with five compositions along the join Qz37Ab63-Qz34Or66 (AQ-OQ) varying the alkali content (Na/K) at constant Si/Al ratio. All glasses were investigated with 1H, 23Na, 27Al and 29Si magic angle spinning (MAS) NMR.29Si MAS spectra of AOQ glasses showed no change upon hydration, suggesting little variation of the Si environments although the large linewidth of the 29Si signal may hide the presence of some Si Q3-OH. The isotropic chemical shift (δiso) of 27Al showed no change upon hydration, regardless of the amount of dissolved water. The 27Al mean quadrupolar coupling constant (Cq) 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 Q3-OH. Nonetheless, the evolution of Cq upon hydration suggests a correlation with OH concentration in the quenched glass. The evolution of 23Na 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, 1H-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 23Na but not for 29Si or 27Al. Therefore, these data suggest that the mixed alkali effect is related to the charge balancing cation rather than a modified aluminosilicate network.