Structural implications of water dissolution in haplogranitic glasses fromNMR 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 th e ternary system Quartz (Qz)-Albite (Ab)-Orthoclase (Or) were synthesized a nd 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 dr y 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, A l-27 and Si-29 magic angle spinning (MAS) NMR. Si-29 MAS spectra of AOQ glasses showed no change upon hydration, suggestin g little variation of the Si environments although the large linewidth of t he Si-29 signal may hide the presence of some Si Q(3)-OH. The isotropic che mical shift (delta (iso)) of Al-27 showed no change upon hydration, regardl ess of the amount of dissolved water. The Al-27 mean quadrupolar coupling c onstant (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 evolutio n 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 h ydration appears to be correlated with total water content or with the conc entration of dissolved H2O molecules. In general, the NMR data are consiste nt with the water solubility model of Kohn et al. (1989), involving the exc hange of charge balancing cations by protons. However, in addition to the p resence 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 (nonli near behaviour) on NMR parameters for Na-23 but not for Si-29 or Al-27. The refore, these data suggest that the mixed alkali effect is related to the c harge balancing cation rather than a modified aluminosilicate network. Copy right (C) 2001 Elsevier Science Ltd.