Abstract:
The compositional dependence of water incorporation mechanisms in melts and glasses belonging to the quartz (Qz)-albite (Ab) join was studied with nuclear magnetic resonance (NMR) spectroscopy. Dry and hydrous glasses (containing 3.8 +/- 0.1 wt% water) ranging in composition from Qz(90)Ab(10) to Qz(28)Ab(72) (in wt%) were synthesised and studied with H-1, Na-23, Al-27 and Si-29 magic angle spinning (MAS) NMR and H-1-Si-29 cross polarisation (CP) MAS NMR at magnetic fields of 8.45 and 14.1 T. The results show that both molecular H2O and OH groups are present in the hydrous glasses, represented by a broad and a narrow component in the static H-1 NMR spectra. The changes in the 23Na MAS NMR spectra of the hydrated glasses indicate that sodium associates with water in all compositions studied. In contrast to the Ab-rich glasses, the Si-29 MAS spectra of Qz-rich glasses (Qz(73)Ab(27) - Qz(90)Ab(10)) change upon water incorporation, indicating the presence of Si-OH groups at least in the Qz-richest sample. The Al MAS data demonstrate that Al is only present in tetrahedral coordination for all glasses studied and that unless delta(iso) for Al Q(4)(4Si) and Al Q(3)(3Si)-OH are identical, AI-OH groups cannot be present in significant concentrations. Thus, in hydrous Ab-rich glasses there is probably no significant depolymerisation as suggested by Kohn et al. (1989a). However, more Al-27 data for appropriate model compounds or reliable calculations of Al-27 shifts and quadrupolar coupling constants are required before this conclusion can be considered to be definitive. For Oz-rich glass compositions a second solubility mechanism involving the formation of Si-OH and depolymerisation of the silicate network is inferred. The data suggest that only in Oz-rich glasses do both mechanisms coexist.