Different water solubility mechanisms in hydrous glasses along the Qz-Ab join:: Evidence from NMR spectroscopy

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 90Ab 10 to Qz 28Ab 72 (in wt%) were synthesised and studied with 1H, 23Na, 27Al and 29Si magic angle spinning (MAS) NMR and 1H- 29Si cross polarisation (CP) MAS NMR at magnetic fields of 8.45 and 14.1 T. The results show that both molecular H 2O and OH groups are present in the hydrous glasses, represented by a broad and a narrow component in the static 1H 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 29Si MAS spectra of Qz-rich glasses (Qz 73Ab 27 - Qz 90Ab 10) change upon water incorporation, indicating the presence of Si-OH groups at least in the Qz-richest sample. The 27Al MAS data demonstrate that Al is only present in tetrahedral coordination for all glasses studied and that unless δ iso for Al Q 4(4Si) and Al Q 3(3Si)-OH are identical, Al-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 27Al data for appropriate model compounds or reliable calculations of 27Al shifts and quadrupolar coupling constants are required before this conclusion can be considered to be definitive. For Qz-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 Qz-rich glasses do both mechanisms coexist.