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

The compositional dependence of water incorporation mechanisms in melts and glasses belonging to the quartz (Qz)-albite (Ab) join was studied with nuc lear magnetic resonance (NMR) spectroscopy. Dry and hydrous glasses (contai ning 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 m olecular 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 change s 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-ri ch 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 g roups at least in the Qz-richest sample. The Al MAS data demonstrate that A l is only present in tetrahedral coordination for all glasses studied and t hat unless delta(iso) for Al Q(4)(4Si) and Al Q(3)(3Si)-OH are identical, A I-OH groups cannot be present in significant concentrations. Thus, in hydro us Ab-rich glasses there is probably no significant depolymerisation as sug gested by Kohn et al. (1989a). However, more Al-27 data for appropriate mod el compounds or reliable calculations of Al-27 shifts and quadrupolar coupl ing constants are required before this conclusion can be considered to be d efinitive. For Oz-rich glass compositions a second solubility mechanism inv olving the formation of Si-OH and depolymerisation of the silicate network is inferred. The data suggest that only in Oz-rich glasses do both mechanis ms coexist. Copyright (C) 2000 Elsevier Science Ltd.