²⁹Si CPMAS NMR investigations of silanol-group minerals and hydrous aluminosilicate glasses

Abstract

One-pulse magic angle spinning (MAS) ²⁹Si and ¹H-²⁹Si cross-polarization (CP) MAS nuclear magnetic resonance (NMR) spectroscopy was performed on minerals with OH groups and on hydrous aluminosilicate glass samples. The silanol-group samples used were krauskopfite, rosenhahnite, thaumasite, ussingite, and KHSi₂O₅ with known Si to H distances and ellenbergerite, the proton positions of which are not as well defined. The Si-H distances from the minerals can be compared with the cross-polarization time constants (τ_SiH) and the proton spin-relaxation times in the rotating frame [τ_1ρ(H)], and a rough correspondence exists between a mineral’s shortest Si-H distance and its τ_SiH value. Also, fast [τ_1ρ(H)] values correspond to large bulk H densities. The CP spectra of the hydrous aluminosilicate glass samples were fitted with two peaks, representing two different Si environments within the glass structure. The contact-time curves of the higher frequency peak imply τ_SiH similar to the mineral samples with short Si-H distances, and this suggests that the glasses could contain a large fraction of either Si-OH groups or protonated bridging O atoms.