MAS NMR-SPECTRA OF HYALITE FROM GRAN-CANARIA

The Si-29 MAS NMR spectrum of hyalite exhibits two signals at chemical shifts of -111.6 and -102.5 ppm which are due to Si tetrahedra surrou nded by four corner-sharing Si tetrahedra (Q(4) configuration) and to Si tetrahedra connected with three Si tetrahedra (Q(3) configuration), respectively. Compared with synthetic non-hydrous silica glass, the Q (4) signal of hyalite is shifted to a higher field, which corresponds to a larger mean intertetrahedral Si-O-Si angle. The half-width of the Q(4) signal is more than 30% smaller than in anhydrous silica glass, indicating a narrower distribution of average Si-O-Si angles in hyalit e. In Si-29{H-1} cross-polarisation MAS NMR experiments, the signal at -102.5 ppm is enhanced, showing that this resonance is caused by sing le protons attached to the tetrahedra. No signal from geminal silanol groups (Q(2)) was observed. The H-1 MAS NMR spectrum consists of three partly overlapping signals at 7.1, 5.6 and 3.9 ppm which presumably c an be ascribed to isolated water molecules, water clusters and two typ es of silanol groups.