A SOLID-STATE SI-29 NUCLEAR-MAGNETIC-RESONANCE STUDY OF OPAL AND OTHER HYDROUS SILICAS

A range of opal and natural silica samples, as well as calcined flint and fused silica, were examined by solid state Si-29 nuclear magnetic resonance (NMR). The chemical shifts from 1-pulse magic angle spinning (MAS-NMR) spectra do not distinguish among opal-AG, opal-C, or opal-CT. However, the Si-29 line widths decrease with increasing structural order-opal-AG 9.8-9.9 ppm (two examples), opal-CT 6.0-7.5 ppm (12 examples), and opal-C < 5 ppm (one example)-and may be used to discriminate among these three phases. Most samples show silanol (Q3) Si-29 resonances in the Si-29-{H-1} cross-polarization (CP) MAS-NMR spectra, but there is no evidence of resonances due to Si(OH)2 (Q2) units. For eight chalcedony and flint samples, in which the silanol groups are believed to be on the silica surface, the Q3 resonances are in the range delta = -98.5 to -99.5 ppm, and the difference in chemical shift between the Q3 and Q4 resonances is in the narrow range of 7.8-8.1 ppm. The chemical shift range for Q3 resonances in the less well resolved spectra of the 15 opal samples is delta = -101.0 to -103.7 ppm, and the corresponding Q3 - Q4 difference is in the range 9.0-12.5 ppm. These opal samples may contain both internal and surface Q3 units, but in view of the increased structural disorder for the opal samples, the difference in their chemical shift parameters compared with the chalcedony and flint samples cannot be used as a basis for establishing the presence or absence of internal silanol groups in opal