29Si and 27Al NMR study of amorphous and paracrystalline opals from Australia

Four opal-AG (amorphous) and two opal-CT (paracrystalline) samples obtained from various regions in Australia were investigated with 29Si NMR and 27Al NMR. The proton cross-polarization 29Si NMR technique was used and the resulting spectra consisted of two main resonances: −102.0 and −111.2 ppm for opal-AG; and −102.5 and −112.2 ppm for opal-CT. These peaks were assigned to the Q3(1OH) and Q4 resonances, respectively. Using very short contact times, a third, very weak peak at −94 ppm was resolved in an opal-CT specimen, which was assigned to silicon in the Q2(2OH) arrangement (i.e. a silicon with twin hydroxyl groups). It was found that the opal-CT samples contained a higher proportion of both geminal and vicinal silanol groups (Q2 and Q3) than the opal-AG samples. The geminal silanol groups present in opal-AG and opal-CT are not restricted to opal-AN. The full-width at half-maximum (FWHM) values were 9.5 ppm for the opal-AG samples, and 6.5 ppm for both opal-CT samples, a result which confirms that opal-CT has a higher degree of short-range structural order than opal-AG. The 27Al NMR spectra of the opals all showed a single resonance at +52 ppm, indicating that the aluminium exists in a tetrahedral arrangement incorporated within the opal structure.