29Si and 21Al 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 Si-29 NMR and Al-27 NMR. The proton cross-polarization Si-29 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 Q(3)(1OH) and Q(4) 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 Q(2)(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 (Q(2) and Q(3)) 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 Al-27 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. (C) 2003 Elsevier B.V. All rights reserved.