NMR, XRD AND IR STUDY ON MICROCRYSTALLINE OPALS

Microcrystalline opal-CT and opal-C were investigated by Si-29 MAS NMR and Si-29 {H-1} cross polarisation MAS NMR spectroscopy, X-ray small angle scattering, X-ray powder diffraction and infrared absorption spectroscopy. The results are compared with those for non-crystalline precious opal (opal-AG), non-crystalline hyalite (opal-AN), moderately disordered cristobalite and with well ordered low-cristobalite and low-tridymite. Opal-C is confirmed to be strongly stacking disordered low-cristobalite with about 20 to 30% probability for tridymitic stacking. More extensively stacking disordered opal-CT does not contain detectable domains of low-cristobalite or low-tridymite. The stacking sequence is close to 50% cristobalite and 50% tridymitic. The local order decreases with increasing stacking disorder, so that the structural state of microcrystalline opals lies between cristobalite, tridymite and non-crystalline opals.