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 spe ctroscopy. The results are compared with those for non-crystalline pre cious opal (opal-AG), non-crystalline hyalite (opal-AN), moderately di sordered cristobalite and with well ordered low-cristobalite and low-t ridymite. Opal-C is confirmed to be strongly stacking disordered low-c ristobalite with about 20 to 30% probability for tridymitic stacking. More extensively stacking disordered opal-CT does not contain detectab le domains of low-cristobalite or low-tridymite. The stacking sequence is close to 50% cristobalite and 50% tridymitic. The local order decr eases with increasing stacking disorder, so that the structural state of microcrystalline opals lies between cristobalite, tridymite and non -crystalline opals.