NMR, XRD and IR study on microcrystalline opals

✍ Scribed by H. Graetsch; H. Gies; I. Topalović

• Book ID: 104662090
• Publisher: Springer-Verlag
• Year: 1994
• Tongue: English
• Weight: 980 KB
• Volume: 21
• Category: Article
• ISSN: 0342-1791
• DOI: 10.1007/bf00203147

✦ Synopsis

Microcrystalline opal-CT and opal-C were investigated by 298i MAS NMR and 298i {1H} 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.