XRD AND SI-29 MAS-NMR STUDY ON SOME NONCRYSTALLINE SILICA MINERALS

Opal-AG, opal-AN (hyalite) and natural silica glass (Libyan desert glass) have been investigated by chemical analyses, near infrared spectroscopy, thermal analyses (DTA and TGA), large angle X-ray scattering and solid-state magic angle spinning NMR spectroscopy. Non-crystalline opals (opal-AG and opal-AN) have a different tetrahedral network than silica glass with larger mean SiOSi angles and a comparatively narrow distribution of SiOSi angles. The difference indicate that non-crystalline opals are almost exclusively built up of interconnected six-membered rings of [SiO4]-tetrahedra while silica glass also contains other structural elements including odd membered rings of [SiO4]-tetrahedra. © 1990.