Cathodoluminescence microanalysis of natural hydrated amorphous SiO₂; opal

Abstract

 Natural amorphous hydrated silicon dioxide (opal) has been investigated for the first time using Cathodoluminescence microanalysis in a Scanning Electron Microscope. Defect centers have been identified and imaged with high sensitivity and high spatial resolution. Intrinsic defects identified include the non bridging oxygen hole center (NBOHC) and the oxygen deficient centers (ODC). Impurities are strongly correlated with the grain structure of the precious opal and a higher than average concentration of impurity defects are observed at grain boundaries and cracks. Impurity associated defect centers include the NBOHC with −OH precursor, and the charge compensated substitutional Al, Ti, and Fe centers. Cathodoluminescence microanalysis provides important information about the defect structure and long term stability of precious opal.