Mineralogic study of the destabilization of Mexican opals

Abstract

This paper presents the first mineralogical study of the destabilization phenomena by whitening in Mexican opals. The Raman spectra of the opal samples are characterized by the well-resolved bands assigned to molecular H₂O, at 3,200 cm^-1 and at 2,900 cm^-1 for the unaltered (orange) and for the destabilized (whitened) parts, respectively. The shape and intensity of the bands at 960 cm^-1 (groups Si–OH) and 1,600 cm^-1, confirm this modification due to molecular H₂O. Water contents, measured by thermal differential analyses (ATD/ATG) decrease in the whitened part (between 6 and 85%). Molecular H₂O present in the destabilized part is more mobile than in the unaltered areas, since its elimination begins below 100 °C. The specific surface (BET method) is always larger in the destabilized whitened part (up to 30 times). Scanning electronic microscopy (SEM) revealed microstructural differences between both parts of the opal: the unaltered (orange) part is more compacted that the destabilized (white) part. These results suggest that the opal instability is the result of structural changes that include the recristalization to cristobalite and the incorporation of molecular H₂O in the structure of volcanic opals.