A Raman, infrared and XRD analysis of the instability in volcanic opals from Mexico

A series of natural volcanic opal samples with the destabilization phenomena from Mexican deposits (states of Queretaro and Jalisco) was investigated by Raman microprobe (RMP), infrared spectrometry and XRD analysis. These techniques show that at low and room temperatures the unaltered transparent opals may be transformed into destabilized white opals, which are a mixture of different polymorphs of tridymite and alpha-cristobalite with various degrees of crystallinity. We found systematic changes in frequencies of both the Raman and the infrared bands, caused by increasing regularities of bond-lengths and bond-angles Si-O-Si groups under the effect of stability. Micro-Raman spectrometry confirms that in the destabilized opal the principal mineral phases are MC (monoclinic ordered)- and MX (incommensurate monoclinic)-tridymites that are characterized by more structural order in comparison with other structural modification of this phase in unaltered opal (POn pseudo-orthorhombic disordered tridymite). XRD investigations show that in the sequence from unaltered to destabilized opal the position of principal maximum (4.30, 4.10 and 2.50 A) shifts towards higher d-spacing. This XRD shifting to higher d-spacing can largely be explained by an increasing amount of tridymite stacking and unresolved superposition of cristobalite and tridymite reflections. The destabilization phenomena in volcanic opals is due to the structural ordering/disordering that is characterized mainly by the formation of the different tridymite polymorphs (MC and MX) in the destabilized opal-CT as well as the decreasing content of molecular water in the structure.