Relationship between nanostructure and optical absorption in fibrous pink opals from Mexico and Peru

Translucent pink opals from Mexico (states of Mapimi and Michoacan) and Peru (Acari area, near Arequipa) are opal-CT, containing from 10 to 40% palygorskite, as demonstrated by XRD, infrared absorption and specific gravity measurements. Their nanostructure is unusual, with bunches of fibres 20 to 30 nm in minimum diameter, related to the fibrous nature of palygorskite crystals, as demonstrated by electron microscopy A complex absorption centred at about 500 nm is the cause of the pink colour. It is proposed that the absorption is due to quinone fossil products associated with the phyllosilicate fibres. The Raman spectrum of monoclinic palygorskite is deduced from that of its mixture with opal. The opal-CT-palygorskite-quinone association is a geological marker of a specific environment, presumably of a fossil lake environment in a volcanic region.