Speciation of iron in obsidians (shiny, black volcanic glasses) from various localities has been determined using X-ray absorption spectroscopy (XANES), optical absorption spectroscopy and transmission electron microscopy (TEM). Optical absorption spectra were measured over a wide range of temperatures (from 10K to 1,073K). XANES measurements show variable Fe2+/Fe3+ ratio depending on formation conditions and provenance of the obsidian. Optical absorption spectra show a spectacular temperature-induced evolution. This is interpreted as arising from the coexistence of distant absorption processes due to different Fe-speciations. Isolated Fe2+ is located in a regular octahedral site, a local environment unknown in silica glasses, whereas isolated Fe3+ is found in both tetrahedral and octahedral coordinations, in the structure of the glassy matrix. The presence of iron intervalence charge-transfers processes is evidenced through the temperature dependence of the intensity of associated bands. The specific coordination of Fe2+ together with the charge-transfer bands are related to the existence of iron oxide clusters (
