Devitrification of natural rhyolitic obsidian glasses: petrographic and microstructural study (SEM+EDS) of recent (Lipari island) and ancient (Sarrabus, SE Sardinia) samples

Microstructural evolution of devitrification of obsidian glasses is a process not fully understood, especially with reference to preferred nucleation sites and anisotropic development of spherulites. Evidence is commonly hidden in advanced devitrification textures. Two sets of naturally devitrified obsidian rocks, calcoalkaline rhyolitic in composition, have been used to test how this process develops. A petrographic and SEM+EDS comparative study of incipient devitrified recent obsidian (from Lipari island) and advanced devitrified ancient rhyolitic rocks (from Sarrabus region, Sardinia island, Italy) allows following successive stages in the generation of K-feldspar spherulites in natural glasses. Spherulites show a preference for epitaxial nucleation over previous minute idiomorphic crystals and a number of processes including rearrangement of initial mineral fibers and interstitial voids lead to the formation of blade-like crystals and denser spherulites. In some cases bubbles with an inferred origin associated to secondary boiling of magmas also favour spherulitic nucleation.