Nature and crystallization stages of spherulites within the obsidian: Acıgöl (Cappadocia- Nevşehir, Turkey)

: The study area comprises a part of the Central Anatolian Cenozoic volcanism within the Cappadocia Volcanic Province (CVP). Obsidian, perlite rhyolite flows, and volcanic ashes are observed in the study area and the spherulites within the obsidian are the objects of this study. The spherulites occupied within the obsidian in the form of round nodules 0.5–10 cm in diameter. The confocal Raman spectra of feldspar minerals within the spherulites are characterized by T-O-T and M-O lattice modes in the range of 100–250 cm –1 , O-T-O deformation and T-O-T lattice modes in the range of 250–350 cm –1 , and O-T-O stretch and O-T-O deformation modes in the range of 400–600 cm –1 and cristobalite minerals are remarkable with their weak peaks at the 296.5 cm –1 and 411.1 cm –1 Raman shifts. The geothermometer studies in the plagioclase minerals which are oligoclase and andesine in composition suggest that the spherulites may have 600–650 °C crystallization temperature. The temperature range shows that the spherulites start the first crystallization at the glass transition (Tg) levels and then continue to grow up to below Tg levels and brown rings are associated with Fe-oxidation development at the last stage.