Experimental devitrification of natural rhyolitic glass was undertaken in an attempt to produce, under known conditions, analogs of naturally devitrified rocks. The results should facilitate the recognition and interpretation of devitrification textures. Previous studies have shown that alkali-rich aqueous solutions increase devitrification rates sufficiently to produce solid pieces of devitrified obsidian in the laboratory, but none of these has provided a systematic study of experimentally produced devitrification textures.
Cylinders of natural volcanic glass, 4.6 mm in diameter and 10 to 20 mm long, were sealed in 5-mm-diameter gold capsules, along with either pure water or alkali-rich solutions. Runs were made in externally heated pressure vessels in the temperature range 240° to 700°C and in the pressure range 0.5 to 4 kb. Textures recognized in the products of the runs closely resembled hydration and devitrification texture s in some rhyolitic rocks. Although hydration fronts and strain birefringence were developed during some experimental runs, no perlitic fractures were observed. Devitrified products contained spherulites, micropoikilitic quartz, orb texture, axiolites, and miarolitic cavities.
Two stages of devitrification are distinguished. The glassy stage is characterized by glassy or felsitic textures with isolated spherulites, and the spherulitic stage by spherulitic textures and micropoikilitic quartz. A hypothetical third stage in this succession, not represented in products from this study, most likely has a granophyric or granitic texture, with no evidence of glassy precursors.