Rhyolite is the general term for a group of
acidic igneous rocks, the effusive equivalent of
granites. It occurs as
lava flows,
breccias, and in volcanic necks and dikes. In the porphyritic varieties the phenocrysts are frequently
quartz or orthoclase
feldspar imbedded in a highly felsitic or glassy ground mass. Rhyolites, including
obsidian, frequently show flow, spherulitic, nodular, and lithophysal structures.
see also: List of minerals