Rhyolite is an igneous, volcanic (extrusive) rock, of felsic (acidic) composition (typically >69% SiO2 -- see the TAS classification. It may have any texture from aphanitic to porphyritic. The mineral assemblage is usually quartz. Biotite and pyroxene are common accessory minerals.
Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, due to their high content of silica and low iron and magnesium contents, rhyolites polymerize quickly and form highly viscous lavas. They can also occur as breccias or in volcanic necks and dikes. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysal structures.
Source: Wikipedia.Org
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