Rhyolite
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Rhyolite is an igneous, volcanic (extrusive) rock, of felsic composition, with aphanitic to porphyritic texture. Mineral assembly is usually quartz, alkali feldspar and plagioclase (in a ratio > 1:2). 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, rhyolite form highly viscous lavas. They can also occur as breccias or in volcanic necks and dikes. Like obsidian, rhyolites frequently show flow, spherulitic, nodular, and lithophysal structures.

See also: igneous rocks, list of rocks, granite

The original article can be found at: http://en.wikipedia.org/wiki/Rhyolite.
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