Rhyolite

Rhyolite, light-colored, fine-grained volcanic rock, occurring principally in volcanic lava. It is chemically identical to granite, and is composed essentially of feldspar and quartz. Of the dark minerals that occur in some specimens of rhyolite, dark brown biotite is the most common. Augite and hornblende also occur in some rhyolites. Some specimens of rhyolite have a striated or streaked appearance and others are uniform in appearance. Porphyritic varieties are found in abundance and often contain embedded crystals of quartz, orthoclase, and oligosclase. The groundmass of the porphyries is partly or completely vitreous. A noncrystalline variety of rhyolite that is completely glassy and usually black in color is called obsidian. Pitchstone resembles obsidian except for its coloring, which is pitchy brown. Pumice, another form of vitreous rhyolite, contains numerous large and small cavities that were caused by expanding gas. These cavities constitute the bulk of the rock, which is light in weight and is used as an abrasive. The term liparites is often applied to rhyolites because many lavas of the Lipari Islands (Isole Eolie), near southern Italy, contain excellent specimens of the rock. The term nevadite is applied to a type of porphyritic rhyolite found in Nevada, in which embedded crystals are numerous and the groundmass is inconspicuous.