High Quality Content by WIKIPEDIA articles! High Quality Content by WIKIPEDIA articles! Rhyolite is an igneous, volcanic (extrusive) rock, of felsic (silica-rich) composition (typically 69% SiO2 see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, alkali feldspar and plagioclase (in a ratio 1:2 see the QAPF diagram). Biotite and hornblende are common accessory minerals. Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolite melts are highly polymerized and form highly viscous lavas.