Rhyolite is an igneous, volcanic rock, of felsic (silica-rich) composition (typically > 69% SiO2 -C see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, sanidine and plagioclase (in a ratio > 2:1 -C see the QAPF diagram).read more >>