A fine-grained, extrusive, igneous rock, often with a sugary texture, consisting of essential quartz, alkali feldspar, and one or more ferromagnesian minerals. Alkali rhyolites are the most common type, being characterized by the ferromagnesian mineral biotite with or without pyroxene, and are found in calc-alkaline terrains. Peralkaline rhyolites are characterized by alkali pyroxenes (aegirine, aegirine–augite) and alkali amphiboles (riebeckite, arfvedsonite), and are found as end-members of alkaline magma series on oceanic islands and rifted continental crust.