Micas are minerals belonging to a large group of silicates of aluminum, iron, calcium, magnesium, and alkali metals, characterized by their easy cleavage into thin, flexible, elastic, and highly lustrous sheets, within the phyllosilicate subgroup. Their crystal system is monoclinic. They are generally found in igneous rocks such as granite and metamorphic rocks such as schist. The most abundant varieties are biotite and muscovite.