MICA mī’ cə, an important group of rockforming minerals characterized by perfect basal cleavage with the production of thin sheets or flakes which are both flexible and elastic. All micas are based on the same sheet-like atomic structure. The principal members of the group are muscovite (hydrated potassium, aluminum silicate) and biotite (hydrated potassium, magnesium, iron, aluminum silicate). Both muscovite, which is generally white to pale silvery-gray in color, and biotite, which is brown to black, are common constituents of granites and metamorphic rocks. They occur in many rocks of the Aqaba Granite Complex which makes up the bed rock on either side of the Red Sea. Muscovite, which is fairly resistant to weathering, also occurs commonly in sedimentary rocks. It shines brightly in sunlight (Latin micare—to gleam or shine) and small flakes would glisten in both desert sands and granite. In some veins cutting granites there are mica flakes up to many feet across.