The term mica refers to a group of minerals having similar physical characteristics, and related, through different, chemical structure. The most characteristic physical properties are a very perfect basal cleavage and flexible and elastic cleavage sheets. Chemically, micas are silicates of aluminum and potassium containing varying amounts of magnesium and iron, and in some varieties, sodium, lithium and other elements. However, only two varieties of mica are used commercially to a great extent. These are muscovite, or potash mica, and phlogopite, or magnesia mica. Lepidolite, or lithia mica, has been used as a source of lithia. Muscovite is the most common mica, and is the only mica mined in the United States. 10 p.