Mica

Mica, term applied to a group of rock-forming minerals, crystallizing in the monoclinic system (see Crystal), and characterized by a perfect basal cleavage that causes them to separate into very thin, somewhat elastic leaves. The micas are complex aluminum silicates, the color varying with the composition. They range in hardness from 2 to 4 and in specific gravity from 2.7 to 3.2. The most important micas are muscovite, phlogopite, lepidolite, and biotite.

Muscovite, also called white mica or common mica, which contains potassium and aluminum, is transparent in thin sheets and translucent in thicker blocks; it is colored in light shades of yellow, brown, green, or red. Phlogopite, which contains potassium, magnesium, and aluminum, is transparent in thin sheets, vitreous or pearly in thick blocks, and is yellowish-brown, green, or white in color. Lepidolite, or lithia mica, which contains potassium, lithium, and aluminum, is usually lilac or pink in color. Biotite, which contains potassium, magnesium, iron, and aluminum, has a splendent luster and is usually dark green, brown, or black in color but is sometimes light yellow.

Muscovite and phlogopite are used as insulating material in the manufacture of electrical apparatus, particularly vacuum tubes. Scrap mica, obtained as waste material in the manufacture of sheet mica, is used as a lubricant when mixed with oils and as a fireproofing material.