Mica

Mica, term applied to a group of rock-forming minerals which forms crystals in the monoclinic system, and is characterized by a perfect basal cleavage that causes them to separate into very thin, somewhat elastic leaves. The micas are complex aluminium silicates, the colour varying with the composition. They range in hardness from 2 to 4 and in relative density 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 aluminium, is transparent in thin sheets and translucent in thicker blocks; it is coloured in light shades of yellow, brown, green, or red. Phlogopite, which contains potassium, magnesium, and aluminium, is transparent in thin sheets, vitreous or pearly in thick blocks, and is yellowish-brown, green, or white in colour. Lepidolite, or lithia mica, which contains potassium, lithium, and aluminium, is usually lilac or pink in colour. Biotite, which contains potassium, magnesium, iron, and aluminium, has a splendent lustre and is usually dark green, brown, or black in colour 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.