Mica (mineral)
Mica is a group of hydrous aluminum silicate minerals known for their unique layered structure, which allows them to cleave into thin sheets. The most prominent types of mica include muscovite, biotite, phlogopite, and lepidolite, each varying in color and chemical composition. Mica is widely distributed globally and can be found in igneous, metamorphic, and sedimentary rocks. Significant sources of sheet mica are located in Brazil, Madagascar, India, and Canada, while the United States is a major producer of scrap mica, primarily from states like North Carolina and Georgia.
The mineral is valued for its excellent electrical insulation properties, making it essential in the electronics and computer industries, particularly in applications like spacers and insulators for vacuum tubes. Additionally, ground mica serves various purposes, including as a coating material in the paint industry, a lubricant, and a pigment extender. Historically, mica has been used since 2000 BCE for decorative and medicinal purposes, with its commercial use evolving significantly since the 19th century. With its unique combination of properties, mica continues to play a vital role in both industrial applications and everyday products.
Mica (mineral)
Where Found
Micas are common rock-forming minerals and are widely distributed throughout the world. They occur in igneous, metamorphic, and sedimentary rocks. They are mined as sheets or flakes and scrap. Sheet mica is primarily found in Brazil, Madagascar, India, and Canada. Muscovite flakes are mined in the United States from igneouspegmatites and metamorphic schists located in North and South Carolina, Connecticut, Georgia, and New Mexico.
Primary Uses
Muscovite sheets are used as electrical insulators in the electronic and computer industries. Scrap mica is ground and used primarily as a coating material and in the paint industry.
Technical Definition
The mica group of minerals is composed mainly of muscovite, KAl2(AlSi3O10)(OH)2; biotite, K(Mg,Fe)3 (AlSi3O10)(OH)2; phlogopite KMg3(AlSi3O10)(OH)2; and lepidolite, K(Li,Al)3(AlSi3O10)(OH)2; although there are thirty known mica minerals. Micas are hydrous aluminum silicate minerals that have a perfect basal cleavage. Micas have a hardness of 2.5 to 4 and show a vitreous to pearly luster. Muscovite is a type of mica that is colorless and transparent in thin sheets and white to light brown or light yellow in thicker blocks. Phlogopite is yellow to brown with a copper-colored reflection off cleavage surfaces. Biotite is primarily black but can appear dark green or brown. Lepidolite has a distinctive lilac to pink color.
Description, Distribution, and Forms
Micas form monoclinic crystals that inevitably show a perfect basal cleavage. Crystals and their cleavage sheets commonly display a hexagonal form. Muscovite and biotite can be found in thick “books” containing layer upon layer of thin cleavage sheets, which can be up to 3 meters across in pegmatites.
Micas are common throughout the world. Muscovite is characteristic of granites and pegmatites. In metamorphic rocks muscovite is the primary constituent of many mica schists. Biotite is found in igneous rocks ranging from granite pegmatites to diorites, gabbros, and peridotites. It also occurs in silica-rich lavas, porphyries, and a wide range of metamorphic rocks. Phlogopite occurs in metamorphosed magnesium limestones, dolomites, and ultrabasic rocks. Lepidolite occurs only in pegmatites.
History
Mining of mica started as early as 2000 b.c.e. in India, where it was used as medicine, decoration, and paint. Commercial mining of mica in the United States began in 1803. Mica was used in store windows, shades for open light flames, and furnace viewing glass. When electronic vacuum tubes were developed in the early 1900’s, mica was used as spacers and insulators in the tubes, thus beginning its use in the electrical industry.
Obtaining Mica
The United States has limited supplies of sheet mica but is the largest producer of scrap mica. Although there are no environmental problems in mining mica, sheet mica is expensive to mine because of the intense hand labor needed to mine and process the sheets.
Uses of Mica
Muscovite and phologopite remain important commercially because they have a low thermal and electrical conductivity and a high dielectrical strength. Sheet mica is used as electrical insulators, retardation plates in neon helium lasers, optical filters, and washers in the computer industry. The isinglass used in furnace and stove doors from the 1800’s to the present is sheet muscovite. Lepidolite is the only mica mined and processed for its composition. It is a source of lithium, which is used in the production of heat-resistant glass.
Scraps and flakes of mica are processed into ground mica and used as a coating on rolled roofing, asphalt shingles, and waterproof fabrics. It is also used in wallpaper to give it a shiny luster, as a lubricant when mixed with oils, and as a pigment extender in paint. A magnesium-rich alteration product of biotite, vermiculite, is used as insulation, packing material, and an ingredient in potting soil.
Bibliography
U.S. Geological Survey.