Muscovite, the common light-coloured mica, and biotite, which is typically black or nearly so, are the most abundant. The mica group represents 37 phyllosilicate minerals that have a layered or platy texture. Gerais, Brazil. characteristic are the thin crystal layers. The many variations come from the diverse ways it formed. The most easily recognizable characteristic are the thin crystal layers. Typically brown, brownish green or reddish brown. These sheets are chemically inert, dielectric, elastic, flexible, hydrophilic, insulating, lightweight, platy, reflective, refractive, … Mica is one such mineral, and it is a silicate mineral. In any low-grade metamorphic rock, a glittery appearance is very often due to a mica mineral, either the white mica muscovite or the black mica biotite. It is attacked readily by hydrochloric acid. Makeup artists know it as "mica shimmer powder," used in everything from eye shadow to lip gloss. Mica formations are associated with volcanoes and hydrothermal vents. Muscovite is the most common mica, found in granites, pegmatites, gneisses, and schists, and as a contact metamorphic rock or as a secondary mineral resulting from the alteration of topaz, feldspar, kyanite, etc. Some of the Mica formations are associated with volcanoes and hydrothermal vents. Let us know if you have suggestions to improve this article (requires login). Muscovites range from colourless, greenish to blue-green to emerald-green, pinkish, and brownish to cinnamon-tan. Crystallography: Monoclinic; prismatic. It can be found in massive crystal layers weighing several hundred pounds. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. It is named in honor of Jean Baptiste Biot, a French physicist who first described the optical effects in the mica minerals. Please select which sections you would like to print: Corrections? Compositions of the common rock-forming micas are given in the table. diverse ways it formed. Lepidolite can also be pink, yellow or gray. The names of the rock-forming micas constitute a good example of the diverse bases used in naming minerals: Biotite was named for a person—Jean-Baptiste Biot, a 19th-century French physicist who studied the optical properties of micas; muscovite was named, albeit indirectly, for a place—it was originally called “Muscovy glass” because it came from the Muscovy province of Russia; glauconite, although typically green, was named for the Greek word for blue; lepidolite, from the Greek word meaning “scale,” was based on the appearance of the mineral’s cleavage plates; phlogopite, from the Greek word for firelike, was chosen because of the reddish glow (colour and lustre) of some specimens; paragonite, from the Greek “to mislead,” was so named because it was originally mistaken for another mineral, talc. Margarite, CaAl2(Si2Al2O10(OH,F)2, is also called calcium or lime mica. All rights reserved. In fact the name mica is believed to have come from the Latin word micare which means “to shine.” Mica is The mica minerals are distinguished by their perfect basal cleavage, which means that they are easily split into thin, often transparent, sheets. 2nd ed. Celadonite, K(Mg,Fe2+)(Al,Fe3+)(Si4O10)(OH)2, is a dark green mica very similar to glauconite in composition and structure, but the two minerals occur in very different settings. Similar Species: Glauconite, commonly found in green pellets in sedimentary deposits, is similar in composition to biotite. For the record, Deer Howie and Zussman give the formula as K(Al,Fe3+)Al1–x(Mg,Fe2+)x[Al1–xSi3+xO10](OH)2. Emeritus Professor of Geology, Central Michigan University, Mount Pleasant. By the rules of nomenclature it is called biotite, but it also has the fine name glimmerite. several variations based on chemical composition and characteristics. Biotite actually is a range of black micas; depending on their iron content they range from eastonite through siderophyllite to phlogopite. Sericite is a name for muscovite with extremely small grains. Usually in irregular foliated masses; often in disseminated scales or in scaly aggregates. With many micas the sheets Mohs hardness of the micas is approximately 21/2 on cleavage flakes and 4 across cleavage. It can be found in massive crystal layers weighing several hundred pounds. RME exports all types of quality such as optical flat mica, backing mica and so on according to the needs of individual buyers. Its name means "shining black" in scientific Greek. Most biotites are 1M and most muscovites are 2M; however, more than one polytype is commonly present in individual specimens. Celadonite is well known to artists as a natural pigment, "green earth," that ranges from bluish green to olive. It's very similar to celadonite, which develops in different settings. The many variations come from the diverse ways it formed. D.G. Supposedly the rock was once a candidate for the California state rock, but serpentinite prevailed. Biotite is regarded as a mixture Today mica windows are still used in cast-iron stoves, but the greater use of muscovite is as insulators in electrical equipment. The two varieties of mica that are commercially important are Muscovite mica and Phlogopite mica.