The modified biotite, produced by hydrothermal or abrasion processes, produces vermiculite, a porous material used as an insulating material in gypsum wall panels and other heat insulation applications. Along with talc, chlorite, serpentine and the clay minerals, the micas are classified as phyllosilicate minerals, "phyllo-" meaning "leaf." Mica is the name given to a group of silicate minerals that have silicon and oxygen Micas have sheet structures whose basic units consist of two polymerized sheets of silica (SiO4) tetrahedrons. With many micas the sheets Mariposite is a green chromium-bearing variety of phengite, first described in 1868 from the Mother Lode country of California, where it is associated with gold-bearing quartz veins and serpentinite precursors. Typically brown, brownish green or reddish brown. B. Biot. The commercially important micas are muscovite and phlogopite, which are used in a variety of applications. 2019]. granites, diorites, gabbros and peridotites. It is a type of phyllosilicate, exhibiting a two-dimensional sheet or layer structure. The general formula for minerals of the mica group is XY 2–3 Z 4 O 10 (OH, F) 2 with X = K, Na, Ba, Ca, Cs, (H 3 O), (NH 4); Y = Al, Mg, Fe 2+, Li, Cr, Mn, V, Zn; and Z = Si, Al, Fe 3+, Be, Ti. Get exclusive access to content from our 1768 First Edition with your subscription. This feature cannot, however, be determined macroscopically; polytypes are distinguished by relatively sophisticated techniques such as those employing X-rays. It has a highly perfect basal cleavage yielding remarkably thin laminae (sheets) which are often highly elastic. Mica definition, any member of a group of minerals, hydrous silicates of aluminum with other bases, chiefly potassium, magnesium, iron, and lithium, that separate readily into thin, tough, often transparent, and usually elastic laminae; isinglass. It can be found in massive crystal layers weighing several hundred pounds. The many variations come from the diverse ways it formed. A rare rock occurs that consists entirely of biotite. Paragonites are colourless to white; biotites may be black, brown, red to red-brown, greenish brown, and blue-green. Manual of Mineralogy… Wiley. You'll see it everywhere you see people because it's used in makeup. Celadonite is best known in the geologic setting shown here: filling openings (vesicles) in basaltic lava, whereas glauconite forms in sediments of the shallow sea. - Our online fossil and mineral rock - An educational site about fossils. It is abundant in metamorphic rocks (both regional and contact), in pegmatites, and also in granites and other intrusive igneous rocks. The general formula for the mica minerals is XY2-3[(Si,Al)4O10](OH,F)2, where X = K,Na,Ca and Y = Mg,Fe,Li,Al. ruby and green. The (2019). These include annite, phlogopite, siderophyllite, fluorophlogopite, fluorannite, eastonite, and many others. It is a name used for a range of black mica minerals with different chemical compositions but with very similar physical properties. It has a specific chemical formula, and is crystalline in structure. Approximate chemical formula K (Mg, Fe). By using ThoughtCo, you accept our. The rest are relatively uncommon, but phlogopite is the most likely of these to be seen in the field. Similar variations in composition are known for the other micas. Layering in the univalent ( potassium , sodium ), or true, micas imparts perfect basal cleavage, allowing crystals to be split into very thin sheets that are tough and flexible. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. as their two major components. The cleavage is a manifestation of the sheet structure described above. Muscovite (also known as common mica, isinglass, or potash mica ) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(AlSi3O10)(F,OH)2, or (KF)2(Al2O3)3(SiO2)6(H2O). INTERESTED IN MORE? The most easily recognizable characteristic are the thin crystal layers. Most biotites are 1M and most muscovites are 2M; however, more than one polytype is commonly present in individual specimens. Mica also has a brilliant shininess One notable occurrence of lepidolite is in greisens, bodies of granite that are altered by fluorine-bearing vapors. The general formula for minerals of the mica group is XY2–3Z4O10(OH, F)2 with X = K, Na, Ba, Ca, Cs, (H3O), (NH4); Y = Al, Mg, Fe2+, Li, Cr, Mn, V, Zn; and Z = Si, Al, Fe3+, Be, Ti. Updates? Micas. The term "sericitic alteration" refers to this kind of metamorphism. These micas vary in chemical composition but are all sheet silicate minerals with very similar physical properties. The formula allows considerable iron substituting for the Mg and Al (that is, both Fe+2 and Fe+3). Lepidolite can also be pink, yellow or gray. The perfect cleavage into thin elastic sheets is probably the most widely recognized characteristic of the micas. The green, chromium-rich variety is called fuchsite; mariposite is also a chromium-rich type of muscovite. Thus, the cross-linked double layer is bound firmly, has the bases of silica tetrahedrons on both of its outer sides, and has a negative charge. Muscovite was once commonly used for windows, and the productive Russian mica mines gave muscovite its name (it was once widely known as "Muscovy glass"). London: DK Publishing. Margarite, CaAl2(Si2Al2O10(OH,F)2, is also called calcium or lime mica. It is also found in immature sedimentary rocks, but changes the clay minerals when worn. The most common include: purple lepidolite, black biotite, brown phlogo… several variations based on chemical composition and characteristics. Craftspeople of all sorts rely on it to add a shimmery or pearly gleam to clay and rubberstamping pigments, among many other uses. The colour and occurrence of this mineral in sediments and sedimentary rocks formed from those sediments generally are sufficient for identification. larger deposits are found in Colorado, USA; Evje, Norway; and Minas It can be colorless or tinted through grays, browns, greens, yellows, or (rarely) violet or red, and can be transparent or translucent. The most easily recognizable Muscovite is in demand for the manufacture of fireproofing and insulating materials and to some extent as a lubricant. Characteristics. Mica's value is based on several of its unique physical properties. Biotite, a silicate mineral in the common mica group. Muscovite mica is found in two colours. Let us know if you have suggestions to improve this article (requires login). Mohs hardness of the micas is approximately 21/2 on cleavage flakes and 4 across cleavage. The name muscovite comes from Muscovy-glass, a name given to the mineral in Elizabethan England due to its use in medieval Russia (Muscovy) as a cheaper alternative to glass in windows. shape. It is named in honor of Jean Baptiste Biot, a French physicist who first described the optical effects in the mica minerals. It is a fundamental component of many metamorphic shingers and forms in suitable compositions in a wide range of pressure and temperature. Glauconite, which most often occurs as pelletlike grains, has no apparent cleavage. Biotite is regarded as a mixture • There are, however, no complete series of solid solutions between any dioctahedral mica and any trioctahedral mica. Photo (c) 2009 Andrew Alden, licensed to (, Rock-Forming Minerals Comprise the Majority of Earth's Rocks, A Few Rocks That Include Silicate Materials, The 12 Most Common Blue, Violet, and Purple Minerals, Picture Guide to Common and Less-Common Minerals, How to Identify the 3 Major Types of Rocks, B.A., Earth Sciences, University of New Hampshire. [online] Available at: [20th October 2020 ], Economically Important Metal Concentrations in Earth’s Crust, Porphyry Deposits: General characteristics and modeling, Mariposite-bearing quartz rock is a popular landscaping stone, itself often called mariposite. Biotite is a name used for a large group of black mica minerals that are commonly found in igneous and metamorphic rocks. Biotite is commonly used to limit the age of rocks through potassium-argon escort or argon-argon escort. Mica - Mica - Origin and occurrence: Micas may originate as the result of diverse processes under several different conditions. Biotite is found in a wide variety of magmatic and metamorphic rocks. |   Privacy Policy. The lustre of the micas is usually described as splendent, but some cleavage faces appear pearly. Sericite is also an industrial mineral, commonly used in makeup, plastics and other products to add a silky shine. Sheets of muscovite 5 meters × 3 meters (16.5 feet × 10 feet) have been found in Nellore, India.[6]. 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. There are 37 different mica minerals. The two varieties of mica that are commercially important are Muscovite mica and Phlogopite mica. Click to Shop ourrocks, minerals, educational materials! Articles from Britannica Encyclopedias for elementary and high school students. Mica has low head conductivity coupled with great dielectric strength. When heated, the vermiculite will expand to most of its initial volume at an accordion-like expansion of its layered structure. Mineralogists consider it part of a series with muscovite, the blend between them being called phengite. that make up the mica group. Biotite. Mica is a metamorphic mineral. Sericite is typically found in low-grade metamorphic rocks like slate and phyllite. Our editors will review what you’ve submitted and determine whether to revise the article. The mica group represents 37 phyllosilicate minerals that have a layered or platy texture. Mica, a chemically complex mineral, is an important electrical insulating material with the added advantage of high resistance to high temperatures. Celadonite is well known to artists as a natural pigment, "green earth," that ranges from bluish green to olive. Sign up for Lesson Plans, discounts & more! It's flaky crystals are brittle rather than flexible. Candy makers use it in luster dust. This variety is mariposite. Where it occurs in larger lumps in pegmatite bodies, lepidolite is an ore of lithium, especially in combination with the pyroxene mineral spodumene, the other relatively common lithium mineral. This specimen is from the Minas Gerais province of Brazil. Crystallography: Monoclinic; prismatic. Thus, as in some of the other groups of minerals (e.g., the garnets), different individual pieces of naturally occurring mica specimens consist of different proportions of ideal end-member compositions.

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