Mica

The mica group comprises 37 phyllosilicate minerals. All crystallize in the monoclinic system, with a tendency towards pseudohexagonal crystals, and are similar in structure but vary in chemical composition. Micas are translucent to opaque with a distinct vitreous or pearly luster, and different mica minerals display colors ranging from white to green or red to black. Deposits of mica tend to have a flaky or platy appearance. The crystal structure of mica is described as TOT-c, meaning that it is composed of parallel TOT layers weakly bonded to each other by cations (c). The TOT layers in turn consist of two tetrahedral sheets (T) strongly bonded to the two faces of a single octahedral sheet (O). The relatively weak ionic bonding between TOT layers gives mica its perfect basal cleavage. The tetrahedral sheets consist of silica tetrahedra, each silicon ion surrounded by four oxygen ions. In most micas, one in four silicon ions is replaced by an aluminium ion, while aluminium ions replace half the silicon ions in brittle micas. The tetrahedra share three of their four oxygen ions with neighbouring tetrahedra to produce a hexagonal sheet. The remaining oxygen ion (the apical oxygen ion) is available to bond with the octahedral sheet. The octahedral sheet can be dioctahedral or trioctahedral. A trioctahedral sheet has the structure of a sheet of the mineral brucite, with magnesium or ferrous iron being the most common cation. A dioctahedral sheet has the structure and (typically) the composition of a gibbsite sheet, with aluminium being the cation. Apical oxygens take the place of some of the hydroxyl ions that would be present in a brucite or gibbsite sheet, bonding the tetrahedral sheets tightly to the octahedral sheet. Tetrahedral sheets have a strong negative charge since their bulk composition is . The octahedral sheet has a positive charge, since its bulk composition is (for a dioctahedral sheet with the apical sites vacant) or (for a trioctahedral site with the apical sites vacant; M represents a divalent ion such as ferrous iron or magnesium) The combined TOT layer has a residual negative charge, since its bulk composition is or . The remaining negative charge of the TOT layer is neutralized by the interlayer cations (typically sodium, potassium, or calcium ions). Because the hexagons in the T and O sheets are slightly different in size, the sheets are slightly distorted when they bond into a TOT layer. This breaks the hexagonal symmetry and reduces it to monoclinic symmetry. However, the original hexahedral symmetry is discernible in the pseudohexagonal character of mica crystals. The short-range order of K+ ions on cleaved muscovite mica has been resolved. File:Mica T.png|View of tetrahedral sheet structure of mica. The apical oxygen ions are tinted pink. File:Mica tO.png|View of trioctahedral sheet structure of mica. The binding sites for apical oxygen are shown as white spheres. File:Mica tOs.png|View of trioctahedral sheet structure of mica emphasizing octahedral sites File:Mica dO.png|View of dioctahedral sheet structure of mica. The binding sites for apical oxygen are shown as white spheres. File:Mica dOs.png|View of dioctahedral sheet structure of mica emphasizing octahedral sites File:Mica tri.png|View of trioctahedral mica structure looking at surface of a single layer File:Mica tri side.png|View of trioctahedral mica structure looking along sheets == Classification ==

Chemically, micas can be given the general formula :X2Y4–6Z8O20(OH, F)4, in which :X is K, Na, or Ca or less commonly Ba, Rb, or Cs; :Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.; :Z is chiefly Si or Al, but also may include Fe3+ or Ti. Structurally, micas can be classed as dioctahedral (Y = 4) and trioctahedral (Y = 6). If the X ion is K or Na, the mica is a common mica, whereas if the X ion is Ca, the mica is classed as a brittle mica. Dioctahedral micas • Muscovite • Paragonite Brittle micas: • Margarite Trioctahedral micas Common micas: • Biotite • Lepidolite • Phlogopite • Zinnwaldite Brittle micas: • Clintonite Interlayer-deficient micas Very fine-grained micas, which typically show more variation in ion and water content, are informally termed "clay micas". They include: • Hydro-muscovite with H3O+ along with K in the X site; • Illite with a K deficiency in the X site and correspondingly more Si in the Z site; • Phengite with Mg or Fe2+ substituting for Al in the Y site and a corresponding increase in Si in the Z site. Sericite is the name given to very fine, ragged grains and aggregates of white (colorless) micas. == Occurrence and production ==

Mica is widely distributed and occurs in igneous, metamorphic and sedimentary regimes. Large crystals of mica used for various applications are typically mined from granitic pegmatites. Similar-sized crystals were also found in Karelia, Russia. Scrap and flake mica is produced all over the world. In 2010, the major producers were Russia (100,000 tonnes), Finland (68,000 t), the United States (53,000 t), South Korea (50,000 t), France (20,000 t) and Canada (15,000 t). The total global production was 350,000 t, although no reliable data were available for China. Most sheet mica was produced in India (3,500 t) and Russia (1,500 t). Flake mica comes from several sources: the metamorphic rock called schist as a byproduct of processing feldspar and kaolin resources, from placer deposits, and pegmatites. Sheet mica is considerably less abundant than flake and scrap mica, and is occasionally recovered from mining scrap and flake mica. The most important sources of sheet mica are pegmatite deposits. Sheet mica prices vary with grade and can range from less than $1 per kilogram for low-quality mica to more than $2,000 per kilogram for the highest quality. In Madagascar and India, it is also mined artisanally, in poor working conditions and with the help of child labour. == Uses ==

The commercially important micas are muscovite and phlogopite, which are used in a variety of applications. Useful properties Mica's value is based on its unique physical properties: the crystalline structure of mica forms layers that can be split or delaminated into thin sheets usually causing foliation in rocks. These sheets are chemically inert, dielectric, elastic, flexible, hydrophilic, insulating, lightweight, platy, reflective, refractive, resilient, and range in opacity from transparent to opaque. Mica is stable when exposed to electricity, light, moisture, and extreme temperatures. It has superior electrical properties as an insulator and as a dielectric, and can support an electrostatic field while dissipating minimal energy in the form of heat; it can be split very thin (0.025 to 0.125 millimeters or thinner) while maintaining its electrical properties, has a high dielectric breakdown, is thermally stable to , and is resistant to corona discharge. Muscovite, the principal mica used by the electrical industry, is used in capacitors that are ideal for high frequency and radio frequency. Phlogopite mica remains stable at higher temperatures (to ) and is used in applications in which a combination of high-heat stability and electrical properties is required. Muscovite and phlogopite are used in sheet and ground forms. They include the sodium-vapor lamp that is the gas-discharge lamp in street lighting. Atomic force microscopy Another use of mica is as a substrate in the production of ultra-flat, thin-film surfaces, e.g. gold surfaces. Although the deposited film surface is still rough due to deposition kinetics, the back side of the film at the mica-film interface is ultra-flat once the film is removed from the substrate. Freshly-cleaved mica surfaces have been used as clean imaging substrates in atomic force microscopy, enabling for example the imaging of bismuth films, plasma glycoproteins, membrane bilayers, and DNA molecules. Peepholes Thin transparent sheets of mica were used for peepholes in boilers, lanterns, stoves, and kerosene heaters because they were less likely to shatter than glass when exposed to extreme temperature gradients. Such peepholes were also fitted in horse-drawn carriages and early 20th-century cars, where they were called isinglass curtains. == Etymology ==

The word mica is derived from the Latin word '', meaning a crumb, and probably influenced by '', to glitter. == Early history ==

Human use of mica dates back to prehistoric times. Mica was known to ancient Indian, Egyptian, Greek, Roman, and Chinese civilizations, as well as the Aztec civilization of the New World. The earliest use of mica has been found in cave paintings created during the Upper Paleolithic period (40,000 BC to 10,000 BC). The first hues were red (iron oxide, hematite, or red ochre) and black (manganese dioxide, pyrolusite), though black from juniper or pine carbons has also been discovered. White from kaolin or mica was used occasionally. A few kilometers northeast of Mexico City stands the ancient site of Teotihuacan. Mica was found in the noble palace complex "Viking Group" during an excavation led by Pedro Armillas between 1942 and 1944. Later, a second deposit was located in the Xalla Complex, but this is not yet proven. Natural mica was and still is used by the Taos and Picuris Pueblos Indians in north-central New Mexico to make pottery. The pottery is made from weathered Precambrian mica schist and has flecks of mica throughout the vessels. Tewa Pueblo Pottery is made by coating the clay with mica to provide a dense, glittery micaceous finish over the entire object. Thin mica flakes are added to a hot starch water solution, and the dupatta is dipped in this water mixture for 3–5 minutes. Then it is hung to air dry. Mica powder '' printing technique adds mica powder to the gelatin solution as adhesive, here printed on the background. Throughout the ages, fine powders of mica have been used for various purposes, including decorations. Powdered mica glitter is used to decorate traditional water clay pots in India, Pakistan and Bangladesh; it is also used on traditional Pueblo pottery, though not restricted to use on water pots in this case. The gulal and abir (colored powders) used by North Indian Hindus during the festive season of Holi contain fine crystals of mica to create a sparkling effect. The majestic Padmanabhapuram Palace, from Trivandrum in India, has colored mica windows. Mica powder is also used as a decoration in traditional Japanese woodblock printmaking, as when applied to wet ink with gelatin as thickener using kirazuri technique and allowed to dry, it sparkles and reflects light. Earlier examples are found among paper decorations, with the height as the Nishi Honganji 36 Poets Collection, codices of illuminated manuscripts in and after ACE 1112. For metallic glitter, Ukiyo-e prints employed very thick solution either with or without color pigments stencilled on hairpins, sword blades or fish scales on . The soil around Nishio in central Japan is rich in mica deposits, which were already mined in the Nara period. Yatsuomote ware is a type of local Japanese pottery from there. After an incident at Mount Yatsuomote a small bell was offered to soothe the kami. Katō Kumazō started a local tradition where small ceramic zodiac bells (きらら鈴) were made out of local mica kneaded into the clay, and after burning in the kiln the bell would make a pleasing sound when rung. Medicine Ayurveda, the Hindu system of ancient medicine prevalent in India, includes the purification and processing of mica in preparing Abhraka bhasma, which is claimed as a treatment for diseases of the respiratory and digestive tracts. == Health impact ==

Mica dust in the workplace is regarded as a hazardous substance for respiratory exposure above certain concentrations. United States The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for mica exposure in the workplace as 20 million parts per cubic foot (706,720,000 parts per cubic meter) over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 3 mg/m3 respiratory exposure over an 8-hour workday. At levels of 1,500 mg/m3, mica is immediately dangerous to life and health. Ireland The Irish defective block crisis relates to mica in construction blocks. == Substitutes ==

Some lightweight aggregates, such as diatomite, perlite, and vermiculite, may be substituted for ground mica when used as filler. Ground synthetic fluorophlogopite, a fluorine-rich mica, may replace natural ground mica for uses that require thermal and electrical properties of mica. Many materials can be substituted for mica in numerous electrical, electronic, and insulation uses. Substitutes include acrylate polymers, cellulose acetate, fiberglass, fishpaper, nylon, phenolics, polycarbonate, polyester, styrene, vinyl-PVC, and vulcanized fiber. Mica paper made from scrap mica can be substituted for sheet mica in electrical and insulation applications. == In popular culture ==

Herman Melville mentions isinglass mica and isinglass dried fish bladder uses in Chapter 68 of Moby Dick, comparing whale skin and blubber's appearance to mica's. In the musical Oklahoma!, the song "The Surrey With the Fringe on Top" describes the surrey as having "isinglass curtains you can roll right down," as mica was commonly used for windows in vehicle side screens. Mentioned in The Book of Life by Deborah Harkness, "her scales fell like isinglass", in reference to the scales of a fire drake named Corra, and in Mark Twain's The Gilded Age where he describes a furnace door which "framed a small square of isinglass..." (chapter seven). It is also mentioned in the first paragraph of Willa Cather’s The Song of the Lark: “the isinglass sides of the hard-coal burner were aglow.” Isinglass is also written about in Little Town on the Prairie (Little House on the Prairie series book 7, Chapter 17: The Sociable) by Laura Ingalls Wilder, "...a tall, shining heater with isinglass windows stood at its center, the chairs around the walls were all of polished woods...", in describing a room that the 'sociable' was being held in. ==See also==