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Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is common in igneous and metamorphic rock and is occasionally found as small flakes in sedimentary rock. It is particularly prominent in many granites,
pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...
s, and schists, and "books" (large individual crystals) of mica several feet across have been found in some pegmatites. Micas are used in products such as drywalls,
paint Paint is any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture. Paint can be made in many ...
s, fillers, especially in parts for automobiles, roofing and shingles, as well as in electronics. The mineral is used in cosmetics and food to add "shimmer" or "frost."


Properties and structure

The mica group is composed of 37
phyllosilicate minerals Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually con ...
. 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
cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s (''c''). The ''TOT'' layers in turn consist of two tetrahedral sheets (''T'') strongly bonded to the two faces of a single octahedral sheet (''O''). It is the relatively weak ionic bonding between ''TOT'' layers that gives mica its perfect basal cleavage. The tetrahedral sheets consist of silica tetrahedra, which are silicon ions surrounded by four oxygen ions. In most micas, one in four silicon ions is replaced by an aluminium ion, while half the silicon ions are replaced by aluminium ions in brittle micas. The tetrahedra each share three of their four oxygen ions with neighboring 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 AlSi3O105-. The octahedral sheet has a positive charge, since its bulk composition is Al(OH)2+ (for a dioctahedral sheet with the apical sites vacant) or M3(OH)24+ (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 Al2(AlSi3O10)(OH)2 or M3(AlSi3O10)(OH)2. 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. 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 :''X''2''Y''4–6''Z''8 O20( 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 Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...
* Lepidolite * Phlogopite *
Zinnwaldite Zinnwaldite, K Li Fe Al(Al Si3) O10(O H, F)2, potassium lithium iron aluminium silicate hydroxide fluoride is a silicate mineral in the mica group. The IMA status is as a series between siderophyllite (KFe2Al(Al2Si2)O10(F,OH)2) and polylithionite ...
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 Phengite is a series name for dioctahedral micas of composition K(AlMg)2(OH)2(SiAl)4O10 it is common for Mg or Fe2+ to substitute for Al on the Y site and a corresponding increase in Si on the Z site in the mica group formula ''X''2''Y''4–6''Z' ...
with Mg or Fe2+ substituting for Al in the ''Y'' site and a corresponding increase in Si in the ''Z'' site.
Sericite Sericite is the name given to very fine, ragged grains and aggregates of white (colourless) micas, typically made of muscovite, illite, or paragonite. Sericite is produced by the alteration of orthoclase or plagioclase feldspars in areas that hav ...
is the name given to very fine, ragged grains and aggregates of white (colourless) 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
pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...
s. The largest documented single crystal of mica ( phlogopite) was found in Lacey Mine, Ontario, Canada; it measured and weighed about . Similar-sized crystals were also found in
Karelia Karelia ( Karelian and fi, Karjala, ; rus, Каре́лия, links=y, r=Karélija, p=kɐˈrʲelʲɪjə, historically ''Korjela''; sv, Karelen), the land of the Karelian people, is an area in Northern Europe of historical significance for ...
, 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), 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).Mica
, USGS Mineral Commodity Summaries 2011
Flake mica comes from several sources: the metamorphic rock called schist as a byproduct of processing feldspar and kaolin resources, from placer deposits, and from 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.Dolley, Thomas P. (2008
"Mica"
in ''USGS 2008 Minerals Yearbook''.
In Madagascar and India, it is also mined artisanally, in poor working conditions and with the help of
child labour Child labour refers to the exploitation of children through any form of work that deprives children of their childhood, interferes with their ability to attend regular school, and is mentally, physically, socially and morally harmful. Such e ...
.


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.


Ground mica

The leading use of dry-ground mica in the US is in the joint compound for filling and finishing seams and blemishes in gypsum wallboard ( drywall). The mica acts as a filler and extender, provides a smooth consistency, improves the workability of the compound, and provides resistance to cracking. In 2008, joint compound accounted for 54% of dry-ground mica consumption. In the paint industry, ground mica is used as a pigment extender that also facilitates suspension, reduces chalking, prevents shrinking and shearing of the paint film, increases the resistance of the paint film to water penetration and weathering and brightens the tone of colored pigments. Mica also promotes paint adhesion in aqueous and oleoresinous formulations. Consumption of dry-ground mica in paint, the second-ranked use, accounted for 22% of the dry-ground mica used in 2008. Ground mica is used in the well-drilling industry as an additive to drilling fluids. The coarsely ground mica flakes help prevent the loss of circulation by sealing porous sections of the drill hole. Well drilling muds accounted for 15% of dry-ground mica use in 2008. The
plastics industry The plastics industry manufactures polymer materials—commonly called plastics—and offers services in plastics important to a range of industries, including packaging, building and construction, electronics, aerospace, and transportation. It is ...
used dry-ground mica as an extender and filler, especially in parts for automobiles as lightweight insulation to suppress sound and vibration. Mica is used in plastic automobile
fascia A fascia (; plural fasciae or fascias; adjective fascial; from Latin: "band") is a band or sheet of connective tissue, primarily collagen, beneath the skin that attaches to, stabilizes, encloses, and separates muscles and other internal organs. ...
and fenders as a reinforcing material, providing improved mechanical properties and increased dimensional stability, stiffness, and strength. Mica-reinforced plastics also have high-heat dimensional stability, reduced warpage, and the best surface properties of any filled plastic composite. In 2008, consumption of dry-ground mica in plastic applications accounted for 2% of the market. The rubber industry used ground mica as an inert filler and mold release compound in the manufacture of molded rubber products such as tires and roofing. The platy texture acts as an anti-blocking, anti-sticking agent. Rubber mold lubricant accounted for 1.5% of the dry-ground mica used in 2008. As a rubber additive, mica reduces gas permeation and improves resiliency. Dry-ground mica is used in the production of rolled roofing and asphalt
shingles Shingles, also known as zoster or herpes zoster, is a viral disease characterized by a painful skin rash with blisters in a localized area. Typically the rash occurs in a single, wide mark either on the left or right side of the body or face. ...
, where it serves as a surface coating to prevent sticking of adjacent surfaces. The coating is not absorbed by freshly manufactured roofing because mica's platy structure is unaffected by the acid in asphalt or by weather conditions. Mica is used in decorative coatings on wallpaper, concrete,
stucco Stucco or render is a construction material made of aggregates, a binder, and water. Stucco is applied wet and hardens to a very dense solid. It is used as a decorative coating for walls and ceilings, exterior walls, and as a sculptural and a ...
, and tile surfaces. It also is used as an ingredient in flux coatings on welding rods, in some special greases, and as coatings for core and mold release compounds, facing agents, and mold washes in foundry applications. Dry-ground phlogopite mica is used in automotive brake linings and clutch plates to reduce noise and vibration (
asbestos Asbestos () is a naturally occurring fibrous silicate mineral. There are six types, all of which are composed of long and thin fibrous crystals, each fibre being composed of many microscopic "fibrils" that can be released into the atmosphere b ...
substitute); as sound-absorbing insulation for coatings and polymer systems; in reinforcing additives for polymers to increase strength and stiffness and to improve stability to heat, chemicals, and ultraviolet (UV) radiation; in heat shields and temperature insulation; in industrial coating additive to decrease the permeability of moisture and hydrocarbons; and in polar polymer formulations to increase the strength of epoxies, nylons, and
polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
s.


Paints and cosmetics

Wet-ground mica, which retains the brilliance of its cleavage faces, is used primarily in
pearlescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterf ...
paints by the automotive industry. Many metallic-looking pigments are composed of a substrate of mica coated with another mineral, usually titanium dioxide (TiO2). The resultant pigment produces a reflective color depending on the thickness of the coating. These products are used to produce automobile paint, shimmery plastic containers, high-quality inks used in advertising and security applications. In the cosmetics industry, its reflective and refractive properties make mica an important ingredient in blushes, eye liner, eye shadow, foundation, hair and body glitter,
lipstick Lipstick is a cosmetic product used to apply coloration and texture to lips, often made of wax and oil. Different pigments are used to produce color, and minerals such as silica may be used to provide texture. The use of lipstick dates back to ...
, lip gloss, mascara, moisturizing lotions, and nail polish. Some brands of toothpaste include powdered white mica. This acts as a mild abrasive to aid polishing of the tooth surface, and also adds a cosmetically pleasing, glittery shimmer to the paste. Mica is added to latex balloons to provide a colored shiny surface.


Built-up mica

Muscovite and phlogopite splittings can be fabricated into various built-up mica products. Produced by mechanized or hand setting of overlapping splittings and alternate layers of binders and splittings, built-up mica is used primarily as an electrical insulation material. Mica insulation is used in high-temperature and fire-resistant power cables in aluminium plants,
blast furnace A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheric ...
s, critical wiring circuits (for example, defense systems, fire and security alarm systems, and surveillance systems), heaters and boilers, lumber kilns, metal smelters, and tanks and furnace wiring. Specific high-temperature mica-insulated wire and cable is rated to work for up to 15 minutes in molten aluminium, glass, and steel. Major products are bonding materials; flexible, heater, molding, and segment plates; mica paper; and tape. Flexible plate is used in electric motor and generator armatures, field coil insulation, and magnet and
commutator In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory. Group theory The commutator of two elements, a ...
core insulation. Mica consumption in flexible plate was about 21 tonnes in 2008 in the US. Heater plate is used where high-temperature insulation is required. Molding plate is sheet mica from which V-rings are cut and stamped for use in insulating the copper segments from the steel shaft ends of a commutator. Molding plate is also fabricated into tubes and rings for insulation in armatures, motor starters, and transformers. Segment plate acts as insulation between the copper commutator segments of direct-current universal motors and generators. Phlogopite built-up mica is preferred because it wears at the same rate as the copper segments. Although muscovite has a greater resistance to wear, it causes uneven ridges that may interfere with the operation of a motor or generator. Consumption of segment plate was about 149 t in 2008 in the US. Some types of built-up mica have the bonded splittings reinforced with cloth, glass,
linen Linen () is a textile made from the fibers of the flax plant. Linen is very strong, absorbent, and dries faster than cotton. Because of these properties, linen is comfortable to wear in hot weather and is valued for use in garments. It also ...
, muslin, plastic, silk, or special paper. These products are very flexible and are produced in wide, continuous sheets that are either shipped, rolled, or cut into ribbons or tapes, or trimmed to specified dimensions. Built-up mica products may also be corrugated or reinforced by multiple layering. In 2008, about 351 t of built-up mica was consumed in the US, mostly for molding plates (19%) and segment plates (42%).


Sheet mica

Technical grade sheet mica is used in electrical components, electronics, in atomic force microscopy and as window sheets. Other uses include diaphragms for oxygen-breathing equipment, marker dials for navigation compasses,
optical filters Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviol ...
, pyrometers, thermal regulators, stove and kerosene heater windows, radiation aperture covers for microwave ovens, and
micathermic heater A micathermic heater is a type of space heater in which the heating element is covered in thin sheets of mica. Micathermic heaters produce both convection heat and radiant heat Thermal radiation is electromagnetic radiation generated by t ...
elements. Mica is birefringent and is therefore commonly used to make quarter and half wave plates. Specialized applications for sheet mica are found in aerospace components in air-, ground-, and sea-launched missile systems, laser devices, medical electronics and radar systems. Mica is mechanically stable in micrometer-thin sheets which are relatively transparent to radiation (such as alpha particles) while being impervious to most gases. It is therefore used as a window on radiation detectors such as
Geiger–Müller tube The Geiger–Müller tube or G–M tube is the sensing element of the Geiger counter instrument used for the detection of ionizing radiation. It is named after Hans Geiger, who invented the principle in 1908, and Walther Müller, who collaborated w ...
s. In 2008, mica splittings represented the largest part of the sheet mica industry in the United States. Consumption of muscovite and phlogopite splittings was about 308 t in 2008. Muscovite splittings from India accounted for essentially all US consumption. The remainder was primarily imported from Madagascar. Small squared pieces of sheet mica are also used in the traditional Japanese '' Kōdō'' ceremony to burn incense: A burning piece of coal is placed inside a cone made of white ash. The sheet of mica is placed on top, acting as a separator between the heat source and the incense, in order to spread the fragrance without burning it.


Electrical and electronic

Sheet mica is used principally in the electronic and electrical industries. Its usefulness in these applications is derived from its unique electrical and thermal properties and its mechanical properties, which allow it to be cut, punched, stamped, and machined to close tolerances. Specifically, mica is unusual in that it is a good electrical insulator at the same time as being a good thermal conductor. The leading use of block mica is as an electrical insulator in electronic equipment. High-quality block mica is processed to line the gauge glasses of high-pressure steam boilers because of its flexibility, transparency, and resistance to heat and chemical attack. Only high-quality muscovite film mica, which is variously called India ruby mica or ruby muscovite mica, is used as a dielectric in capacitors. The highest quality mica film is used to manufacture capacitors for
calibration standard In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known a ...
s. The next lower grade is used in
transmitting capacitor In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
s. Receiving capacitors use a slightly lower grade of high-quality muscovite. Mica sheets are used to provide structure for heating wire (such as in
Kanthal Kanthal may refer to : * the historical name of Pratapgarh State, a princely state in India, until it was renamed after its capital Prtabgarh in 1698 * Kanthal (alloy) Kanthal is the trademark for a family of iron-chromium-aluminium (FeCrAl) al ...
or
Nichrome Nichrome (also known as NiCr, nickel-chromium or chromium-nickel) is a family of alloys of nickel, chromium, and often iron (and possibly other elements) commonly used as resistance wire, heating elements in devices like toasters, electrical kettl ...
) in heating elements and can withstand up to . Single ended self-starting lamps are insulated with a mica disc and contained in a borosilicate glass gas discharge tube (arc tube) and a metal cap. They include the sodium-vapor lamp that is the
gas-discharge lamp Gas-discharge lamps are a family of artificial light sources that generate light by sending an electric discharge through an ionized gas, a plasma. Typically, such lamps use a noble gas (argon, neon, krypton, and xenon) or a mixture of thes ...
in street lighting.


Atomic force microscopy

Another use of mica is as a substrate in the production of ultraflat, 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 ultraflat once the film is removed from the substrate. Freshly-cleaved mica surfaces have been used as clean imaging substrates in
atomic force microscopy Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the op ...
, enabling for example the imaging of bismuth films, plasma
glycoprotein Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...
s, 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
carriage A carriage is a private four-wheeled vehicle for people and is most commonly horse-drawn. Second-hand private carriages were common public transport, the equivalent of modern cars used as taxis. Carriage suspensions are by leather strapping an ...
s 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 Prehistory, also known as pre-literary history, is the period of human history between the use of the first stone tools by hominins 3.3 million years ago and the beginning of recorded history with the invention of writing systems. The use of ...
times. Mica was known to ancient Indian,
Egyptian Egyptian describes something of, from, or related to Egypt. Egyptian or Egyptians may refer to: Nations and ethnic groups * Egyptians, a national group in North Africa ** Egyptian culture, a complex and stable culture with thousands of years of ...
, Greek, Roman, and
Chinese Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of va ...
civilizations, as well as the Aztec civilization of the New World. The earliest use of mica has been found in
cave painting In archaeology, Cave paintings are a type of parietal art (which category also includes petroglyphs, or engravings), found on the wall or ceilings of caves. The term usually implies prehistoric origin, and the oldest known are more than 40,000 ye ...
s created during the Upper
Paleolithic The Paleolithic or Palaeolithic (), also called the Old Stone Age (from Greek: παλαιός ''palaios'', "old" and λίθος ''lithos'', "stone"), is a period in human prehistory that is distinguished by the original development of stone too ...
period (40,000 BC to 10,000 BC). The first hues were red (
iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...
,
hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
, or red
ochre Ochre ( ; , ), or ocher in American English, is a natural clay earth pigment, a mixture of ferric oxide and varying amounts of clay and sand. It ranges in colour from yellow to deep orange or brown. It is also the name of the colours produced ...
) and black (
manganese dioxide Manganese dioxide is the inorganic compound with the formula . This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for is for dry-cell ...
,
pyrolusite Pyrolusite is a mineral consisting essentially of manganese dioxide ( Mn O2) and is important as an ore of manganese.. It is a black, amorphous appearing mineral, often with a granular, fibrous, or columnar structure, sometimes forming reniform ...
), 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. The most striking structure of Teotihuacan is the towering Pyramid of the Sun. The pyramid contained considerable amounts of mica in layers up to thick. Natural mica was and still is used by the
Taos Taos or TAOS may refer to: Places * Taos, Missouri, a city in Cole County, Missouri, United States * Taos County, New Mexico, United States ** Taos, New Mexico, a city, the county seat of Taos County, New Mexico *** Taos art colony, an art colo ...
and Picuris Pueblos Indians in north-central New Mexico to make pottery. The pottery is made from weathered
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
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. Mica flakes (called ''abrak'' in Urdu and written as ابرک) are also used in Pakistan to embellish women's summer clothes, especially ''dupattas'' (long light-weight scarves, often colorful and matching the dress). 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

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 Gulal or abir ( bn, আবীর) or abhir ( or, ଅଭୀର) is the traditional name given to the coloured powders used for the typical Hindu rituals, in particular for the Holi festival or Dol Purnima (though commonly associated with the r ...
'' (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 Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...
as thickener using ''
kirazuri ''Kirazuri'' is a method used in Japanese woodblock printmaking using mica powder, or to add sparkle. When used for the background in portraits, depending on the base color, it is called ''Shiro-kirazuri'' (white), ''Kuro-kirazuri'' (literally ...
'' 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 stenciled on hairpins, sword blades or fish scales on . The soil around Nishio in central Japan is rich in mica deposits, which was already mined in the Nara period.
Yatsuomote ware Yatsuomote ware () is a type of Japanese pottery originally from , Mikawa Province in the Chūbu region of Japan during the late Edo period. Around 1825 Katō Hachiemon (加藤八右衛門) opened his kiln at the southern foot of Mount Yatsuom ...
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 The term immediately dangerous to life or health (IDLH) is defined by the US National Institute for Occupational Safety and Health (NIOSH) as exposure to airborne contaminants that is "likely to cause death or immediate or delayed permanent advers ...
.


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 Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, tough materials, and some grades are optically transparent. They are easily work ...
,
polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
, styrene, vinyl-PVC, and
vulcanized fiber Vulcanized fibre or red fibre is a laminated plastic composed of only cellulose. The material is a tough, resilient, hornlike material that is lighter than aluminium, tougher than leather, and stiffer than most thermoplastics. The newer wood-lamin ...
. Mica paper made from scrap mica can be substituted for sheet mica in electrical and insulation applications.


See also

*
Mica fish Mica fish are lenticular, elongate lozenge, parallelogram-shaped, or lens-shaped (roughly fish-shaped) single mica crystals that are often used as shear sense indicators. They commonly occur in micaceous quartzitic mylonites. They characteristica ...
*


References


Sources


External links

*
Mineral Galleries data




* * Scientific American,
Mica
, 22-Oct-1881, pp.257 {{Authority control Phyllosilicates Dielectrics Articles containing video clips Industrial minerals