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Micas ( ) are a group of
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

mineral
s whose outstanding physical characteristic is that individual mica
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosco ...

crystal
s can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is common in
igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the p ...
and
metamorphic rock , a type of metamorphic rock Metamorphic rocks arise from the transformation of existing rock (geology), rock to new types of rock, in a process called metamorphism upright=1.35, Schematic representation of a metamorphic reaction. Abbreviat ...

metamorphic rock
and is occasionally found as small flakes in
sedimentary rock Sedimentary rocks are types of rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compoun ...

sedimentary rock
. It is particularly prominent in many
granite Granite () is a coarse-grained (phaneritic A phanerite is an igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Lat ...

granite
s,
pegmatite A pegmatite is an igneous rock showing a very coarse Texture (geology), texure, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a si ...
s, and
schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fair ...

schist
s, and "books" (large individual crystals) of mica several feet across have been found in some pegmatites. Micas are used in products such as
drywall Drywall (also known as plasterboard, wallboard, sheet rock, gypsum board, buster board, custard board, or gypsum panel) is a panel made of (), with or without additives, typically extruded between thick sheets of facer and backer , used in the ...

drywall
s,
paint Paint is any pigmented liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid flow, flow in which the material densi ...

paint
s, fillers, especially in parts for automobiles, roofing and shingles, as well as in electronics. The mineral is used in cosmetics to add "shimmer" or "frost."


Properties and structure

The mica group includes 37
phyllosilicate minerals
phyllosilicate minerals
. All crystallize in the
monoclinic In crystallography Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids (see crystal structure). The word "crystallography" is derived from the Greek language, Greek words ''crystallon'' "c ...

monoclinic
system, with a tendency towards pseudohexagonal
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosco ...

crystal
s, 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 An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are u ...
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 Brucite is the mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it i ...

brucite
, with magnesium or ferrous iron being the most common cation. A dioctahedral sheet has the structure and (typically) the composition of a
gibbsite Gibbsite, Al(OH)3, is one of the mineral forms of aluminium hydroxide Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer Polymorphism (materials science), polymorp ...

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
O
O
20(
OH
OH
, F)4, in which :''X'' is
K
K
,
Na
Na
, or
Ca
Ca
or less commonly
Ba
Ba
,
Rb
Rb
, or
Cs
Cs
; :''Y'' is
Al
Al
,
Mg
Mg
, or
Fe
Fe
or less commonly
Mn
Mn
,
Cr
Cr
,
Ti
Ti
,
Li
Li
, etc.; :''Z'' is chiefly
Si
Si
or Al, but also may include
Fe3+
Fe<sup>3+</sup>
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 Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ...

Muscovite
*
Paragonite Paragonite is a mineral, related to muscovite. Its empirical formula is sodium, Naaluminum, Al2 Ohydrogen, H)2, Alsilicon, Si3oxygen, O10 A wide solvus separates muscovite from paragonite, such that there is little solid solution along the vect ...
Brittle micas: *
Margarite:''This article refers about a member of the mica group, and should not be confused with "Margarita (disambiguation), margarita".'' Margarite is a calcium rich member of the mica group of the Silicate minerals, phyllosilicates with formula: calcium ...


Trioctahedral micas

Common micas: *
Biotite Biotite is a common group of phyllosilicate Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science conce ...

Biotite
*
Lepidolite Lepidolite is a lilac ''Syringa vulgaris'', the lilac or common lilac, is a species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. ...

Lepidolite
*
Phlogopite Phlogopite is a yellow, greenish, or reddish-brown member of the mica Micas ( ) are a group of minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This c ...

Phlogopite
* 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 Illite is a group of closely related non-expanding minerals. Illite is a secondary mineral precipitate, and an example of a , or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – octahedron (O) – silica ...

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 A sample of sericite Sericite is the name given to very fine, ragged grains and Aggregate (geology), aggregates of white (colourless) micas, typically made of muscovite, illite, or paragonite. Sericite is produced by the alteration of orthoclase or ...
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 Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the p ...
,
metamorphic Metamorphic rocks arise from the transformation of existing rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the mineral ...
and
sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentatio ...

sedimentary
regimes. Large crystals of mica used for various applications are typically mined from
granitic A granitoid is a generic term for a diverse category of coarse-grained igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languag ...
pegmatite A pegmatite is an igneous rock showing a very coarse Texture (geology), texure, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a si ...
s. The largest documented single crystal of mica (
phlogopite Phlogopite is a yellow, greenish, or reddish-brown member of the mica Micas ( ) are a group of minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This c ...

phlogopite
) was found in Lacey Mine,
Ontario ("Loyal she began, loyal she remains") , Label_map = yes , image_map = Ontario in Canada 2.svg , map_alt = Map showing Ontario's location east/central of Canada. , coordinates = , cap ...

Ontario
,
Canada Canada is a country in the northern part of North America North America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, ...

Canada
; it measured and weighed about . Similar-sized crystals were also found in
Karelia Karelia (Karelian language, Karelian and fi, Karjala, ; rus, Каре́лия, links=y, r=Karélija, p=kɐˈrʲelʲɪjə, historically ''Korjela''; sv, Karelen), the land of the Karelians, Karelian people, is an area in Northern Europe of ...

Karelia
,
Russia Russia ( rus, link=no, Россия, Rossiya, ), or the Russian Federation, is a country spanning Eastern Europe Eastern Europe is the eastern region of . There is no consistent definition of the precise area it covers, partly because th ...

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 Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fair ...

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 ex ...

child labour
.


Uses

The commercially important micas are
muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ...

muscovite
and
phlogopite Phlogopite is a yellow, greenish, or reddish-brown member of the mica Micas ( ) are a group of minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This c ...

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 mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no g ...
in rocks. These sheets are chemically inert,
dielectric In electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force i ...

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 A corona discharge is an electrical discharge An electric discharge is the release and transmission of electricity in an applied electric field An electric field (sometimes E-field) is the physical field that surrounds each electric cha ...

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 Gypsum is a soft sulfate mineralThe sulfate minerals are a class of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the ...

gypsum
wallboard (
drywall Drywall (also known as plasterboard, wallboard, sheet rock, gypsum board, buster board, custard board, or gypsum panel) is a panel made of (), with or without additives, typically extruded between thick sheets of facer and backer , used in the ...

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 A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compound , CH4; is among the simplest organic compou ...
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 fluid Driller pouring anti-foaming agent down the drilling string on a drilling rig In geotechnical engineering#REDIRECT geotechnical engineering {{Redirect category shell, 1= {{R from other capitalisation ..., drilling fluid, also called drilli ...
s. 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 industryThe plastics industry manufactures polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Rep ...
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 ; adjective fascial; from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Thro ...
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 Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous The viscosity of a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or externa ...

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 stripe 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 Construction aggregate, aggregates, a binder (material), 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, ...
, 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 (pronounced: or ) is a naturally occurring fibrous silicate mineral Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fair ...
substitute); as sound-absorbing insulation for coatings and
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

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 paints by the automotive industry. Many metallic-looking pigments are composed of a substrate of mica coated with another mineral, usually
titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical ...
(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 liner or eyeliner is a cosmetic used to define the eyes. It is applied around the contours of the eye(s) to create a variety of aesthetic effects. History Eye liner was first used in Ancient India, Ancient Egypt Ancient Egypt was ...

eye liner
,
eye shadow Eye shadow (or eyeshadow) is a cosmetic applied primarily to the eyelids to make the wearer's eyes stand out or look more attractive. Eye shadow can also be applied under eyes or to brow bones. Eye shadow can add depth and dimension to one's ey ...
,
foundation Foundation may refer to: * Foundation (nonprofit), a type of charitable organization ** Foundation (United States law), a type of charitable organization in the U.S. ** Private foundation, a charitable organization that, while serving a good cause ...

foundation
, hair and body glitter,
lipstick Lipstick is a cosmetic that applies color, texture, and protection to the lips. Many colors and types of lipstick exist. Some lipsticks are also lip balms, to add both color and hydration. Although the name originally applied to the baton (s ...

lipstick
,
lip gloss Lip gloss is a cosmetic used primarily to give lips a glossy lustre, and sometimes to add a subtle color. It is distributed as a liquid or a soft solid (not to be confused with lip balm, which generally has medical or soothing purposes or lipsti ...
,
mascara Mascara is a Cosmetics, cosmetic commonly used to enhance the eyelashes. It may darken, thicken, lengthen, and/or define the eyelashes. Normally in one of three forms—liquid, powder, or cream—the modern mascara product has various formulas; ...
, 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.


Insulator

Mica is also used as an insulator in concrete block and home attics and can be poured into walls (usually in retrofitting uninsulated open top walls). Mica may also be used as a soil conditioner, especially in potting soil mixes and in gardening plots. Greases used for
axle An axle or axletree is a central shaft for a rotating A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation p ...

axle
s are composed of a compound of fatty oils to which mica,
tar Tar is a dark brown or black viscous The viscosity of a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (matter), ...

tar
or
graphite Graphite (), archaically referred to as plumbago, is a Crystallinity, crystalline form of the element carbon with its atoms arranged in a Hexagonal crystal system, hexagonal structure. It occurs naturally in this form and is the most stable for ...

graphite
is added to increase the durability of the grease and give it a better 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 used for to produce industrial metals, generally , but also others such as or . ''Blast'' refers to the combustion air being "forced" or supplied above atmospheric pressure. In a blast furnace, fuel (), , and ( ...
s, critical wiring circuits (for example, defense systems, fire and security alarm systems, and surveillance systems), heaters and boilers, lumber
kiln , Wrecclesham Wrecclesham is a village on the southern outskirts of the large town of Farnham Farnham is a market town in Surrey, England, within the Borough of Waverley Borough Council, Waverley.OS Explorer map 145:Guildford and Farnham ...

kiln
s, 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 Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gene ...
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 h ...

linen
,
muslin Muslin () is a cotton fabric A textile is a flexible material made by creating an interlocking network of yarn Yarn is a long continuous length of interlocked fibres, suitable for use in the production of textiles, sewing, crochetin ...
, 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 optical instruments, instruments that use or Photodetector, detect it. Optics usually describes the ...
,
pyrometer Image:Pyrometer 040824.jpg, upA sailor checking the temperature of a ventilation system. A pyrometer is a type of remote-sensing thermometer used to measure the temperature of distant objects. Various forms of pyrometers have historically existed. ...
s, thermal regulators, stove and kerosene heater windows, radiation aperture covers for microwave ovens, and elements. Mica is
birefringent Birefringence is the optical property of a material having a refractive index In optics, the refractive index (also known as refraction index or index of refraction) of a optical medium, material is a dimensionless number that describes how fas ...

birefringent
and is therefore commonly used to make quarter and half
wave plate A waveplate or retarder is an optics, optical device that alters the Polarization (waves), polarization state of a light wave travelling through it. Two common types of waveplates are the ''half-wave plate'', which shifts the polarization direction ...
s. Specialized applications for sheet mica are found in aerospace components in air-, ground-, and sea-launched missile systems,
laser A laser is a device that emits light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as h ...

laser
devices, medical electronics and radar systems. Mica is mechanically stable in micrometer-thin sheets which are relatively transparent to radiation (such as
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Proton ...

alpha particle
s) 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 A Geiger counter is an instrument used for detecting and measuring ionizing radiation. Also known as a Geiger–Müller counter (or Geiger–Muller counter), it ...
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

Micanite or mica for isolated mounting of transistors (top, right) and mica discs. 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 standards. The next lower grade is used in transmitting capacitors. 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 (alloy), Kanthal or Nichrome) 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 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, enabling for example the imaging of bismuth films, plasma glycoproteins, cell membrane, 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 used in "isinglass curtains" in horse-drawn carriages and early 20th-century cars.


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 prehistory, prehistoric times. Mica was known to ancient History of India, Indian, Ancient Egypt, Egyptian, Ancient Greece, Greek,Ancient Rome, Roman, and History of China, 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. 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 Pueblo, Taos and Picuris Pueblo, New Mexico, Picuris Pueblos Indians in north-central New Mexico to make pottery. The pottery is made from weathered Precambrian mica
schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fair ...

schist
, and has flecks of mica throughout the vessels. Tewa people, 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'' (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 printing in Japan, 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 Nishi Honganji Sanju-rokunin Kashu, 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 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 IDLH, immediately dangerous to life and health.


Substitutes

Some lightweight aggregate (geology), 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, phenolic resin, phenolics, polycarbonate,
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, polyvinyl chloride, vinyl-PVC, and vulcanized fiber. Mica paper made from scrap mica can be substituted for sheet mica in electrical and insulation applications.


See also

* Mica fish *


References


Sources


External links

*
Mineral Galleries data




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