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Wikipedia

Mica

Micas (/ˈmkəz/ MY-kəz) 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.[6] It is particularly prominent in many granites, pegmatites, and schists,[7] and "books" (large individual crystals) of mica several feet across have been found in some pegmatites.[8]

Mica
General
CategoryPhyllosilicates
Formula
(repeating unit)
AB2–3(X, Si)4O10(O, F, OH)2
IMA symbolMca[1]
Identification
Colorpurple, rosy, silver, gray (lepidolite); dark green, brown, black (biotite); yellowish-brown, green-white (phlogopite); colorless, transparent (muscovite)
CleavageAlmost perfect
Fractureflaky
Mohs scale hardness2.5–4 (lepidolite); 2.5–3 biotite; 2.5–3 phlogopite; 2–2.5 muscovite
Lusterpearly, vitreous
StreakWhite, colorless
Specific gravity2.8–3.0
Diagnostic featurescleavage
References[2][3][4][5]
Sheets of mica
Photomicrographs of a thin section containing phlogopite. In cross-polarized light on left, plane-polarized light on right.
Dark mica from eastern Ontario

Micas are used in products such as drywalls, paints, fillers, especially in parts for automobiles, roofing and shingles, as well as in electronics. The mineral is used in cosmetics and food[9] to add "shimmer" or "frost."

Properties and structure

The mica group is composed of 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.[10]

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). It is the relatively weak ionic bonding between TOT layers that gives mica its perfect basal cleavage.[11]

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 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.[12]

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.[13]

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).[11]

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.

Classification

Chemically, micas can be given the general formula[14]

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

Brittle micas:

Trioctahedral micas

Common micas:

Brittle micas:

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 (colourless) micas.

Occurrence and production

Mica embedded in metamorphic rock

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.[6]

The largest documented single crystal of mica (phlogopite) was found in Lacey Mine, Ontario, Canada; it measured 10 m × 4.3 m × 4.3 m (33 ft × 14 ft × 14 ft) and weighed about 330 tonnes (320 long tons; 360 short tons).[17] Similar-sized crystals were also found in Karelia, Russia.[18]

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).[19] 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.[20]

In Madagascar[21] and India,[22] 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 500 °C (932 °F), 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 900 °C (1,650 °F)) 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.[20]

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.[20]

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 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 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.[20]

Dry-ground mica is used in the production of rolled roofing and asphalt shingles, 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, 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 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 polyesters.[20]

Paints and cosmetics

 
Mica flakes embedded in a fresco for glitter

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 (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, 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.[20]

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 furnaces, 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.[20]

Flexible plate is used in electric motor and generator armatures, field coil insulation, and magnet and commutator 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, 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%).[20]

Sheet mica

 
Muscovite windows

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, pyrometers, thermal regulators, stove and kerosene heater windows, radiation aperture covers for microwave ovens, and micathermic heater 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 tubes.

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.[20]

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.[20]

Mica sheets are used to provide structure for heating wire (such as in Kanthal or Nichrome) in heating elements and can withstand up to 900 °C (1,650 °F).

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.[23][24] They include the sodium-vapor lamp that is the gas-discharge lamp in street lighting.[25][26][23][24]

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,[27] enabling for example the imaging of bismuth films,[28] plasma glycoproteins,[29] membrane bilayers,[30] and DNA molecules.[31]

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.[32][33][34]

Etymology

The word mica is derived from the Latin word mica, meaning a crumb, and probably influenced by micare, to glitter.[35]

Early history

 
Hand carved from mica from the Hopewell tradition

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.[36]

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 to 1944.[37][38] Later, a second deposit was located in the Xalla Complex,[38] another palatial structure east of Street of the Dead. There is a claim mica was found within the Pyramid of the sun, which originates from Peter Tompkins in his book Mysteries of the Mexican Pyramids.[39] But it 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.[20]

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).[40][41] 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

 
Kirazuri printing technique adds mica powder to the gelatin solution as adhesive, here printed on the background.[42]

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, 65 km (40 mi) from Trivandrum in India, has colored mica windows.

Mica powder is also used as a decoration in traditional Japanese woodblock printmaking,[43] 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 stenciled on hairpins, sword blades or fish scales on carp streamers (鯉のぼり, Koinobori).

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.[44][45][46]

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.[47][48]

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.[49]

Substitutes

Some lightweight aggregates, such as diatomite, perlite, and vermiculite, may be substituted for ground mica when used as filler. Ground synthetic fluorophlogopite,[50] 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.[19]

See also

References

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Sources

  This article incorporates public domain material from Mica. United States Geological Survey.

External links

  •   Media related to Mica at Wikimedia Commons
  • Mineral Galleries data
  • Mindat
  • CDC – NIOSH Pocket Guide to Chemical Hazards
  • "Mica" . Encyclopædia Britannica (11th ed.). 1911.
  • Scientific American, "Mica", 22-Oct-1881, pp. 257

mica, this, article, about, mineral, other, uses, disambiguation, kəz, group, silicate, minerals, whose, outstanding, physical, characteristic, that, individual, mica, crystals, easily, split, into, extremely, thin, elastic, plates, this, characteristic, descr. This article is about the mineral or gem For other uses see Mica disambiguation Micas ˈ m aɪ k e z MY kez 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 6 It is particularly prominent in many granites pegmatites and schists 7 and books large individual crystals of mica several feet across have been found in some pegmatites 8 MicaGeneralCategoryPhyllosilicatesFormula repeating unit AB2 3 X Si 4O10 O F OH 2IMA symbolMca 1 IdentificationColorpurple rosy silver gray lepidolite dark green brown black biotite yellowish brown green white phlogopite colorless transparent muscovite CleavageAlmost perfectFractureflakyMohs scale hardness2 5 4 lepidolite 2 5 3 biotite 2 5 3 phlogopite 2 2 5 muscoviteLusterpearly vitreousStreakWhite colorlessSpecific gravity2 8 3 0Diagnostic featurescleavageReferences 2 3 4 5 Sheets of mica Photomicrographs of a thin section containing phlogopite In cross polarized light on left plane polarized light on right Dark mica from eastern Ontario Micas are used in products such as drywalls paints fillers especially in parts for automobiles roofing and shingles as well as in electronics The mineral is used in cosmetics and food 9 to add shimmer or frost Contents 1 Properties and structure 2 Classification 2 1 Dioctahedral micas 2 2 Trioctahedral micas 2 3 Interlayer deficient micas 3 Occurrence and production 4 Uses 4 1 Useful properties 4 2 Ground mica 4 3 Paints and cosmetics 4 4 Built up mica 4 5 Sheet mica 4 5 1 Electrical and electronic 4 5 2 Atomic force microscopy 4 5 3 Peepholes 5 Etymology 6 Early history 6 1 Mica powder 6 2 Medicine 7 Health impact 7 1 United States 8 Substitutes 9 See also 10 References 11 Sources 12 External linksProperties and structure EditThe mica group is composed of 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 10 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 It is the relatively weak ionic bonding between TOT layers that gives mica its perfect basal cleavage 11 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 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 12 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 13 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 11 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 View of tetrahedral sheet structure of mica The apical oxygen ions are tinted pink View of trioctahedral sheet structure of mica The binding sites for apical oxygen are shown as white spheres View of trioctahedral sheet structure of mica emphasizing octahedral sites View of dioctahedral sheet structure of mica The binding sites for apical oxygen are shown as white spheres View of dioctahedral sheet structure of mica emphasizing octahedral sites View of trioctahedral mica structure looking at surface of a single layer View of trioctahedral mica structure looking along sheetsClassification EditChemically micas can be given the general formula 14 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 Edit Muscovite 15 ParagoniteBrittle micas Margarite 16 Trioctahedral micas Edit Common micas Biotite 15 Lepidolite Phlogopite ZinnwalditeBrittle micas ClintoniteInterlayer deficient micas Edit 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 colourless micas Occurrence and production Edit source source source source source source source source source source source source source source Mica embedded in metamorphic rock 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 6 The largest documented single crystal of mica phlogopite was found in Lacey Mine Ontario Canada it measured 10 m 4 3 m 4 3 m 33 ft 14 ft 14 ft and weighed about 330 tonnes 320 long tons 360 short tons 17 Similar sized crystals were also found in Karelia Russia 18 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 19 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 20 In Madagascar 21 and India 22 it is also mined artisanally in poor working conditions and with the help of child labour Uses EditThe commercially important micas are muscovite and phlogopite which are used in a variety of applications Useful properties Edit 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 500 C 932 F 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 900 C 1 650 F 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 20 Ground mica Edit 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 20 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 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 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 20 Dry ground mica is used in the production of rolled roofing and asphalt shingles 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 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 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 polyesters 20 Paints and cosmetics Edit Mica flakes embedded in a fresco for glitter 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 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 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 20 Built up mica Edit 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 furnaces 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 20 Flexible plate is used in electric motor and generator armatures field coil insulation and magnet and commutator 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 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 20 Sheet mica Edit Muscovite windows 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 pyrometers thermal regulators stove and kerosene heater windows radiation aperture covers for microwave ovens and micathermic heater 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 Muller tubes 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 20 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 Edit Silver mica capacitors 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 20 Mica sheets are used to provide structure for heating wire such as in Kanthal or Nichrome in heating elements and can withstand up to 900 C 1 650 F 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 23 24 They include the sodium vapor lamp that is the gas discharge lamp in street lighting 25 26 23 24 Atomic force microscopy Edit 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 27 enabling for example the imaging of bismuth films 28 plasma glycoproteins 29 membrane bilayers 30 and DNA molecules 31 Peepholes Edit 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 32 33 34 Etymology EditThe word mica is derived from the Latin word mica meaning a crumb and probably influenced by micare to glitter 35 Early history Edit Hand carved from mica from the Hopewell tradition 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 36 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 to 1944 37 38 Later a second deposit was located in the Xalla Complex 38 another palatial structure east of Street of the Dead There is a claim mica was found within the Pyramid of the sun which originates from Peter Tompkins in his book Mysteries of the Mexican Pyramids 39 But it 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 20 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 40 41 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 Edit Kirazuri printing technique adds mica powder to the gelatin solution as adhesive here printed on the background 42 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 65 km 40 mi from Trivandrum in India has colored mica windows Mica powder is also used as a decoration in traditional Japanese woodblock printmaking 43 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 stenciled on hairpins sword blades or fish scales on carp streamers 鯉のぼり Koinobori 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 44 45 46 Medicine Edit 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 47 48 Health impact EditMica dust in the workplace is regarded as a hazardous substance for respiratory exposure above certain concentrations United States Edit 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 49 Substitutes EditSome lightweight aggregates such as diatomite perlite and vermiculite may be substituted for ground mica when used as filler Ground synthetic fluorophlogopite 50 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 19 See also EditMica fish Minerals portalReferences Edit Warr L N 2021 IMA CNMNC approved mineral symbols Mineralogical Magazine 85 3 291 320 Bibcode 2021MinM 85 291W doi 10 1180 mgm 2021 43 S2CID 235729616 Mica Archived 2015 01 16 at the Wayback Machine Minerals Education Coalition The Mica Group Archived 2015 03 02 at the Wayback Machine Rocks And Minerals 4 U Mica Archived 2015 03 17 at the Wayback Machine mineralszone com Amethyst Galleries THE MICA GROUP Archived 2014 12 30 at the Wayback Machine galleries com a b Nesse William D 2000 Introduction to mineralogy New York Oxford University Press pp 244 249 ISBN 9780195106916 Nesse 2000 pp 245 246 248 Jahns R H 1946 Mica deposits of the Petaca district Rio Arriba County New Mexico PDF New Mexico Bureau of Mines and Mineral Resources Bulletin 25 60 Retrieved 29 June 2021 CFR Code of Federal Regulations Title 21 U S Food and Drug Administration U S Food and Drug Administration July 20 2022 Retrieved November 1 2022 Nesse 2000 pp 244 250 a b Nesse 2000 p 238 Nesse 2000 p 235 Nesse 2000 pp 235 237 W A Deer R A Howie and J Zussman 1966 An Introduction to the Rock Forming Minerals Longman ISBN 0 582 44210 9 a b Mineralogy Phyllosilicates Colgate University 1997 Archived from the original on 19 September 2015 Retrieved 18 April 2016 Nesse 2000 pp 249 250 Rickwood P C 1981 The largest crystals PDF American Mineralogist 66 885 907 Archived PDF from the original on 2013 08 25 The giant crystal project site Archived from the original on 2009 06 04 Retrieved 2009 06 06 a b Mica Archived 2011 10 30 at the Wayback Machine USGS Mineral Commodity Summaries 2011 a b c d e f g h i j k Dolley Thomas P 2008 Mica Archived 2011 10 30 at the Wayback Machine in USGS 2008 Minerals Yearbook Latest Terre des Hommes Archived from the original on January 21 2020 O Driscoll Dylan 4 October 2017 Overview of child labour in the artisanal and small scale mining sector in Asia and Africa K4D Helpdesk Report Institute of Development Studies Retrieved 10 December 2020 a b The Low Pressure Sodium Lamp a b The Low Pressure Sodium Lamp Lighting Comparison LED vs High Pressure Sodium Low Pressure Sodium www stouchlighting com The Sodium Lamp How it works and history edisontechcenter org Eaton P and West W 2010 Substrates for AFM pp 87 89 in Atomic Force Microscopy Oxford University Press ISBN 978 0 19 957045 4 Weisenhorn A L 1991 Atomically resolved images of bismuth films on mica with an atomic force microscope Journal of Vacuum Science amp Technology B Microelectronics and Nanometer Structures 9 2 1333 Bibcode 1991JVSTB 9 1333W doi 10 1116 1 585190 Marchant R E Lea A S Andrade J D Bockenstedt P 1992 Interactions of von Willebrand factor on mica studied by atomic force microscopy PDF Journal of Colloid and Interface Science 148 1 261 272 Bibcode 1992JCIS 148 261M doi 10 1016 0021 9797 92 90135 9 hdl 2027 42 30333 Singh S Keller D J 1991 Atomic force microscopy of supported planar membrane bilayers Biophysical Journal 60 6 1401 10 Bibcode 1991BpJ 60 1401S doi 10 1016 S0006 3495 91 82177 4 PMC 1260200 PMID 1777565 Thundat T Allison D P Warmack R J Brown G M Jacobson K B Schrick J J Ferrell T L 1992 Atomic force microscopy of DNA on mica and chemically modified mica Scanning Microscopy 6 4 911 8 PMID 1295085 Isinglass curtains are referenced in the 1943 musical Oklahoma s song The Surrey with the Fringe on Top isinglass n Oxford English Dictionary Online ed Oxford University Press Subscription or participating institution membership required Wilke Joanne 2007 Eight Women Two Model Ts and the American West University of Nebraska Press p 28 ISBN 978 0803260191 Kirkpatrick E M ed 1983 Chambers 20th Century Dictionary Schwarz Davidson Seaton Simpson Sherrard New ed Edinburgh W amp R Chambers Ltd p 793 ISBN 0550102345 Haze Xaviant 2016 11 21 Ancient Giants of the Americas Suppressed Evidence and the Hidden History of a Lost Race Red Wheel Weiser ISBN 9781632659323 Acosta Jorge R 1970 ARCHAEOLOGICAL EXPLORATIONS IN TEOTIHUACAN Artes de Mexico 134 12 ISSN 0300 4953 a b Cowgill George L 2015 Ancient Teotihuacan early urbanism in Central Mexico New York NY p 180 ISBN 978 0 521 87033 7 OCLC 898206006 Tompkins Peter 1987 Mysteries of the Mexican pyramids Harper amp Row p 202 OCLC 1150839351 Dehlvi Sadia October 14 2007 Tradition and modernity Dawn com Archived from the original on October 20 2013 Ramzi Shanaz March 31 2005 Fashion through the ages Dawn com Archived from the original on October 20 2013 喜多川歌麿筆 Kitagawa Utamaro 1790s 茶托を持つ難波屋おきた Okita of Naniwaya with a tea cup Colbase Tokyo National Museum 国立博物館所蔵品統合検索システム in Japanese retrieved 2019 11 28 ビクセン Vixen 総合光学機器メーカー 浮世絵 gt 雲母摺と空摺 Ukiyoe gt Kirazuri and karazuri 単眼鏡が広げる美術鑑賞の世界 Mono scope enhances appreciation of art world in Japanese Retrieved 2019 11 28 きらら鈴 愛知県 きらら鈴とは コトバンク きらら鈴 を受け継ごうとする お母さん たちがいます 旬な地元ネタ まいぷれ 西尾 碧南 高浜 Abhraka Bhasma Preparation Indications and Properties Archived 2015 10 05 at Wikiwix Ayurmedinfo com Abhraka Bhasma Properties and uses Archived 2015 10 04 at the Wayback Machine ayurtimes com CDC NIOSH Pocket Guide to Chemical Hazards Mica containing less than 1 quartz www cdc gov Archived from the original on 2015 12 08 Retrieved 2015 11 27 Fluorphlogopite synthetic mica Borosilicate and quartz glass mica sealing level gauges armature Continental Trade www continentaltrade com pl Archived from the original on 2018 02 12 Sources Edit This article incorporates public domain material from Mica United States Geological Survey External links Edit Media related to Mica at Wikimedia Commons Mineral Galleries data Mindat CDC NIOSH Pocket Guide to Chemical Hazards Mica Encyclopaedia Britannica 11th ed 1911 Scientific American Mica 22 Oct 1881 pp 257 Retrieved from https en wikipedia org w index php title Mica amp oldid 1136286866, wikipedia, wiki, book, books, library,

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