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Rock with mica
Mica sheet
Mica flakes

The mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic with a tendency towards pseudo-hexagonal crystals and are similar in chemical composition. The highly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.

The word "mica" is thought to be derived from the Latin word micare, meaning "to glitter", in reference to the brilliant appearance of this mineral (especially when in small scales).

Contents

Mica classification

Chemically, micas can be given the general formula[1]

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.

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Trioctahedral micas

Common micas:

Brittle micas:

Interlayer deficient micas

Very fine-grained micas with typically 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.

Occurrence

Mica output in 2005

The British Geological Survey reports that as of 2005, Kodarma district in Jharkhand state in India had the largest deposits of mica in world. China was the top producer of mica with almost a third of the global share, closely followed by the USA, South Korea and Canada. Large deposits of sheet Mica were mined in New England from the 19th Century to the 1960s. Large mines existed in Connecticut, New Hampshire, and Maine.

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.

Until the 19th century, large crystals of mica were quite rare and expensive as a result of the limited supply in Europe. However, their price dramatically dropped when large reserves were found and mined in Africa and South America during the early 1800s. The largest sheet of mica ever mined in the world came from a mine in Denholm, Quebec, Canada.[2]

Scrap and flake mica is produced all over the world. Flake mica comes from several sources: the metamorphic rock called schist as a by-product of processing feldspar and kaolin resources, from placer deposits, and from pegmatites. Sheet mica is considerably less abundant than flake and scrap mica. Sheet mica is occasionally recovered from mining scrap and flake mica. The most important sources of sheet mica are pegmatite deposits.

Properties and uses

Electronics

A number of mica insulator slices for TO-3 and TO-264 packages.

Mica has a high dielectric strength and excellent chemical stability, making it a favoured material for manufacturing capacitors for radio frequency applications. It has also been used as an insulator in high voltage electrical equipment, and between the bars of commutators in Direct Current motors and generators. Mica is used because it can be split into very thin slices, and this keeps its thermal resistance low while retaining sufficient dielectric strength to prevent current from flowing across it at moderate voltages. The insulation is usually necessary when the heat sink is earthed (grounded) while the electronic component's metal surfaces will be connected to a power supply or signal line. If they were in direct contact this could form a short circuit. Heat sink insulation can also be necessary to prevent the heat sink from acting like an antenna if the component is connected to a rapidly varying signal.

Isinglass

Thin transparent sheets of mica called "isinglass" were used for peepholes in boilers and lanterns because they were less likely to shatter compared to glass when exposed to extreme temperature gradients. Such peepholes were also used in "isinglass curtains" in horse-drawn carriages[3] and early 20th century cars. A book about a journey in a Model T Ford car describes isinglass curtains as follows:

"Oiled canvas side curtains were put up over the windows for wind, rain, and cold (there were no heaters) and were held in place with rods that fit into the doors and twisting button snaps around the perimeter... 'Isinglass' peepholes in the curtains allowed limited visibility. Isinglass was made of thin sheets of cracked mica."[4]

Other

Resin-bonded mica or micanite or built up micanite from splittings or agglomerated micanite made from pulped mica powder are called micanites. All these products have additives, like resins, and the resultant products, while having inferior insulation properties to natural "mica insulators" are called micanites or micafoliums.

Mica is also birefringent and is commonly used to make quarter and half wave plates.

Illites or clay micas have a low cation exchange capacity for 2:1 clays. K+ ions between layers of mica prevent swelling by blocking water molecules.

Because mica can be pressed into a thin film, it is often used on Geiger-Müller tubes to detect low penetrating alpha particles.

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. The shimmer from mica is also used in makeup, as it gives a translucent "glow" to the skin or helps to mask imperfections.

Mica is used in the production of pearlescent pigments. 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.

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). Because mica is resistant to heat, it is used instead of glass in windows for stoves and kerosene heaters.

Another use of mica is in the production of ultraflat thin film surfaces (e.g. gold surfaces) using mica as substrate. Although the deposited film surface is still rough due to deposition kinetics, the back side of the film at mica-film interface provides ultraflatness, when the film is removed from the substrate.

It is also used as an insulator in concrete block, home attics, and can be poured into walls (usually in retrofitting uninsulated open top walls).

May also be used as a soil conditioner, especially in potting soil mixes and in gardening plots.

Muscovite mica is the most common substrate for sample preparation for the atomic force microscope. Freshly-cleaved mica surfaces have been used as clean imaging substrates in atomic force microscopy, enabling for example the imaging of bismuth films,[5] plasma glycoproteins,[6] membrane bilayers,[7] and DNA molecules.[8]

Greases used for axles are composed of a compound of fatty oils to which mica, tar or graphite is added to increase the durability of the grease and give it a better surface.

Hand carved from mica from the Hopewell tradition

Mica in ancient times

Human use of mica dates back to pre-historic times. Mica was known to ancient Egyptian, Greek and Roman civilizations, Chinese civilization, 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 visual and striking structure of Teotihuacan is the towering Pyramid of the Sun. The pyramid contained considerable amounts of mica in layers up to 30 cm (12 in) thick. The mica found in the pyramid of the sun has been identified as mined in Brazil, that is 3400 km from the pyramid complex.[9]

Throughout the ages, fine powders of mica have been used for various purposes, including decorative purposes. The colored Gulal and Abeer used by Hindus of north India during holi festival contain fine small crystals of mica. The majestic Padmanabhapuram palace, 65 km (40 mi) from Trivandrum in India, has colored mica windows.

References

  1. ^ Deer, W. A., R. A. Howie and J. Zussman (1966) An Introduction to the Rock Forming Minerals, Longman, ISBN 0-582-44210-9
  2. ^ "Denholm" (in French). MRC de La Vallée-de-la-Gatineau. http://www.mrcvg.qc.ca/textes/mun-denholm.shtml. Retrieved 2008-08-27. 
  3. ^ Isinglass curtains are referred to in the 1955 musical Oklahoma's song The Surrey with the Fringe on Top.
  4. ^ Joanne Wilke, Eight Women, Two Model Ts and the American West, University of Nebraska Press, (2007), ISBN 0803260199
  5. ^ Weisenhorn, A. L.; et al. (1991). "Atomically resolved images of bismuth films on mica with an atomic force microscope". Journal of Vacuum Science & Technology, B: Microelectronics and Nanometer Structures 9 (2): 1333–1335. doi:10.1116/1.585190. 
  6. ^ Marchant, Roger E.; Lea, A. Scott; Andrade, Joseph D.; Bockenstedt, Paula (1992). "Interactions of von Willebrand factor on mica studied by atomic force microscopy". Journal of Colloid and Interface Science 148 (1): 261–272. doi:10.1016/0021-9797(92)90135-9. 
  7. ^ Singh, Seema; Keller, David J. (1991). "Atomic force microscopy of supported planar membrane bilayers". Biophysical Journal 60 (6): 1401–1410. doi:10.1016/S0006-3495(91)82177-4. 
  8. ^ Thundat, T.; Allison, D. P.; Warmack, R. J.; Brown, G. M.; Jacobson, K. B.; Schrick, J. J.; Ferrell, T. L. (1992). Scanning Microscopy 6: 911. PMID 1295085. 
  9. ^ Fagan, Garrett G. (2006). Archaeological Fantasies: How Pseudoarchaeology Misrepresents the Past and Misleads the Public. New York: Routledge. pp. 102. ISBN 0415305934. http://books.google.com/books?id=sIYpx9mzd4gC&pg=PA102. 

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

MICA, a group of widely distributed rock-forming minerals, some of which have important commercial applications. The principal members of the group are muscovite, biotite, phlogopite and lepidolite. The name mica is probably derived from the Latin micare, to shine, to glitter; the German word glimmer has the same meaning. The mineral was probably included with selenite under Pliny's term lapis specularis. Mineralogical Characters. - The micas are characterized by a very easy cleavage in a single direction and by the high degree of flexibility, elasticity and toughness of the extremely thin cleavage flakes. They all crystallize in the monoclinic system, often, however, in forms closely resembling those of the rhombohedral or orthorhombic systems. Crystals have usually the form of hexagonal or rhomb-shaped scales, plates or prisms, with plane FIG. I. FIG. 2.

angles of 60° and 120°, and, with the exception of the basal planes, are only rarely bounded by smooth and well-defined faces. The crystal represented in fig. r is bounded by the basal pinacoid c (ooi) parallel to which is the perfect cleavage, the clinopinacoid b ((D10) parallel to the plane of symmetry, and the pyramids m (221) and o (112). The angles between these pyramids and the basal plane are 852° and 73° respectively. The prism (110) at 90° from the basal plane is not developed as a crystal face, but is a plane of twinning, the two individuals of the twin being united parallel to the basal plane (fig. 2). The different species of mica have very nearly the same forms and interfacial angles, and they not infrequently occur intergrown together in parallel position. The best developed crystals are those of Vesuvian biotite.

When a cleavage flake of mica is struck a sharp blow with a blunt needle-point a six-rayed star of cracks or "percussion figure" is developed: the rays intersect at angles of approximately 60°, and the pair most prominently developed are parallel to the plane of symmetry of the crystal. A similar six-rayed system of cracks, bisecting the angles between the rays of the previous set, is produced when a blunt punch is gradually pressed against a sheet of mica; this is known as the "pressure figure." These cracks coincide with planes of easy separation or of gliding in the crystal; they are especially useful in helping to determine the crystallographic orientation of a cleavage flake of mica when crystal faces are absent. Sheets of mica which have been subjected to earth-movements are frequently cracked and ridged parallel to these directions, and are then valueless for economic purposes.

In their optical characters the micas exhibit considerable variations. The indices of refraction are not high, the mean index being about I. 58-1.60, but the double refraction is very strong (o.04 - o.05) and is negative in sign. The angle between the optic axes varies from 70 0 -50° in muscovite and lepidolite to Io - o° in biotite and phlogopite; the latter are thus frequently practically uniaxial. The acute bisectrix of the optic axes never deviates from the normal to the basal plane by more than a degree or two, hence a cleavage flake of mica will always show an optic figure in convergent light when placed on the stage of a polarizing microscope. The plane of the optic axes may be either perpendicular or parallel to the plane of symmetry of the crystal, and according to its position two classes of mica are distinguished. To the first class, with the optic axial plane perpendicular to the plane of symmetry, belong muscovite, lepidolite, paragonite, and a rare variety of biotite called anomite; the second class includes zinnwaldite, phlogopite, lepidomelane and most biotites. Dark coloured micas are strongly pleochroic. Accurate determinations of the optical orientation, as well as the symmetry of the etching figures on the cleavage planes, seem to suggest that the micas, except muscovite, may be anorthic rather than monoclinic in crystallization.

The different kinds of mica vary from perfectly colourless and transparent - as in muscovite - through shades of yellow, green, red and brown to black and opaque - as in lepidomelane; the former have a pearly lustre and the latter a submetallic lustre on the cleavage surfaces. Sheets of mica very often show coloured rings and bands (Newton's rings), due to the interference of light at the surfaces of internal cleavage cracks. The spec. gra y. varies between 2.7 and 3.1 in the different species. The hardness is 2 -3; smooth cleavage surfaces can be just scratched with the finger-nail. The micas are bad conductors of heat and electricity, and it is on these properties that many of their technical applications depend.

Inclusions of other minerals are frequently to be observed in mica. Flattened crystals of garnet, films of quartz, and needles of tourmaline are not uncommon. Cleavage sheets are frequently disfigured and rendered of little value by brown, red or black spots and stains, often with a dendritic arrangement of iron oxides. Minute acicular inclusions, probably of rutile, arranged parallel to the rays of the percussion figure, give rise to the phenomenon of "asterism" in some micas, particularly phlogopite: a candle-flame or spot of light viewed through a cleavage sheet of such mica appears as a six-rayed star.

Table of contents

Chemical Composition

The micas are extremely complex and variable in composition. They are silicates, usually orthosilicates, of aluminium together with alkalis (potassium, sodium, lithium, rarely rubidium and caesium), basic hydrogen, and, in some species magnesium, ferrous and ferric iron, rarely chromium, manganese and barium. Fluorine is also often an essential constituent, and titanium is sometimes present.

The composition of the several species of mica is given by the following formulae, some of which are only approximate. It will be seen that they may be divided into two groups - alkali-micas (potash-mica, &c.) and ferromagnesian micas - which correspond roughly with the division into light and dark micas.

Muscovite.. H2K Al3(S104)3 Paragonite.. H 2 Na A13(S104)3 Lepidolite... KLi[Al(OH,F)2]Al(S103) 3 Zinnwaldite.. (K,Li)3[Al(OH,F)2]FeAl2S15016 Biotite. .. (H,K)2(Mg,Fe)2(A1,Fe)2(S104) 3 Phlogopite.. [H,K, (MgF)13Mg3A1(S104)3 The water which is present in muscovite. to the extent of 4 to 6%, and rather less in the other species, is expelled only at a high temperature; it is therefore water of constitution, existing as basic hydrogen or as hydroxyl replacing fluorine.

Roscoelite is a mica in which the aluminium is largely replaced by vanadium (V203, 30%); it occurs as brownish-green scaly aggregates, intimately associated with gold in California, Colorado and Western Australia.

Various attempts have been made to explain the variations in composition of the micas. G. Tschermak, in 1878, regarded them as isomorphous mixtures of the following fundamental molecules: H 2 KA1 3 (SiO 4) 3, corresponding with muscovite; Mg 6 Si 3 0 12, a hypothetical polymer of olivine; and H4S15012, a hypothetical silicic acid. F. W. Clarke (1889-1893) supposes them to be substitution derivatives of normal aluminium orthosilicate A14(S104)3, in which part of the aluminium is replaced by alkalis, magnesium, iron and the univalent groups (MgF), (A1F2),(AlO), (MgOH); an excess of silica is explained by the isomorphous replacement of H 4 SiO 4 by the acid H4S130s.

Artificially formed crystals of the various species of mica have been observed in furnace-slags and in silicate fusions.

Occurrence

Mica occurs as a primary and essential constituent of igneous rocks of almost all kinds; it is also a common product of alteration of many mineral silicates, both by weathering and by contactand dynamo-metamorphic processes. In sedimentary rocks it occurs as detrital material.

Muscovite and biotite are commonly found in siliceous rocks, whilst phlogopite is characteristic of calcareous rocks. The best crystallized specimens of any mica are afforded by the small brilliant crystals of biotite, which encrust cavities in the limestone blocks ejected from Monte Somma, Vesuvius. Large sheets of muscovite, such as are of commercial value, are found only in the very coarsely crystallized pegmatite veins traversing granite, gneiss or micaschist. These veins consist of felspar, quartz and mica, often with smaller amounts of other crystallized minerals, such as tourmaline, beryl and garnet; they are worked for mica in India, the United States (South Dakota, Colorado and Alabama), and Brazil (Goyaz, Bahia and Minas Geraes). The commercially valuable micas of Canada and Ceylon are mainly phlogopite (q.v.), which has a rather different mode of occurrence. The mica mined in India is practically all muscovite. The principal mining districts are those of Hazaribagh in Bengal and Nellore in Madras; in the former district the mica has usually a ruby tint, whilst in the latter it is more often greenish. In the Inikurti mine, Nellore, "books" of mica measuring 10 ft. across, and up to 15 ft. across the folia have been found, and rectangular sheets measuring 30 by 24 in. and free from cracks and flaws have frequently been obtained.

Uses

On account of its transparency and its resistance to fire and sudden changes of temperature, mica has been much used for the windows of stoves and lanterns, for the peep-holes of furnaces, and the chimneys of lamps and gas-burners. At one time it was used for window panes of houses and the port-holes of Russian men-of-war, being commonly known as "Muscovy glass." Spangles of mica are much used for decorative purposes of various kinds, and the mineral was formerly known as glacies Mariae (Ger., Frauenglas) because of its use for decorating statues of the Virgin. The lapis specularis of Pliny, scattered over the Circus Maximus to produce a shining whiteness, was probably mica. Large quantities of ground mica are used in the manufacture of wall-paper, and to produce a frosted effect on toys, stage scenery, &c. Powdered mica is also used in the manufacture of paints and paper, as a lubricant, and as an absorbent of nitro-glycerine and disinfectants. Sheets of mica are used as a surface for painting, especially in India; for lantern slides; for carrying photographic films; as a protective covering for pictures and historical documents; for mounting soft and collapsible natural history specimens preserved in spirit; for the vanes of anemometers; mirrors of delicate physical instruments; for various optical and many other purposes. Being a bad conductor of heat it is used for the packing and jackets of boilers and steam-pipes. Other applications depend on the strength of its resistance to acids.

The most extensive application of mica at the present day is for electrical purposes. Being a bad conductor of electricity it is of value as an insulator, and the smooth flexible sheets are much used in the construction of armatures of dynamos and in other electrical machinery. For various purposes a manufactured material known as "micanite" or "micanite cloth" is much used; this consists of small sheets of mica cemented with shellac or other insulating cement on cloth or paper.

Muscovite and phlogopite are practically the only species used commercially, the former being the more common. Phlogopite is rarely found as colourless transparent sheets and is therefore almost exclusively used for electrical purposes. Many other uses of mica might be mentioned. The potassium it contains renders it of value as a manure. The species lepidolite is largely used for the manufacture of lithium and rubidium salts.

Mining, Preparation and Value

Mica mining is an industry of considerable importance, especially in India; but here the methods of mining are very primitive and wasteful. In working downwards in open quarries and in tortuous shafts and passages much of the mica is damaged, and a large amount of labour is expended in hauling waste material to the surface. Since the mineral occurs in definite veins, a more satisfactory and economical method of working would be that adopted in metalliferous mines, with a vertical shaft, cross-cuts, and levels running along the strike of the vein: the mica could then be extracted by overhead stopping, and the waste material used for filling up the worked-out excavations.

In dressing mica the "books" are split along the cleavage into sheets of the required thickness, and the sheets trimmed into rectangles with a sharp knife, shears or guillotine, stained and damaged portions being rejected. The dressed sheets are sorted according to size, transparency, colour and freedom from spots or stains. Scrap mica is ground to powder or used in the manufacture of micanite.

The price of mica varies very considerably according to the size, transparency and quality of the sheets. An average price for cut sheets of all sizes is about 4s. per lb, but for large sheets it may be as high as 54s. per lb.

REFERENCES

For the mineralogical characters see the textbooks of J. D. Dana and C. Hintze; for economic questions, the following: T. H. Holland, "The Mica Deposits of India," Memoirs of the Geological Survey of India (1902), xxxiv. 11 -121; G. P. Merrill, The Non-Metallic Minerals (New York, 1904), pp. 163180; "The Mining and Preparation of Mica for Commercial Purposes," Bulletin of the Imperial Institute (London, 1904), ii. 278-291; F. Cirkel, "Mica: its Occurrence, Exploitation and Uses" (Canadian Dept. of the Interior, Mines Branch, Ottawa 1905, 1 4 8 pp.). (L. J. S.)


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Wiktionary

Up to date as of January 15, 2010
(Redirected to mica article)

Definition from Wiktionary, a free dictionary

A sheet of mica
See also mică

Contents

English

Pronunciation

Noun

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Wikipedia

mica (uncountable)

  1. The name of a group of hydrous aluminosilicate minerals characterized by highly perfect cleavage, so that they readily separate into very thin leaves, more or less elastic.

Translations

See also

Anagrams


Galician

Noun

mica f. (usually uncountable)

  1. mica

Italian

Adverb

mica

  1. (colloquial) not
    Mica male! - Not bad!
  2. (colloquial) hardly, you know
    Mica sono stupido - I’m hardly stupid; I’m not stupid, you know

Anagrams

  • Anagrams of acim
  • cima

Romanian

Pronunciation

  • IPA: ['mi.ka]

Adjective

mica

  1. definite feminine singular nominative form of mic.
  2. definite feminine singular accusative form of mic.

Simple English

The mica group of minerals are aluminosilicates with various metals. They split into fine sheets.

Mica includes several closely related materials having perfect basal cleavage.

All are monoclinic with a tendency towards pseudo-hexagonal crystals and are similar in chemical composition. The highly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.



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