Mica 21061 226180431 2008-07-17T05:11:11Z ClueBot 4928500 Reverting possible vandalism by [[Special:Contributions/117.198.33.92|117.198.33.92]] to version by Mpatel. False positive? [[User:ClueBot/FalsePositives|Report it]]. Thanks, [[User:ClueBot]]. (442232) (Bot) {{otheruses}} [[Image:mica-in-rock-from-alstead.jpg|thumb|Rock with mica]] [[Image:MicaSheetUSGOV.jpg|thumb|Mica sheet]] [[Image:mica-from-alstead.jpg|thumb|Mica flakes]] The '''mica''' group of sheet [[silicate]] ([[silicate minerals|phyllosilicate]]) [[mineral]]s includes several closely related materials having highly perfect [[basal cleavage]]. All are [[monoclinic]] with a tendency towards pseudo-hexagonal [[crystal]]s and are similar in chemical composition. The highly perfect cleavage, which is the most prominent characteristic of mica, is explained by the [[hexagon]]al sheet-like arrangement of its [[atom]]s. The word "mica" is thought to be derived from the [[Latin]] word ''micare'', to glitter, in reference to the brilliant appearance of this mineral (especially when in small scales). Mica has a [[lamellar]] form with a shiny luster. ==Mica classification== Chemically, micas can be given the general formula:<ref>Deer, W. A., R. A. Howie and J. Zussman (1966) ''An Introduction to the Rock Forming Minerals'', Longman, ISBN 0-582-44210-9</ref> :''X''<sub>2</sub>''Y''<sub>4-6</sub>''Z''<sub>8</sub>O<sub>20</sub>(OH,F)<sub>4</sub> :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 Fe<sup>3+</sup> or Ti Structurally the micas can be classed as ''di-octahedral'' (''Y'' = 4) and ''tri-octahedral'' (''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''. ===Tri-octahedral micas=== Common micas: *[[Phlogopite]] *[[Biotite]] *[[Zinnwaldite]] *[[Lepidolite]] Brittle micas: *[[Clintonite]] ===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 H<sub>3</sub>O<sup>+</sup> 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 Fe<sup>2+</sup> substituting for Al in the ''Y'' site and a corresponding increase in Si in the ''Z'' site ==Occurrence== [[Image:Mica output2.PNG#Summary|thumb|right|Mica output in 2005]] The [[British Geological Survey]] reports that as of 2005, 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 1960's. Large mines existed in CT, NH and Maine. Mica is widely distributed and occurs in [[igneous]], [[metamorphic rocks|metamorphic]] and [[sedimentary]] regimes. Large crystals of mica used for various applications are typically mined from [[granitic]] [[pegmatite]]s. Until the 19th century, large crystals of mica were quite rare and expensive as a result of the limited supply in Europe. However, its price dramatically dropped when large reserves were found and mined in Africa and South America during the early 1800s. 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 == 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 [[Electrical insulation|insulator]] in high voltage electrical equipment. It is also [[birefringence|birefringent]] and is commonly used to make quarter and half [[wave plate]]s. Because mica is resistant to [[heat]] it is used instead of glass in windows for [[stove]]s and [[kerosene heater]]s. It is also used to separate electrical conductors in [[cable]]s that are designed to have a [[fire-resistance rating]] in order to provide [[circuit integrity]]. The idea is to keep the [[metal]] conductors from fusing in order to prevent a short-circuit so that the cables remain operational during a fire, which can be important for applications such as emergency lighting. 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. [[Aventurine]] is a variety of quartz with mica inclusions used as a gemstone. Pressed Mica sheets are often used in place of glass in [[greenhouse]]s. [[Muscovite]] mica is the most common substrate for sample preparation for the [[atomic force microscope]]. 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 sheets<!--Mica heating boards? --> are used to provide structure for heating wire (like [[Kanthal]], [[Nichrome]], etc..) in [[heating element]]s and can withstand up to 900 °C. <ref>{{cite web|title=Precision Pressed Products|url=http://www.precisionv-1mica.com/pp5.html}} 071103 precisionv-1mica.com</ref> <ref>{{cite web|title=S &amp; J Trading Inc : Mica Products|url=http://www.sjmica.com/cat3_prd8.htm}} 071103 sjmica.com</ref> <ref>{{cite web|title=Mica Products|url=http://www.indiamart.com/nscmica/mica-products.html}} 071103 indiamart.com</ref><!-- How are these manufactured? --> 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. [[mica insulator|Mica slices]] are used in electronics to provide electric insulation between a heat generating component and the heat sink used to cool it<ref>See pictures and use on [http://www.bigdaddy-enterprises.com/electronics/e_heatsinks.htm] [http://www.aavidthermalloy.com/products/standard/access/kits.shtml] [http://www.vitacom.ro/Products/OEM/250/MICA-TO220/ISOLATION-TO220.html] [http://www.alliedelec.com/Search/ProductDetail.asp?SKU=935-9422&MPN=NTE422&R=935-9422&sid=480D2A80500DE17F]</ref> . The same word is sometimes used by technicians to designate a synthetised gum (usually blue or grey) which is used for the same purpose, but which does not actually consist of silicate mineral (language abuse). == Role in primitive life == Helen Hansma, a research scientist affiliated with the [[University of California]], has proposed that the unique properties of Mica enabled the formation of life in the oceans of the distant past.<ref>[http://www.sciencedaily.com/releases/2007/12/071204102500.htm Life On Earth May Have Originated As The Organic Filling In A Multilayer Sandwich Of Mica Sheets<!-- Bot generated title -->]</ref> <ref>[http://www.npr.org/templates/story/story.php?storyId=17007216 NPR: Life on Earth May Have Begun In Between Rocks<!-- Bot generated title -->]</ref> In atomic force microscopy, biological samples are placed on mica because it is atomically flat. Even bare DNA molecules may be seen as small ridges. Inspecting mica under the microscope, bits of algae and other organic materials suggested to her the possibility early life molecules could have evolved within mica sheets in a communal fashion eons before the evolution of cell membranes or [[lipid]] [[Vesicle_%28biology%29|vesicles]]. Mica might have provided a secure place with time and space and protection from the open ocean. Further research might provide additional predictions about energy and entropy for life. Mica is old rock, some of earliest evidence for life's most primitive cells is in [[Akilia Island]], [[Greenland]], where [[biotite]], a common mica, is also found. [[Potassium]] ions, which hold the sheets of mica together, are is also required by cells. [[Abiogenesis|Primordial soup]] with [[amino acid|amino acids]] and simple building blocks of life might have seeped between the water-loving mica sheets. The large planer area between sheets might have facilitated the building of long chain molecules. Negative spaces holding the potassium ions on mica are 0.5 nm apart, as are the single stranded DNA molecules (letters of genetic code), as well as amino acids in proteins. Clay also provides spacing that might encourage this process, but the planer area might better encourage the process. Expansion and contraction caused by temperature changes and ocean currents might provide mechanical energy to help rearrange the molecules and trigger the formation of chemical bonds. == Mica in ancient times == [[Image:Hand Hopewell mica.jpg|thumb|175px|Hand carved from mica from the [[Hopewell tradition]]]] Human use of mica dates back to [[pre-historic]] times. Mica was known to ancient [[Ancient Egypt|Egyptian]], [[Ancient Greece|Greek]] and [[Ancient Rome|Roman]] civilizations, [[History of China|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 colours were red, iron oxide (hematite, a form of red ochre) and black (manganese dioxide), though black from juniper or pine carbons has also been discovered. White from kaolin or mica was used occasionally. A few kilometeres 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 locally mined mica in layers up to 30cm thick.<ref>Garrett G. Fagan, ed., ''Archaeological Fantasies: How Pseudoarchaeology Misrepresents the Past and Misleads the Public'', p. 102, ISBN 0415305934 </ref> Throughout the ages, fine powders of mica have been used for various purposes, including decorative purposes. The coloured [[Gulal]] and [[Abeer]] used by [[Hindus]] of north India during [[holi]] festival contain fine small crystals of mica. The majestic [[Padmanabhapuram Palace|Padmanabhapuram palace]], 65 km from [[Trivandrum]] in India, has coloured mica windows. ==References== {{reflist}} *[http://www.galleries.com/minerals/silicate/micas.htm Mineral Galleries data] *[http://www.mindat.org/min-6728.html Mindat] ==See also== *[[Mica insulator]] {{Commonscat|Mica}} [[Category:Phyllosilicates]] [[Category:Insulators]] [[Category:dielectrics]] [[ca:Mica]] [[cs:Slída]] [[da:Glimmer]] [[de:Glimmergruppe]] [[et:Vilgud]] [[es:Mica]] [[eo:Glimo]] [[fr:Mica]] [[gl:Mica]] [[ko:운모]] [[it:Mica]] [[he:נציץ]] [[lt:Žėrutis]] [[mk:Микашист]] [[ml:അഭ്രം]] [[mr:अभ्रक]] [[nah:Mētzcuitlatl]] [[nl:Mica]] [[ja:雲母]] [[no:Glimmer]] [[nn:Glimmer]] [[pl:Miki]] [[pt:Mica]] [[ro:Mică]] [[ru:Слюды]] [[sh:Tinjci]] [[fi:Kiille]] [[sv:Glimmer]] [[uk:Слюда]] [[zh:云母]]