View source for Muscovite

{{Short description|Phyllosilicate mineral in the dioctahedral mica group}}
{{Other uses}}
{{Use American English|date=December 2021}}<!-- Appears to be current dialect in use ("color" not "colour", etc.) -->
{{Infobox mineral
| name        = Muscovite
| category    = [[Phyllosilicate]] [[minerals]], dioctahedral [[mica]] [[Mineral group|group]]
| image       = Muscovite-Albite-122887.jpg
| imagesize   = 
| caption     = Muscovite with [[albite]] from [[Doce River|Doce]] valley, [[Minas Gerais]], Brazil (dimensions: 6 × 5.3 × 3.9 cm)
| formula     = KAl<sub>2</sub>(AlSi<sub>3</sub>O<sub>10</sub>)(F,OH)<sub>2</sub>
| IMAsymbol   = Ms<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref>
| molweight   = 
| strunz      = 9.EC.15 
| dana        = 71.02.02a.01
| system      = [[Monoclinic]]
| class       = Prismatic (2/m) <br/><small>(same [[H-M symbol]])</small>
| symmetry    = ''C2/c'' 
| unit cell   = a = 5.199&nbsp;[[Ångstrom|Å]], b = 9.027&nbsp;Å, <br/>c = 20.106&nbsp;Å, β = 95.78°; Z&nbsp;=&nbsp;4
| color       = White, grey, silvery
| habit       = Massive to platy
| twinning    = Common on the [310], less common on the {001}
| cleavage    = Perfect on the {001}
| fracture    = Micaceous
| tenacity    = Elastic
| mohs        = 2–2.5 parallel to {001} <br/>4 right angle to {001}
| luster      = Vitreous, silky, pearly
| polish      = 
| refractive  = n<sub>α</sub> = 1.552–1.576 <br/>n<sub>β</sub> = 1.582–1.615 <br/>n<sub>γ</sub> = 1.587–1.618
| opticalprop = Biaxial (−)
| birefringence = δ = 0.035 – 0.042
| dispersion  = r > v weak
| pleochroism = Weak when colored
| fluorescence= None
| absorption  = 
| streak      = White
| gravity     = 2.76–3
| density     = 
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| diaphaneity = Transparent to translucent
| other       = 
| references  = <ref>[https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Muscovite Mineralienatlas]</ref><ref>[http://mindat.org/min-2815.html Muscovite mineral information and data Mindat]</ref><ref>[http://webmineral.com/data/Muscovite.shtml Muscovite Mineral Data Webmineral]</ref><ref>[http://rruff.geo.arizona.edu/doclib/hom/muscovite.pdf Handbook of Mineralogy]</ref>
}}

'''Muscovite''' (also known as '''common mica''', '''isinglass''', or '''potash mica'''<ref>[http://www.britannica.com/eb/article-9054407/muscovite Encyclopædia Britannica]</ref>) is a hydrated [[Silicate minerals#Phyllosilicates|phyllosilicate]] mineral of [[aluminium]] and [[potassium]] with formula KAl<sub>2</sub>(Al[[silicon|Si]]<sub>3</sub>[[oxygen|O]]<sub>10</sub>)([[fluorine|F]],O[[hydrogen|H]])<sub>2</sub>, or ([[potassium fluoride|KF]])<sub>2</sub>([[aluminate|Al<sub>2</sub>O<sub>3</sub>]])<sub>3</sub>([[silicate|SiO<sub>2</sub>]])<sub>6</sub>([[hydrate|H<sub>2</sub>O]]). It has a highly perfect [[basal cleavage]] yielding remarkably thin laminae (sheets) which are often highly [[Elasticity (physics)|elastic]]. Sheets of muscovite {{convert|5|x|3|m|ft|sp=us}} have been found in [[Nellore]], India.<ref>{{cite journal| url = http://www.minsocam.org/ammin/AM66/AM66_885.pdf| journal = American Mineralogist| volume = 66| pages = 885–907| year= 1981| title= The largest crystals| author = P. C. Rickwood}}</ref>

Muscovite has a [[Mohs hardness]] of 2–2.25 parallel to the [[Miller index|[001] face]], 4 perpendicular to the [001] and a [[specific gravity]] of 2.76–3. It can be colorless or tinted through grays, violet or red, and can be transparent or translucent. It is [[anisotropic]] and has high [[birefringence]]. Its crystal system is [[monoclinic]]. The green, [[chromium]]-rich variety is called [[fuchsite]]; [[mariposite]] is also a chromium-rich type of muscovite.

Muscovite is the most common [[mica]], found in [[granite]]s, [[pegmatite]]s, [[gneiss]]es, and [[schist]]s, and as a contact [[metamorphic rock]] or as a secondary [[mineral]] resulting from the alteration of [[topaz]], [[feldspar]], [[kyanite]], etc. It is characteristic of [[peraluminous rock]], in which the content of aluminum is relatively high.<ref>Blatt, Harvey and Robert J. Tracy, ''Petrology,'' Freeman, 2nd ed., 1995, p. 516 {{ISBN|0-7167-2438-3}}</ref> In pegmatites, it is often found in immense sheets that are commercially valuable. Muscovite is in demand for the manufacture of [[fireproofing]] and insulating materials and to some extent as a [[lubricant]].

== Naming ==
The name muscovite comes from ''Muscovy-glass'', a name given to the mineral in [[Elizabethan England]] due to its use in medieval Russia ([[Grand Duchy of Moscow|Muscovy]]) as a cheaper alternative to [[glass]] in windows. This usage became widely known in England during the sixteenth century with its first mention appearing in letters by [[George Turberville]], the secretary of England's ambassador to the [[Russian tsardom|Russian]] [[tsar]] [[Ivan the Terrible]], in 1568.

[[File:Muscovite window.jpg|thumb|upright|Muscovite window]]

== Distinguishing characteristics ==
Micas are distinguished from other minerals by their pseudohexagonal crystal shape and their perfect cleavage, which allows the crystals to be pulled apart into very thin elastic sheets. [[Pyrophyllite]], and [[talc]] are softer than micas and have a greasy feel, while [[chlorite group|chlorite]] is green in color and its cleavage sheets are inelastic. The other common mica mineral, [[biotite]], is almost always much darker in color than muscovite. [[Paragonite]] can be difficult to distinguish from muscovite but is much less common, though it is likely mistaken for muscovite often enough that it may be more common that is generally appreciated.<ref name=Nesse2000/>  Muscovite mica from Brazil is red due to manganese(3+).<ref>{{Cite web |title=Minerals Colored by Metal Ions |url=http://minerals.gps.caltech.edu/color_causes/Metal_Ion/index.html |access-date=2023-03-01 |website=minerals.gps.caltech.edu}}</ref>

== Composition and structure ==
Like all [[mica]] minerals, muscovite is a [[phyllosilicate]] (sheet silicate) mineral with a ''TOT-c'' structure. In other words, a crystal of muscovite consists of layers (''TOT'') bonded to each other by [[potassium]] [[cations]] (''c'').<ref name=Nesse2000>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=9780195106916 |pages=235–238}}</ref>

Each layer is composed of three sheets. The outer sheets ('T' or tetrahedral sheets) consist of [[silicon–oxygen tetrahedron|silicon-oxygen tetrahedra]] and [[aluminium]]-oxygen tetrahedra, with three of the oxygen anions of each tetrahedron shared with neighboring tetrahedra to form a hexagonal sheet. The fourth oxygen anion in each tetrahedral sheet is called an ''apical'' oxygen anion.<ref name=Nesse2000/> There are three silicon cations for each aluminium cation but the arrangement of aluminium and silicon cations is largely disordered.<ref>{{cite journal |last1=Guggenheim |first1=Stephen |last2=Chang |first2=Yu-Hwa |last3=Koster van Groos |first3=August F. |title=Muscovite dehydroxylation; high-temperature studies |journal=American Mineralogist |date=1 June 1987 |volume=72 |issue=5–6 |pages=537–550 |url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/72/5-6/537/42040/Muscovite-dehydroxylation-high-temperature-studies |access-date=15 December 2021}}</ref>

The middle octahedral (''O'') sheet consists of aluminium cations that are each surrounded by six oxygen or [[hydroxide]] anions forming an octahedron, with the octahedrons sharing anions to form a hexagonal sheet similar to the tetrahedral sheets. The apical oxygen anions of the outer ''T'' sheets face inwards and are shared by the octahedral sheet, binding the sheets firmly together. The relatively strong binding between oxygen anions and aluminium and silicon cations within a layer, compared with the weaker binding of potassium cations between layers, gives muscovite its perfect basal cleavage.<ref name=Nesse2000/>

In muscovite, alternate layers are slightly offset from each other, so that the structure repeats every two layers. This is called the 1''M'' polytype of the general mica structure.<ref name=Nesse2000/>

<gallery>
File:Mica T.png|View of tetrahedral sheet of muscovite. The apical oxygen ions are tinted pink.
File:Mica dO.png|View of octahedral sheet of muscovite. The binding sites for apical oxygen are shown as white spheres.
File:Mica dOs.png|View of octahedral sheet of muscovite with ion sizes altered to emphasizing octahedral sites
File:Muscovite structure 100.jpg|Structure of muscovite, viewed in the [100] direction (along the layers of the crystal)
</gallery>

The formula for muscovite is typically given as {{chem2|KAl2(AlSi3O10)(OH)2}}, but it is common for small amounts of other elements to substitute for the main constituents. [[Alkali metal]]s such as [[sodium]], [[rubidium]], and [[caesium]] substitute for potassium; [[magnesium]], [[iron]], [[lithium]], [[chromium]], [[titanium]], or [[vanadium]] can substitute for aluminium in the octahedral sheet; [[fluorine]] or [[chlorine]] can substitute for hydroxide; and the ratio of aluminium to silicon in the tetrahedral sheets can change to maintain charge balance where necessary (as when magnesium cations, with a charge of +2, substitute for aluminium ions, with a charge of +3).{{sfn|Nesse|2000|p=244}}

Up to 10% of the potassium may be replaced by sodium, and up to 20% of the hydroxide by fluorine. Chlorine rarely replaces more than 1% of the hydroxide. Muscovite in which the mole fraction of silicon is greater than aluminium, and magnesium or iron replaces some of the aluminium to maintain charge balance, is called [[phengite]].{{sfn|Nesse|2000|p=244}}

Chromium-rich and vanadium-rich muscovite are known respectively as [[fuchsite]] and [[roscoelite]].

==Uses==
{{main|Mica#Uses}}
Muscovite can be cleaved into very thin transparent sheets that can substitute for glass, particularly for high-temperature applications such as industrial furnace or oven windows. It is also used in the manufacture of a wide variety of electronics and as a [[Filler (materials)|filler]] in paints, plastic, and [[wallboard]]. It lends a silky luster to [[wallpaper]]. It is also used in [[tire]] manufacture as a [[mold release agent]], in [[drilling mud]], and in various [[cosmetics]] for its luster.{{sfn|Nesse|2000|p=246}}

==Gallery==
<gallery mode="packed">
File:Beryl-Muscovite-171688.jpg|Muscovite with [[beryl]] (var. morganite) from Paprok, Afghanistan
File:Alurgite St Marcel.jpg|Muscovite (var. alurgite), from Prabornaz Mine, Aosta Valley, Italy
File:Muscovite crystals.JPG|Tabular muscovite crystals in a gneiss in [[thin section]] viewed under [[cross-polarized light]] at 2x magnification.
File:Muskovit (Astrolith).jpg|Astrolite polytype of Muscovite
</gallery>
{{stereo image
 |image   = Fuschite3d.jpg
 |caption = Small specimen of Muscovite (fuchsite) from Brazil.
 |width   = 500
 |height  = 140
}}
{{clear right}}

==References==
{{Reflist}}

== External links ==
* {{Commons category-inline|Muscovite}}

{{Phyllosilicates}}
{{Authority control}}

[[Category:Potassium minerals]]
[[Category:Radioactive minerals]]
[[Category:Gemstones]]
[[Category:Aluminium minerals]]
[[Category:Monoclinic minerals]]
[[Category:Minerals in space group 15]]
[[Category:Potash]]
[[Category:Medieval Russian architecture]]
[[Category:Windows]]
[[Category:Dioctahedral mica group]]