Editing Muscovite (section)

== 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]].