Editing Mica (section)

== Properties and structure ==

The mica group comprises 37 [[Silicate minerals#Phyllosilicates|phyllosilicate minerals]]. All crystallize in the [[monoclinic]] system, with a tendency towards pseudohexagonal [[crystal]]s, 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.{{sfn|Nesse|2000|pp=244–250}}

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 [[cation]]s (''c''). The ''TOT'' layers in turn consist of two tetrahedral sheets (''T'') strongly bonded to the two faces of a single octahedral sheet (''O''). The relatively weak ionic bonding between ''TOT'' layers gives mica its perfect basal cleavage.{{sfn|Nesse|2000|p=238}}

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 aluminium ions replace half the silicon ions in brittle micas. The tetrahedra share three of their four oxygen ions with neighbouring tetrahedra to produce a hexagonal sheet. The remaining oxygen ion (the ''apical'' oxygen ion) is available to bond with the octahedral sheet.{{sfn|Nesse|2000|p=235}}

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.{{sfn|Nesse|2000|pp=235–237}}

Tetrahedral sheets have a strong negative charge since their bulk composition is {{chem2|AlSi3O10(5-)}}. The octahedral sheet has a positive charge, since its bulk composition is {{chem2|Al(OH)(2+)}} (for a dioctahedral sheet with the apical sites vacant) or {{chem2|M3(OH)2(4+)}} (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 {{chem2|Al2(AlSi3O10)(OH)2−}} or {{chem2|M3(AlSi3O10)(OH)2−}}. The remaining negative charge of the TOT layer is neutralized by the interlayer cations (typically sodium, potassium, or calcium ions).{{sfn|Nesse|2000|p=238}}

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. The short-range order of K<sup>+</sup> ions on cleaved muscovite mica has been resolved.<ref>{{Cite journal |last1=Franceschi |first1=Giada |last2=Kocán |first2=Pavel |last3=Conti |first3=Andrea |last4=Brandstetter |first4=Sebastian |last5=Balajka |first5=Jan |last6=Sokolović |first6=Igor |last7=Valtiner |first7=Markus |last8=Mittendorfer |first8=Florian |last9=Schmid |first9=Michael |last10=Setvín |first10=Martin |last11=Diebold |first11=Ulrike |date=2023-01-13 |title=Resolving the intrinsic short-range ordering of K<sup>+</sup> ions on cleaved muscovite mica |journal=Nature Communications |language=en |volume=14 |issue=1 |pages=208 |doi=10.1038/s41467-023-35872-y |pmid=36639388 |issn=2041-1723|pmc=9839703 |arxiv=2308.14055 |bibcode=2023NatCo..14..208F }}</ref>

<gallery>
File:Mica T.png|View of tetrahedral sheet structure of mica. The apical oxygen ions are tinted pink.
File:Mica tO.png|View of trioctahedral sheet structure of mica. The binding sites for apical oxygen are shown as white spheres.
File:Mica tOs.png|View of trioctahedral sheet structure of mica emphasizing octahedral sites
File:Mica dO.png|View of dioctahedral sheet structure of mica. The binding sites for apical oxygen are shown as white spheres.
File:Mica dOs.png|View of dioctahedral sheet structure of mica emphasizing octahedral sites
File:Mica tri.png|View of trioctahedral mica structure looking at surface of a single layer
File:Mica tri side.png|View of trioctahedral mica structure looking along sheets
</gallery>