Editing Biotite (section)

===Structure===
Like other micas, biotite has a crystal structure 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''). It is the relatively weak ionic bonding between ''TOT'' layers that gives biotite its perfect basal cleavage.{{sfn|Nesse|2000|p=238}}

The tetrahedral sheets consist of silica tetrahedra, which are silicon ions surrounded by four oxygen ions. In biotite, one in four silicon ions is replaced by an aluminium ion. The tetrahedra each share three of their four oxygen ions with neighboring 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 in biotite is a trioctahedral sheet having the structure of a sheet of the mineral [[brucite]], with magnesium or ferrous iron being the usual cations. Apical oxygens take the place of some of the hydroxyl ions that would be present in a brucite 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 AlSi<sub>3</sub>O<sub>10</sub><sup>5-</sup>. The trioctahedral sheet has a positive charge, since its bulk composition is M<sub>3</sub>(OH)<sub>2</sub><sup>4+</sup> (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 M<sub>3</sub>(AlSi<sub>3</sub>O<sub>10</sub>)(OH)<sub>2</sub><sup>−</sup>. The remaining negative charge of the TOT layer is neutralized by the interlayer potassium 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 biotite crystals.

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