with the octahedral sheet.

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.

Tetrahedral sheets have a strong negative charge, since their bulk composition is AlSi₃O₁₀^5-. The trioctahedral

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