In phyllosilicate (sheet silicate) crystal chemistry, the octahedral sheet can be occupied either by trivalent cations (Al³⁺, Fe³⁺) filling only two-thirds of available octahedral sites (dioctahedral, e.g., muscovite) or by divalent cations (Mg²⁺, Fe²⁺) filling all three octahedral sites (trioctahedral, e.g., biotite, chlorite), since full site occupancy with divalent cations balances the required charge with all sites filled.
Trioctahedral sheet silicates are defined by having divalent cations (such as Mg²⁺ or Fe²⁺) occupying the octahedral sites, and because these cations carry a lower...
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