In the ideal case, the smallest structural unit contains three octahedra. If two such sites are occupied with cations and one site is vacant, then the octahedral sheet is considered “dioctahedral”. If all three sites are occupied, the sheet is considered “trioctahedral”. (Quot Guggenheim et al., 2006; see also references therein). A trioctahedral sheet generally contains predominantly divalent cations.
Cf., dioctahedral sheet