Trioctahedral

Trioctahedral refers to the occupancy of the octahedral sheet in phyllosilicate minerals, where all three octahedral sites per formula unit are occupied by cations. In contrast, dioctahedral minerals have only two of the three octahedral sites filled, leaving one site vacant. In trioctahedral minerals the octahedral sheet is typically built from divalent cations such as Mg2+ and Fe2+, though trivalent cations like Al3+ can participate in mixed compositions.

Classic examples of trioctahedral phyllosilicates include the serpentine group (antigorite, chrysotile, lizardite) and talc, where the octahedral sheet is fully occupied. Some smectites, such as saponite, are also trioctahedral. The occupancy pattern influences crystal chemistry and physical properties, including layer charge distribution, swelling behavior, and cation exchange capacity.

Determining whether a mineral is trioctahedral or dioctahedral involves chemical analysis and crystallographic studies, as occupancy can vary with composition and formation conditions. The distinction aids in mineral classification and provides insights into the geological history and formation environments of rocks and clay minerals, such as serpentinization, hydrothermal processes, and diagenesis.

See also: 2:1 phyllosilicates, dioctahedral, trioctahedral, serpentine, talc, smectites.