dioctahedral

Dioctahedral refers to a classification of certain layered silicate minerals, particularly phyllosilicates, in which the octahedral sheet contains two cations occupying two of the three available octahedral sites per formula unit. The remaining octahedral site is vacant, giving the octahedral layer a two-in-three occupancy pattern. In dioctahedral minerals, the octahedral sites are typically occupied by aluminum(III), often with limited substitution by other cations, which influences the layer charge and chemistry of the mineral.

In the common 2:1 layered silicate structure, dioctahedral phyllosilicates consist of a tetrahedral silicon-oxygen sheet on each side of a central octahedral sheet that is only two-thirds filled. This occupancy pattern contrasts with trioctahedral minerals, where all three octahedral sites are occupied. The difference in octahedral occupancy, along with associated substitutions, affects crystal chemistry, swelling behavior, and physical properties such as cation exchange capacity.

Typical dioctahedral minerals include members of the kaolinite group, such as kaolinite, halloysite, dickite, and nacrite, which are widely studied for their weathering and clay-formation processes. Pyrophyllite is also commonly cited as a dioctahedral phyllosilicate. These minerals are important in sedimentary geology, soil science, and ceramics due to their low or moderate layer charge and characteristic X-ray diffraction patterns.

In contrast, trioctahedral minerals have all octahedral sites filled, as seen in many talc and some serpentinite group minerals. The distinction between dioctahedral and trioctahedral forms helps in identifying clays and understanding their formation conditions and industrial uses.