Final answer:
The basis for dividing sheet silicates into trioctahedral and dioctahedral groups concerns the occupancy of octahedral sites by cations within the mineral layer structure, which affects their properties.
Step-by-step explanation:
The division between trioctahedral and dioctahedral sheet silicates is based on the specific occupation of octahedral sites by cations within the silicate structure. Sheet silicates, or phyllosilicates, have a two-dimensional layered structure. In these layers, silicon-oxygen tetrahedra are bonded to form a sheet, and each tetrahedron shares three of its oxygen atoms with adjacent tetrahedra.
In dioctahedral sheet silicates, two out of three possible octahedral sites are occupied by cations (usually Al3+), which allows for the substitution of silicon in the tetrahedra by trivalent cations like aluminum. On the other hand, in trioctahedral sheet silicates, all three octahedral sites are typically filled by divalent cations (such as Mg2+ or Fe2+), allowing for the occupancy of every octahedral position within the layer.
This fundamental difference in cation occupancy affects the physical and chemical properties of the minerals and is used to categorize them into these two groups: dioctahedral and trioctahedral sheet silicates.