A group of 2:1 layer silicates with a high cation exchange capacity, about 110 cmolc kg-1for soil smectites, and variable interlayer spacing. Formerly called the montmorillonite group. The group includes dioctahedral members montmorillonite, beidellite, and nontronite, and trioctahedral members saponite, hectorite, and sauconite.