Smectite Group

Monoclinic clay-like minerals. Smectite is a group name for platy phyllosilicates of 2:1 layer and a layer charge of ~ -0.2 to -0.6 per formula unit. Generally for natural samples, the d(001) spacing is approximately 14.4-15.6 Å, although other spacing may occur depending on H2O retention and interlayer occupancy. The group is further divided into subgroups that are either trioctahedral (according to Bailey, 1980, this subgroup name is "saponite") or dioctahedral (subgroup name of "montmorillonite", according to Bailey, 1980), and these subgroups are further divided into mineral species. Prior to circa 1975, the smectite group was called the montmorillonite-saponite group. Smectite minerals have large specific surface areas (10 - 700 m2/g) and exhibit a high expansion (swelling) capability in the presence of H2O. Smectite and vermiculite minerals are often referred to as "swelling" or "expandable" clay minerals. Smectite is commonly a primary constituent of bentonite (see bentonite for respective genesis information) and pelitic sediments (e.g., shales) and occurs in soils. Positive identification of minerals in the smectite group may need data from dehydration experiments (thermogravimetric analysis, TGA, or differential thermo-analysis, DTA), and X-ray powder patterns before and after treatment by heating and with organic liquids. The most common members are montmorillonite, nontronite and saponite. A possible V-dominant member is coded as 'UM1979-21-SiO:AlHNaV'.