The smectites (or montmorillonite–saponite group) are defined as layer-lattice minerals with a 2:1 (triphormic) structure that undergo interlayer swelling when exposed to water or water-vapour, or any one of several organic liquids; this swelling can be detected by alteration of the 001 spacing measured by X-ray diffraction.
The structural models for the elementary layers are derived from that of pyrophyllite for the dioctahedral family, or that of talc for the trioctahedral family, depending on the characteristic isomorphous substitution (MacEwan, 1961; Caillère and Hénin, 1963). The four principal minerals of the group are indicated in Table 6.1. To these must be added nontronite, a dioctahedral mineral in which most of the aluminium is replaced by Fe3+. The charge on the nontronite layers could be due to either tetrahedral or octahedral substitution, to give a beidellite or a montmorillonite type, respectively, but current opinion favours classification as the beidellite type, with tetrahedral substitution corresponding to the formula (Si8 -x Alx)iv(Fe4)viO20(OH)4.
The formulae given in Table 6.1 must be considered idealized because there may be other minor substitutions that do not alter the distribution of charge (e.g. Fe3+ for AI3+, or Fe2+ for Mg2+), and because tetrahedral and octahedral substitution can coexist in a mineral. The species is therefore generally classified according to the major substitutions; for instance, a montmorillonite in which more than half the exchange capacity is due to tetrahedral substitution is preferably classed as beidellite (Hofmann et al., 1955).