Transformation mechanisms and interstratification in conversion of smectite to kaolinite: An HRTEM study

The transformation of smectite into kaolinite and kaolinite-smectite inters tratification were studied in samples belonging to the Argiles Plastiques f ormation of the Paris basin, by high-resolution transmission electron micro scopy (HRTEM). Two original smectite phases, 1) beidellite with I-nm-thick layers, and 2) beidellite-montmorillonite with 1.25-nm-thick layers, are pr ogressively transformed into kaolinite-smectite mixed-layer minerals, and i nto kaolinite. As the percentage of kaolinite layers increases in the inter layered minerals, the kaolinite-smectite layer sequences, initially disorde red, become locally more ordered, with the presence of KS and KKS units rep eated 2 to 4 times (K = kaolinite layer, S = smectite layer). Two solid-sta te mechanisms seem to be responsible for the formation of kaolinite: 1) the transformation of 1 smectite layer into 1 kaolinite layer, denoted S-->K, by stripping of a tetrahedral sheet and the adjacent interlayer region; 2) the intercalation of 1 kaolinite layer into smectite, denoted 0 (zero)-->K. Structural and chemical incidences of these mechanisms are discussed.