Chemistry of illite/smectite and end-member illite

Chemical data from three different series of diagenetic illite/smectites (I/S), analyzed statistically by two regresion techniques, indicate that the content of fixed-K per illite layer is not constant, but ranges from ∼0.55 per O₁₀(OH)₂ for illite layers in randomly interstratified I/S (R=0; > 50% smectite layers) to ∼1.0 per O₁₀(OH)₂ for illite layers formed in ordered I/S (R>0; <50% smectite layers). By extrapolation of the experimental data, the following chemical characteristics were obtained for end-member illite derived from the alteration of smectite in bentonite: average fixed-K per illite layer = 0.75 per O₁₀(OH)₂; total charge = about −0.8; cation-exchange capacity = 15 meq/100 g; surface area (EGME) = 150 m²/g.