Alkali Cation Selectivity and Fixation by Clay Minerals

-Two variables must be considered when calculating exchange free energies (AG~ for 2:1 clays: (1) anionic field strength, as expressed by equivalent anionic radius (ra), and (2) interlayer water content, as expressed by interlayer molality. For smectites that are in a state of high hydration, interlayer molality is determined by the cations undergoing exchange. Thus AG~ for an exchanging cation pair can be calculated solely from measurements of r~. r~ is related to layer charge per half unit cell (C) and ab unit cell area(A)by:ra = (-A/8~rC)lJz. Thelayerchargenecessaryforcationfixationcanbepredictedbycalculating the ra at which cation exchange with an illite structure expresses a AG~ equal to that of exchange with a smectite structure. The theory can also be applied qualitatively to understand the high selectivity of illite for Cs +, the fixation of K + rather than Na § in shales during diagenesis, the stability of illite over muscovite in the weathering environment, and cation segregation in smectite. Key Words---Cation exchange, Cation fixation, Cation selectivity, Equivalent anionic radius, Free energy of exchange, Illite, Smectite.