Abstract: Equivalent conductivities of adsorbed cations were determined in clays saturated with Na⁺, Cs⁺, Ca⁺⁺ and with mixtures of these cations. Measurements were also made on Ca⁺⁺ clays which had been forced by previous drying into bundles of platelets or tactoids. The average mobility of adsorbed Ca⁺⁺ and Cs⁺ is much lower than that of adsorbed Na⁺.

It was concluded that the average mobility of adsorbed Ca⁺⁺ is low because most of this Ca⁺⁺ is on the internal surfaces of tactoids. Ca⁺⁺ adsorbed between these internal surfaces appears to have a mobility much lower than Ca⁺⁺ on the external surfaces which has a mobility of the same order of magnitude as Na⁺. Polarization of adsorbed Cs⁺ accounts at least partially for its low mobility in these clays.

Demixing of adsorbed cations (segregation with Na⁺ dominant between some platelets and Ca⁺⁺ between others) is suggested as an initial step leading to breakup of a Na⁺−Ca⁺⁺ clay mass into tactoids. The tactoid model, with Ca⁺⁺ and Na⁺ preferentially on the internal and external surfaces respectively, furnishes an explanation of the instability of clay and soil aggregates with 15% exchangeable sodium.