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The effect of salt concentration and exchanger composition on Na/Ca exchange in montmorillonite and illite suspensions (0.02 to 0.08 g clay/g water) was studied, where the equivalent fraction of exchangeable Ca, ECa, ranged from 0.6 to 1. A theoretical evaluation based on double‐layer theory for montmorillonite indicates that the formation of tactoids and internal surfaces at the Ca3+ end of the exchange isotherm can cause the Vanselow selectivity coefficient, KV, to depend on exchangeable cation composition and total electrolyte concentration. Whereas for montmorillonite, KV increased with ECa (0.6 < ECa < 1), as predicted for internal surfaces, the reverse was true for illite, where external surfaces predominate. Likewise, for a given Na adsorption ratio, dilution of the equilibrium solution for illite (0.6 < ECa < 1) and for montmorillonite (ECa ≃ 0.6) decreased KV as predicted for external surfaces.The average Gapon selectivity coefficient for the montmorillonite (0.011) and illite clays (0.023) with ECa ≤ 0.8, suggests that the value for soils, (0.0148) reflects their mixed mineralogy.

The flocculation value, the minimum salt concentration which causes flocculation within 24 hours, was determined for mixtures of Na‐ and Ca‐montmorillonite (API‐25) and Fithian illite (API‐35). The equivalent fraction of exchangeable Na+, ENa, varied from 0 to 1. We prepared Na/Ca clay suspensions by adding the appropriate amounts of homoionic dry clays to a series of Na/Ca chloride solutions (0.2 to 80 molc m−8; 0.2 to 80 meq liter−1) with appropriate sodium adsorption ratio and shaking the mixture for 1 hour. The flocculation value of Ca‐montmorillonite (0.25 molo m−3) increased rapidly with small amounts of exchangeable Na+: the flocculation value for an ENa of 0, 0.2, 0.4, and 1 was 0.25, 6, 9, and 12 molc m−3, respectively. The flocculation values for Ca‐ and Na‐illite were 0.25 and 55 molo m−3, respectively, and increased linearly with ENa. Van der Waals attraction forces are mainly responsible for flocculation of Ca‐montmorillonite and illite systems. Edge‐to‐face attraction is dominant in flocculating Na‐montmorillonite. The large effect of small amounts of exchangeable Na+ in montmorillonite suspensions results from demixing of exchangeable ions whereby Na+ is the predominant cation on the external surfaces when ENa < 0.20.

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