Adsorption isotherms have been determined for mixed Ca-Na montmorillonites, with various values of f, the proportion of (½ Ca) to Na (Fig. 1). These are coincident from f=0 to f=0·3, and then diverge. This is interpreted as meaning that “demixing” occurs at f=0·3, a system being formed in which some interlayer spaces are occupied mainly by Ca, others mainly by Na.

X-ray analysis confirms this view. Plots of the water uptake against f, for various relative humidities (Figs. 2–5) show that, for R. H.'s lower than 75%, the demixing is incomplete.

The observations can be explained by the occurrence of a limited number of doubly-charged sites (by Al-for-Si exchange spots being accidentally contiguous). For the montmorillonite used, the statistical probability of occurrence of such sites is found to be 0·2 if the layers have symmetry C-2/m and 0·33 for C-2, so that the observations are in favour of the latter symmetry.