A bit of clay physics

The important difference between sodium and calcium Bentonites is the relative proportions of the two cations in the inter-laminar region. In calcium Bentonite, the divalent calcium ion predominates. The ionic radius of Ca²⁺ is 99 pm (Aylward and Findlay, 1987), which is almost identical to that of Na⁺. Although the ions occupy the same volume in space the charge density of the Ca²⁺ ion is twice that of the Na⁺ ion. In physical terms this impacts greatly on physiochemical characteristics of the two minerals, since the negatively charged Bentonite lattice sheets are more tightly held together by the calcium ions.

(from Perry A~ Arledge, author of Living Clay, Nature's Own Miracle Cure)