Distinction between the non-expanding three-layer clay minerals, illites, and the expanding three-layer clay minerals montmorillonites is accomplished by glycolation treatment. Distinction among the montmorillonites can be accomplished if the interlayer cation, type and amount of adsorbed liquid, and particle size are kept constant. With these variables constant, the expansion of montmorillonites can be correlated with particle size and/or charge on the tetrahedral layer.

Sodium saturated montmorillonite samples taken from 0–100 per cent relative humidity generally exhibit three hydration levels. Coarse fractions and fractions that possess high charge densities per unit area, compared to fine fractions and fractions that possess low charge densities per unit area, expand the most at low humidities and expand the least at high humidities. This is explained by cation or surface hydration and the strength of the bond between the layers. Introduction of potassium into the interlayer positions of the montmorillonite lattice decreases the ability of the silicate layers to expand. Examination of the diffraction patterns reveals that physical combinations of particles with different hydrations, interlayer mixtures of hydrates, and interlayer mixtures within the physical combinations have been produced.