A clay mineral that in its natural state expands to 15Å with water and to 17 Å with ethylene glycol was treated with various concentrations of KCl. At the highest KC1 concentration all the material was rendered nonexpanding. There was a threshold KC1 concentration at which potassium ions were absorbed in sufficient quantities to prevent expansion of the clay in water. At concentrations less than the threshold the material is characterized by a random interlayer mixture of expanding and nonexpanding layers. The sequence produced with increasing KC1 concentration is: expanding clay → interlayer mixture → nonexpanding clay.

Solvation with ethylene glycol is more effective than solvation with water for much higher potassium ion population densities in the interlayer space. At potassium ion population densities intermediate between those at the concentration thresholds for water solvation and for ethylene glycol solvation, there are layers that will expand with glycol and not with water.

Low potassium ion population densities correspond to low surface charge densities of potassium saturated clays, and the clays expand like montmorillonite. The high potassium ion population densities correspond to high surface charge densities of potassium-saturated clays and illite and the clays do not expand. With intermediate potassium ion population densities corresponding to intermediate surface charge densities for potassium-saturated clays, the clays expand with ethylene glycol but not with water.