Interlayer expansion that occurs in phyllosilicate minerals responding to the effects of temperature, pressure and composition (e.g., chemical potential) of an electrolyte surrounding the mineral with respect to the interlayer H2O (i.e., the “osmotic effect” or “osmotic pressure”). At equilibrium, the chemical potential of H2O in the electrolyte and in the interlayer is related to the salt concentration of the electrolyte (the osmotic pressure is zero). At a given temperature, pressure, or composition away from equilibrium, there will be a difference in chemical potential between the electrolyte and clay interlayer and the resulting osmotic pressure will be the driving force to affect the layer-to-layer distance. Under certain conditions, the resulting osmotic pressure drives the clay layers further apart than the interlayer distances common to intracrystalline swelling.
Cf., intracrystalline swelling