Pseudoternary systems of negatively charged clay (montmorillonite), anionic polyelectrolyte (sodium polyacrylate), counterions, and water exhibit a local segregation of two distinct phases on a colloidal scale. The microscopic osmotic equilibrium condition in the diphasic dispersion sets the spacing observed between the sodium montmorillonite platelets : the effect of polyelectrolyte is equivalence to an external depletion force equilibrating electrostatic repulsion. In the case of calcium montmorillonite clay, the osmotic stress effect is only observed when sufficient polyelectrolyte is added to saturate the ion condensation of all calcium clay counterions on the polyelectrolyte chain.