Abstract: There is disclosed a method for improving the rheological properties of a clay mineral which, when dispersed with water, either releases into solution or retains on its surface multivalent cations, which method includes the step of treating the clay mineral in aqueous suspension with an aluminosiliceous material having a cation exchange capacity of at least 50 meq/100 g, said aluminosiliceous material being employed in an amount sufficient to react with a significant proportion of the exchangeable cations in the clay mineral. Also disclosed is an aqueous suspension of a clay mineral, a method for preparing a paper coating composition and a method for improving the rheological properties of a clay mineral.