measure of the ability of an insoluble material to undergo displacement of ions previously attached and loosely incorporated into its structure by oppositely charged ions present in the surrounding solution. zeolite minerals used in water softening, for example, have a large capacity to exchange sodium ions (Na⁺) for calcium ions (Ca²⁺) of hard water. High cation-exchange capacities are characteristic of clay minerals (clay mineral) and numerous other natural and synthetic substances possessing ion-exchanging properties.