A natural hydrated silicate of aluminum and either sodium or calcium or both, of the type Na2O•Al2O3•xSiO2•xH2O. Both natural and artificial zeolites are used extensively for water softening, as detergent builders, and cracking catalysts. For the former purpose the sodium or potassium compounds are required, since their usefulness depends on the cationic exchange of the sodium of the zeolite for the calcium or magnesium of hard water. When the zeolite has become saturated with calcium or magnesium ion it is flooded with strong salt solution, a reverse exchange of cations takes place and the material is regenerated. The natural zeolites are analcite, chabazite, heulandite, natrolite, stilbite, and thomosonite.Synthetic zeolites are made either by a gel process (sodium silicate and alumina) or a clay process (kaolin), which forms a matrix to which the zeolite is added. These processes are quite complex, involving substitution of various rare-earth oxides. The effectiveness of zeolites depends on their pore size, which may be as small as 4–5 Å. Other applications of zeolites are as adsorbents, desiccants, and in solar collectors, where they function as both heating and cooling agent. See Molecular Sieve; Aluminosilicate; ion-exchange resin; Cage Zeolite.
