Zeolite, a mineral composed chiefly of aluminum and silica combined with water. There are several kinds, most of which contain sodium or calcium. Zeolites that contain sodium are used to soften hard water (water containing such minerals as calcium and magnesium). When hard water comes in contact with sodium-containing zeolites, a chemical reaction occurs in which sodium ions are exchanged for calcium or magnesium ions. The supply of sodium in the zeolites is eventually used up and must be replenished by rinsing the zeolites with saltwater. The calcium and magnesium ions taken up by the zeolites are exchanged for sodium ions in the saltwater and are washed away. Zeolites are also used as catalysts and as drying agents, and those with water-softening properties are used in some detergents.

There are a number of synthetic compounds, called ion exchange resins, that have properties similar to those of natural zeolites.