Zeolites are useful because of their molecular structure: like quartz they are framework silicates, but the silica units are widely separated, with the spaces between occupied by water molecules and common cations, mostly sodum and calcium. They readily take up and release water, and they readily exchange sodium for calcium and vice versa, properties that made them useful as drying agents and water softeners (in which Na replaces Ca).
There are dozens of zeolite minerals; this gallery shows the most important and distinctive ones. "Zeolite" means "boiling stone" in scientific Greek, because all of these minerals readily melt and boil in a blowpipe flame.
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