Zeolites

Zeolites are a group of similar-looking minerals that form at shallow depths and relatively low temperatures, less than 200°C. The best-known specimens form in cavities (vesicles or vugs) in basalt—that is, they commonly form amygdules—but they are commercially important in altered tuff beds and saline lake deposits.

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.