Dictionary.net

Zeolites. A group of crystalline, hydrated alkali-aluminum silicates. They occur naturally in sedimentary and volcanic rocks, altered basalts, ores, and clay deposits. Some 40 known zeolite minerals and a great number of synthetic zeolites are available commercially. (From Merck Index, 11th ed)

[Gr.] (Geol.) Hydrated silicates of alumina; e.g. natrolite, mesotype, etc. , found in the cavities of volcanic rocks.