Zeolite

Article By:

Moore, Paul B. Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois.

Herron, Norman Central Research and Development, DuPont Research, Wilmington, Delaware.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.754600

Any mineral belonging to the zeolite family of minerals and synthetic compounds characterized by an aluminosilicate tetrahedral framework, ion-exchangeable large cations, and loosely held water molecules permitting reversible dehydration. The general formula can be expressed as X_y^1+,2+Al_x³⁺Si_1−x⁴⁺O₂ ·nH₂O. Since the oxygen atoms in the framework are each shared by two tetrahedrons, the (Si,Al):O ratio is exactly 1:2. The amount of large cations (X) present is conditioned by the aluminum/silicon (Al:Si) ratio and the formal charge of these large cations. Typical large cations are the alkalies and alkaline earths such as sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), strontium (Sr²⁺), and barium (Ba²⁺). The large cations, coordinated by framework oxygens and water molecules, reside in large cavities in the crystal structure; these cavities and channels may even permit the selective passage of organic molecules. Thus, zeolites are extensively studied from theoretical and technical standpoints because of their potential and actual use as molecular sieves, catalysts, and water softeners. See also: Molecular sieve; Silicate minerals; Water softening

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