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hydrated sodium aluminosilicate mineral, Na2Al2Si3O102H2O, in the zeolite family. It has been found in the form of colourless or white, glassy, slender crystals or fibrous masses filling cavities or fissures in basaltic rocks, as in Trentino, Italy; Brevik, Nor.; Belfast, N.Ire.; the Faroe Islands; and northeastern New Jersey, U.S. Natrolite was one of the first zeolite minerals for which cation-exchange properties (dissolved sodium, potassium, calcium, and magnesium readily replacing one another in the molecular structure) were discovered. It is used in softening water.