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a mineral of the zeolite family with chemical composition Na2[Al2Si3O10].2H2O; an aluminosilicate, it is classed according to crystallochemical structure as a tectosilicate. Natrolite crystallizes in the orthorhombic and tetragonal systems to form elongated prismatic or acicular crystals; more rarely, it occurs as compact cryptocrystalline masses. It is usually colorless; sometimes it is grayish or yellowish. Natrolite’s hardness on Mohs’ scale measures 5–5.5, and its density is 2,200–2,500 kg/m3.
Natrolite absorbs H2O, NH3, CO2, H2S, and other substances. It is also known for its ability to exchange base ions in solution for base ions in the structure, a property that has found wide application in industry, primarily in the manufacture of artificial zeolites. Natrolite forms in association with other zeolites and with calcite, quartz, and chlorites in the cavities and fissures of igneous rocks by precipitation from hot aqueous solutions. It is also formed as a result of the hydrothermal changes of nepheline and may be found in the weathering zone of nepheline syenites, as well as in contemporary marine deposits.
M. D. DORFMAN