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Zeolites are naturally occurring aluminosilicate minerals. They have 3- dimensional framework structures of interconnected channels and cages that contain exchangeable alkali and alkaline earth cations and adsorbed water. Like phyllosilicates, zeolites have the ability to exchange cations and undergo reversible hydration-dehydration. However, unlike phyllosilicates, cation exchange and hydration-dehydration can occur In zeolites without significant structural change. Zeolites were discovered in 1756, when the Swedish mineralogist, Cronstedt, tested a mineral that appeared to froth when heated with the mineralogist's blowpipe. Cronstedt named this mineral 'zeolite' using the Greek words zein and lithas, literally meaning 'boiling stones' . Since their discovery, more than 60 species have been found to occur naturally in soils, sediments, and rocks. The most common, concentrated, and extensive zeolite deposits occur in sedimentary rocks, particularly of volcanic origin. Zeolites have unique cation exchange, ion selectivity, adsorption, and molecular sieving properties imparted by their open framework structure and composition. These properties make zeolites useful in a variety of environmental and agricultural applications, thus introducing zeolites to soils and sediments where they may not naturally occur. Natural zeolites have been used for sorption and removal of organic molecules, radionuclides, heavy metals, and ammonium from waters, soils, and sediments and as slow-release fertilizers, plant growth media, and animal feed supplements. In addition, hundreds, if not thousands, of synthetic zeolite species have been produced in the laboratory for specific catalytic or ion-sieving industrial applications. The goal of this chapter is to introduce readers to the environmentally important group of zeolite minerals. The effective use of zeolites in environmental and agricultural applications depends on thorough understanding of the properties and occurrences of natural zeolites in soil environments. This chapter focuses on natural zeolites, concentrating on the most common zeolites that occur in soils or are most likely to be applied to soils by humans, and only briefly considers synthetic zeolites at the end of the chapter.