Clinoptilolite is a zeolite, which makes it a member of the tectosilicate group. Zeolites are framework silicates in which all silica tetrahedra are completely polymerized with other silica or with atoms proxying for silicon in tetrahedral sites. One of the noteworthy features of zeolites is the large pores or channels that are formed within the structures. These channels are filled with water and with exchangeable cations to balance negative charges in the zeolite structural framework. Because of this porous structure and the substitution of Al³⁺ for Si⁴⁺ in the tetrahedral framework, clinoptilolite and other zeolites have significant cation exchange capacities (CEC), on the order of 100 to 300 cmol of charge per kg. As with most zeolites, however, exchange selectivity is restricted by channel size. Larger ions may be selectively excluded. These “ion sieving” properties of zeolites are important properties exploited in a number of industrial and other uses.

Distribution

Clinoptilolite may occur as well-formed crystals in vugs and cavities of basalts, or in sedimentary rocks, particularly in saline-lake sediments of Tertiary Age. It and the other siliceous zeolites are common alteration products of SI-glass in marine or freshwater environments. Clinoptilolite is one of the most common zeolites found in the US and elsewhere. Due to their high exchange capacities and ion selectivities, zeolites are used as ion exchangers or ion sieves in a wide variety of applications. The most common use is as an ion exchanger in water softeners, where zeolites with high specific selectivity for Ca2+ and Mg2+ are used to exchange Na+ ions for them to produce “soft” water.

Koyama, K., and Y. Takeuchi. 1977. Z. Kristallogr. 145:216-239.