MINERALS/Zeolites 591 Zeolites W S Wise, University of California–Santa Barbara, Santa Barbara, CA, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction Zeolites are aluminosilicate minerals that occur as low-temperature (generally less than 200 C) alteration products of volcanic and feldspathic rocks They are well known in cavities of basalt, having crystallized as a result of diagenetic or hydrothermal alteration (Figure 1) Some zeolites completely replace rhyolitic tuff in saline alkaline lacustrine environments or through groundwater percolation Thick sequences of sediment from arc-source terranes contain several different kinds of zeolite, which formed through diagenetic alteration and very low-grade metamorphism These occurrences of zeolites have economic significance, because they either produce useful rock or affect the porosity of reservoir rocks A zeolite is a hydrated aluminosilicate mineral with a structure characterized by a framework of linked tetrahedra, each consisting of four oxygen atoms surrounding a silicon or aluminium cation This threedimensional network has open cavities in the forms of channels and cages, which are occupied by water molecules and non-framework cations The channels are large enough to allow the passage of guest ionic and molecular species This feature allows certain Figure Crystals of laumontite and calcite in a cavity in an altered basalt near Kailua, northeast Oahu, Hawaii This area is within the former conduit system of the 2.6 Ma Koolau volcano It is likely that the pervasive rock alteration and zeolite formation is a result of hydrothermal activity when the volcano was active The long dimension of the cavity is cm zeolites to exchange non-framework cations with the surrounding solution or to act as molecular sieves For example, clinoptilolite (Table 1) can selectively remove radiogenic strontium from nuclear wastewater, and chabazite can remove carbon dioxide from methane in the gas generated in landfill sites Each zeolite mineral or, in many cases, zeolite series has a different framework arrangement Table lists the currently recognized zeolite mineral species and series, their generalized formulae, and structure-type codes Many more zeolites have been synthesized in the laboratory The Structure Commission of the International Zeolite Association assigns a threeletter code to each different arrangement, including the synthetic ones At the time of writing there are 145 structure types, of which 42 occur as minerals For updates see the International Zeolite Association website http://www.iza-structure.org/ Structure of Zeolites All silicates have silicon (or other elements such as aluminium) located in the small space between four oxygen anions This site is called a tetrahedral site because the cation is strongly bonded to the four oxygen anions In framework silicates each oxygen anion is shared between two tetrahedral sites Therefore, the resulting framework has two oxygen anions for each silicon cation, giving the composition Si4ỵ O22 , as in the mineral quartz (see Minerals: Quartz) Most other framework silicate minerals have aluminium in some of the tetrahedral sites, which results in a charge imbalance For example, in some feldspars (see Minerals: Feldspars) the framework has the composition [AlSi3O8] , and the charge is balanced by the addition of alkali cations in sites near the oxygen anions bonded to the aluminium tetrahedral site The zeolite minerals all have framework arrangements with channels or cages (large pockets) In order to illustrate some of the variety among the zeolites, we will examine three different framework structures: natrolite (NAT), chabazite (CHA), and heulandite (HEU) Natrolite forms long prismatic crystals, in which the basic unit of the structure is the [Al2Si3O10]2 chain (Figure 2) Continuous repetition of this unit forms long chains parallel to the prism length Crosslinking of these chains forms channels parallel to the chains To balance the charge in the framework, caused by the inclusion of aluminium in two of the