What are zeolites?
General

Zeolites are among the most common products of chemical interaction between groundwaters and the Earth’s crust during diagenesis and low-grade metamorphism. The unique crystal structures of zeolites result in large molar volume, high cation-exchange capacities, and reversible dehydration (Neuhoff et al. 2000).
The zeolite mineralogy of low-grade meta-basalts is a sensitive indicator for the thermobarometric evolution of oceanic crust and continental flood basalts. Discrete temperature-, pressure-, and depth controlled zones characterised by assemblages with silica minerals, phyllosilicates and zeolites frequently serve as metamorphic indicator in low-grade meta-basalts. Natural zeolites commonly form at low temperatures by reactions of aqueous solutions with volcanic rocks, and volcanogenic sediments. Consequently, ion-exchange capabilities of zeolites can affect ground water composition and quality in volcanic aquifers (e.g. White et al 1980).
History and definition of zeolites
The Swedish mineralogist CRONSTEDT introduced the term “zeolite” in 1756 for certain silicate minerals in allusion to their behaviour on heating in a borax bead (Greek: zeo = to boil; lithos = stone). Hey (1930) concluded that zeolites in general have aluminosilicate frameworks with loosely bonded alkali or alkali-earth cations, or both. He pointed out the consequential requirements that the molar ratio Al2O3 : (Ca, Sr, Ba, Na2, K2)O = 1 and that O : (Si + Al) = 2 in the empirical formula.
Another characteristic features of zeolites are the potential for reversible low-temperature dehydration, the ability of dehydration and reversibly absorption of other molecules.

Definition of zeolites after the International Mineralogical Association, Commission on New Minerals and Mineral Names (Coombs et al. 1998):
“A zeolite mineral is a crystalline substance with a structure characterized by a framework of linked tetrahedra, each consisting of four O atoms surrounding a cation. This framework contains open channels and cages. These are usually occupied by H2O molecules and extra-framework cations that are commonly exchangeable. The channels are large enough to allow the passage of guest species. In the hydrated phases, dehydration occurs at temperatures mostly below about 400°C and is largely reversible. The framework may be interrupted by (OH, F) groups; these occupy a tetrahedron apex that is not shared with adjacent tetrahedral” (Coombs et al.  1998).
Structure
The basic feature of all zeolite structures is an aluminosilicate framework (tectosilicate) composed of (Si, Al)O4 tetrahedra, each oxygen of which is shared between two tetrahedron (ARMBRUSTER & GUNTER 2001). The net negative charge on the tectosilicate framework is balanced by the incorporation of cations (interchannel cations) in ~2 to 10 Å cages or channels. In most cases Ca2+, Na+ or K+ and less frequently Li+, Mg2+, Sr2+ and Ba2+ are situated in cavities within the framework structures. This feature can also be observed in feldspar and feldspathoid minerals. But in contrast to this feldspar and feldspathoid minerals the zeolite aluminosilicate framework contain open cavities and open channels  (i.e. they have lower densities) through which ions can be either extracted or introduced ions (Armbruster & Gunter  2001).
Their compositions are represented by the structural formula (1):

    (A+z)y/z(B+3)y(Si)xO2(x+y)  · nH2O        (1)
Where A represent interchannel cations (such as Na+, K+, Ca2+, Ba2+, Sr2+, Mg2+ and Fe2+), B are tetrahedral coordinated trivalent cations in the zeolite framework (Al3+ and Fe3+), z is the charge on the interchannel cations, n is the number of moles of interchannel molecular water, and x and y are the stoichiometric coefficients for trivalent cations and Si4+ in tetrahedral sites, respectively. The quantities y/z and 2(x+y) represent the stoichiometries of the interchannel cations and framework oxygens, respectively, necessary for maintaining charge balance in the tectosilicate lattices of zeolites (Armbruster & Gunter  2001).
An additional feature, which differentiated the zeolites still further from the feldspar and feldspathoid minerals, is the presence of water molecules within the structural channels. These are relatively loosely bound to the framework and cations, and like the cations they can be removed and replaced without disrupting framework bonds (Deer et al. 1992).
Currently, three classification schemes are used widely for zeolite structures. Two of these are based upon specifically defined aspects of crystal structure, whereas the third has a more historical basis, placing zeolites with similar properties (e.g. morphology) into the same group (Armbruster & Gunter  2001).
The first structural classification of zeolites is based on the framework topology, with distinct framework receiving a three-letter code (Meier et al. 1996).
The second structural method for the classification of zeolites is based on a concept termed “secondary building units” (SBU). The primary building unit for zeolites is the tetrahedron and the SBUs are the geometric arrangements of tetrahedra (Breck 1974, Armbruster & Gunter  2001). Quite often, these SBUs tend to control the morphology of the zeolites.
The third broad classification scheme is similar to the SBU classification of Breck (1974), except that it includes some historical context of how the zeolites were discovered and named. This scheme uses a combination of zeolite group names which have specific SBUs and is the widely used by geologists. This is the classification scheme used by Gottardi & Galli (1985) .

Fig. 1 The structural units, finite or infinite, which may be used to assemble the frameworks of zeolites. a: The chain of fibrous zeolites; b: The singly connected 4-ring chain; c: The doubly connected 4-ring chain; d: The 6-ring (single); e: The 6-ring (double); f: The 4-4-1-1 heulandite unit. In each drawing, the balls represent tetrahedra (SiO44- or AlO45-) and the bars represent oxygens shared by the tetrahedra (after Breck 1974 and Gottardi & Galli 1985).
Three types of solid solution in zeolites are consistent with the stoichiometry of formula (1). These solutions are not strictly coupled and can occur independently from other substitutions as long as charge balance is maintained.
The first of these is the solid solution within the tetrahedral sites. Tetrahedral substitution of Si4+ and Al3+ observed in zeolites is highly variable, whereas the substitution Fe3+ for Si4+ or Al3+ is limited (Neuhoff et al. 2000).
Secondly, solid solutions among interchannel cations are often quite extensive, as evidenced by the large ion-exchange capacities of some zeolites (e.g. Colella 1996). Total interchannel ion charge is necessarily a function of Al3+ and Fe3+ content because the interchannel ions compensate for charge imbalances resulting from the presence of trivalent ions in tetrahedral sites. Zeolites with high Si/Al ratios commonly are richer in monovalent interchannel cations than are more aluminous samples of the same species. Twice as many monovalent ions as divalent ions are necessary to compensate for charge imbalances caused by Al3+ in the framework, and the additional monovalent ions often occlude H2O molecules present in isostructural zeolites with divalent interchannel cations (e.g. natrolite and scolecite, Ca-heulandite and Na-heulandite) (Neuhoff et al. 2000).
The third type of solid solution in zeolites is a consequence of the loose bounding nature of molecular water in zeolites, whereas the total water content is a sensitive function of temperature, total pressure and the partial hydrostatic pressure (Neuhoff et al. 2000).  
Application
Zeolites have many useful purposes. They can perform ion exchange, filtering, odor removal, chemical sieve and gas absorption tasks. The most well known use for zeolites is in water softeners. Calcium in water can cause it to be "hard" and capable of forming scum and other problems.  Zeolites charged with the much less damaging sodium ions can allow the hard water to pass through its structure and exchange the calcium for the sodium ions. This process is reversable.  In a similar way zeolites can absorb ions and molecules and thus act as a filter for odor control, toxin removal and as a chemical sieve.  Zeolites can have the water in their structures driven off by heat with the basic structure left intact.  Then other solutions can be pushed through the structure. The zeolites can then act as a delivery system for the new fluid. This process has applications in medicine, livestock feeds and other types of research. Zeolites added to livestock feed have been shown to absorb toxins that are damaging and even fatal to the growth of the animals, while the basic structure of the zeolite is biologically neutral. Aquarium hobbyists are seeing more zeolite products in pet stores as zeolites make excellent removers of ammonia and other toxins. Most municipal water supplies are processed through zeolites before public consumption. These uses of zeolites are extremely important for industry, although synthetic zeolites are now doing the bulk of the work.