Zeolites are a group of hydrated aluminosilicates of the alkali or alkaline earth metals (principally sodium, potassium, magnesium and calcium).  Zeolites have three-dimensional crystalline frameworks of tetrahedral silica or alumina anions strongly bonded at all corners.  The zeolite structures contain (-Si-O-Al-) linkages that form surface pores of uniform diameter and enclose regular internal cavities and channels of discrete sizes and shapes, depending on the chemical composition and crystal structure of the specific zeolite involved.  Pore sizes range from about 2 to 4.3 angstroms.   The enclosed cavities contain both the metal cations and water molecules.  The cations are loosely bound to the lattice and thus can engage in ion exchange.  The water molecules can also be reversibly driven off in most zeolites.  Views of a typical zeolite structure are shown in the following diagram:

Atomic	Tetrahedral	Crystal
SOURCE: Natural and Synthetic Zeolites, Information Circular 9140, US Department of the Interior, Bureau of Mines, 1987, p.3.

Chabazite and clinoptilolite are the two out of the 48 minerals in the zeolite group which have commercial applications.  These deposits of high purity zeolite minerals occur in Cenozoic age tuffaceous sediments principally in the Western United States.  Chabazite and Clinoptilolite which formed over a long period of time are the end product of the chemical reaction between volcanic ash consisting of glass shards and alkaline water.  Because of their high silica to alumina ratios ranging from 2 to 1 for chabazite to 5 to 1 for clinoptilolite, these minerals are stable and less likely to dealuminate in acidic solutions than are synthetic zeolites.

Zeolites are crystalline hydrated aluminosilicates, of the alkali and alkaline earth metals.  Their crystalline framework is arranged in an interconnecting lattice structure. The arrangement of these elements in a zeolite crystal, creates a porous framework silicate structure with interconnecting channels that range in size from 2.5 to 4.3 angstroms, depending on the zeolite mineral. This structure allows zeolites to perform the following functions consistently within a broad range of chemical and physical environments:

• Gas adsorption: the ability to selectively adsorb specific gas molecules
• Water adsorption/desorption: the ability to reversibly adsorb/desorb water without any chemical or physical change in the zeolite matrix
• Ion exchange: the ability to exchange cations in the crystal structure for other cations such as lead, thallium, cesium or strontium based on the specific exchange selectivity of the zeolite mineral.

The adsorption functions identified above are accomplished when gas molecules of different sizes are allowed to pass through the channels, and depending upon the size of the channel are separated as to size, in a process known as molecular sieving.

Each zeolite mineral has a distinct ion exchange selectivity and capacity. This process occurs when water molecules can pass through the channels and pores allowing cations present in the solution to be exchanged for cations in the structure. Several factors must be considered in this process. These include solution strength, pH, temperature and the presence of other competing cations in the solution. These factors can affect both the ion exchange selectivity and capacity of the specific zeolite mineral.