In general, zeolite structures are fine grained and are comprised of negatively charged, three dimensional (Si,Al)O₄ corner-sharing tetrahedral networks (“framework”) which form structural cavities and “extraframework sites”. The tetrahedra, with Al substituting Si, have a negative charge that is balanced by exchangeable cations in the extraframework sites. H₂O, which is polar, interacts with both the exchangeable cations and the framework, varying in number depending on relative humidity. Hydration and dehydration is generally continuous and reversible, and quite dynamic, although laumontite exhibits non-continuous behavior. Zeolites occur authigenicly, in low-temperature secondary alterations, such as in soils, as hydrothermal alteration products, in altered volcanics, in sediments, and many other environments. See also Part 1 of the Glossary. There are about 75 natural zeolites, and the more common species are given here:

chabazite (Ca_0.5,Na,K)₄(Al₄Si₈O₂₄) ^. 12(H₂O)

clinoptilolite (Na,K,Ca_0.5)₆(Al₆Si₃₀O₇₂) ^. 20(H₂O)

erionite K₂(Ca_0.5,Na)₇(Al₉Si₂₇O₇₂) ^. 28(H₂O)

laumontite Ca₄(Al₈Si₁₆O₄₈) ^. 16(H₂O)

mordenite Na₃Ca₂K(Al₈Si₄₀O₉₆) ^. 28(H₂O)

phillipsite K₂(Na,Ca_0.5)₃(Al₅Si₁₁O₃₂) ^. 12(H₂O)

stilbite NaCa₄(Al₉Si₂₇O₇₂) ^. 30(H₂O)