Zeolites are a group of tectosilicates with just over 90 members whose structure is composed of SiO₄ and AlO₄ tetrahedra, with a (Si+Al)/O ratio equal to 1/2. What makes zeolites so industrially interesting is that, unlike most other tectosilicates, their structures have large vacancies, constituting very porous and permeable minerals, as the spaces are interconnected with each other, forming channels. These open spaces can house large cations such as Na, K, Ba, Ca, H₂O, NH₄, carbonate and nitrate ions. The combined group has enormous structural complexity, with over 120 structural variations.

There are 4 groups of “zeolites”:

Hydrothermal zeolites are well known and constitute large, very well-formed crystals, which generally occur crystallized at higher temperatures in cavities and fractures of volcanic rocks. These zeolites are mainly collected from the Deccan basalts in India and are available in the collector minerals trade, fetching high prices in some cases. Some of these zeolites are heulandite, stilbite, natrolite, mesolite, scholecite, mordenite, stellerite, eistylbite, chabazite, analcime, and laumontite. Its characteristics under the microscope must be searched in the respective files, available in this Database.

Sedimentary zeolites are cryptocrystalline, massive in appearance, usually whitish in color and with low density. They can only be identified by X-Ray Diffractometry. Under the microscope it is not possible to recognize any diagnostic features.

These sedimentary zeolites are the most common autigenic silicate minerals in sedimentary rocks, being found in rocks of the most diverse ages, lithologies and depositional environments. The recognition of this abundance only occurred after the generalization of X-Ray Diffraction techniques. They are valuable indicators of the depositional and post-depositional (diagenetic) environments of the host rocks. Geologically, zeolites have much in common with clay minerals; all occurrences of zeolites in sedimentary rocks are closely associated with clay minerals or are interspersed with clay-mineral-rich strata in the same stratigraphic sequences. The most common minerals in sedimentary occurrences are analcime, mordenite and clinoptilolite. They usually form during diagenesis due to alteration of volcanic material of acidic (rhyolitic) composition, such as volcanic glass and volcanic ash.

These cryptocrystalline zeolites constitute ores of increasing importance, currently with more than 40 industrial uses. Among them, we can mention the exchange of ions, their use as molecular sieves, use for odor removal (absorption of NH₄) and absorption of gases, use as water softeners (exchanges the Ca from water for Na) and additive in animal feed. . These industrial uses have only been recognized in the last few decades.

Synthetic zeolites are materials produced in the laboratory, with well-defined characteristics regarding the size and disposition of vacancies (open spaces – size and orientation of the channels). New variants are produced every year.

Minerals with a “zeolitic” structure are included in the term “industrial zeolites”:

                        phosphates like kehoeite, pahasapaite and tiptopite and

                        silicates like apophyllite, hsianghualite, lovdarite, viseita, partheita, prehnite, roggianite, gyrolite, maricopaite, okenite, tacharanite and tobermoreite.

Detailed information on the various ways in which natural zeolites occur can be found on the website of the Commission on Natural Zeolites: http://www.iza-online.org/

Details on occurrence, forms and many others can be found in Rudy Tschernich’s excellent book, “Zeolites of the World”. This book is available for free download from www.mindat.org.