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Claus Hedegaard

Zeolites

Zeolites are a group of interesting minerals, receiving considerable attention from collectors and industry alike. They are so-called hydrated framework silicates, capable of exchanging cations with the environment, and giving off water when heated. Let us take that once more in smaller pieces.
The term 'zeolite' was coined by the Swedish nobleman and mineralogist Axel Fredrick Cronstedt (1722-1756), who would have written it 'zeolith' corresponding to the etymology. The word combines the Greek 'zeon' meaning to boil and 'lithos' meaning stone. The minerals expel water when heated and seem to boil and puff up when heated in molten borax during chemical analysis - they are 'boiling stones.' The name caught on -- you might even say too well! -- and the term 'zeolite' was sometimes used for a wide range of minerals having nothing to do with zeolites of modern usage. At times even Tourmaline and Lazurite (Lapis Lazuli) were considered zeolites, though lacking the properties described by Cronstedt.
It is quite common for minerals to contain water, to be hydrated. This water can be expelled by heating, but the loss of water is irreversible or at least associated with a structural transformation. That is, most hydrated minerals loose water and change structure when heated. Zeolites are different. They can actually loose all the water without changing structure, and even resorb the same amount of water again. Note, they are not just 'porous rocks acting like sponges.' A fixed number of water molecules will attach to each zeolite molecule, and they fit in certain specific positions in the crystal lattice.
The key to the reversible water loss lies in the 'framework silicate' structure. Zeolites have a mesh of silicon, aluminum, and oxygen, linked in a rigid three- dimensional lattice. This lattice has comparatively large voids where water molecules may attach, as may cations. 'Cations' are positive ions, generally metals, and these metals characterise the individual zeolite species. Just like the water, these metal ions can move in and out of the lattice without altering the basic structure of the zeolite. There is one important difference between the water molecules and the cations, though. The cations have a positive electric charge, and charge has to be preserved; if a given positive charge is removed from the lattice, a similar positive charge has to be put into the lattice.
This is precisely what makes zeolites industrially important, they are able to exchange cations with the surroundings, that is, work as ion changers. Particularly in 'water softening' we wish to get rid of divalent cations of a.o. calcium and magnesium, that form grey precipitates with soap, but we will accept monovalent cations of f.ex. sodium, potassium, or hydrogen, that do not form precipitates. By treating water with zeolites, we can exchange the calcium and magnesium ions in the water for f.ex. sodium, the water gets 'softer' and will not form precipitates with soap. Over 100 different zeolites are manufactured synthetically for different uses. Some are very specific, and preferably exchange certain metals, leaving others, others are quite general and exchange just about anything for hydrogen. Specifically designed zeolites are very important for separating some metals, like rare earth elements, and for extracting traces of metals from very dilute solutions.
The ability to exchange ions has really messed up zeolites, or at least the mineralogical description of them, until recently. That ability allows you to get practically any combination of cations in zeolites, that are morphologically and crystallographically identical, and even similar material from the same deposit will often show a range of compositions. Consequently, quite a number of species have been proposed over the years, and they are often quite difficult to tell apart. A few years ago, the International Mineralogical Association (IMA) formed a sub-committee to characterise and redefine zeolites properly. They did, and all was good. Sort of ... Admittedly, their conclusion is far more manageable than the proposed amphibole nomenclature earlier, but a couple of old friends were lost in battle, some real odd-balls (like structurally similar phosphates!) were included, and your ability to make a quick- and-dirty visual identification was hampered by a sudden need for detailed chemical analyses.
Anyway, quoting from Mandarino (1999) or Coombs et al. (1997), the following individual species of silicate zeolites are now approved: Amicite, Ammonioleucite, Bikitaite, Analcime, Barrerite, Bellbergite, Boggsite, Chiavennite, Cowlesite, Edingtonite, Epistilbite, Garronite, Gaultite, Gismondine, Gobbinsite, Gonnardite, Goosecreekite, Gottardite, Harmotome, Hsinghualite, Kalborsite, Laumontite, Lovdarite, Maricopaite, Mazzite, Merlinoite, Mesolite, Montesommaite, Mordenite, Mutinaite, Natrolite, Offretite, Partheite, Perlialite, Pollucite, Roggianite, Scolecite, Stellerite, Tetranovaite, Thomsonite, Tschernichite, Tschšrnerite, Wairakite, Wellsite, Willhendersonite, and Yugawaralite.
Many of the old, supposedly well-known, species show up as series: Brewsterite, Chabazite, Clinoptilolite, Dachiardite, Erionite, Faujasite, Ferrierite, Gmelinite, Heulandite, Levyne, Paulingite, Phillipsite, and Stilbite. The individual species will then be characterised as f.ex. Ferrierite-Na, Ferrierite-Ca, and Ferrierite-Mg, depending on whether sodium, calcium, or magnesium dominates. At Nevada Mining and Minerals, we will continue to supply f.ex. 'Ferrierite' with no qualifier, simply because an individual chemical analysis is hardly cost effective. It may give you a new name, but does not improve quality of the specimen.
Paranatrolite and Tvedalite may also be valid zeolite species, but await additional structural research. Pahasapaite and Weinebeneite are structurally similar phosphates, that IMA in its wisdom decided to consider zeolites [firmly believing structure is secondary to chemical composition, this appears an unnecessary inflation of the zeolite group to us]. The species Herschelite, Sodium Dachiardite, and Tetranatrolite were discredited as species. Note that several silicates occurring with zeolites are often erroneously considered to be zeolites. This is particularly the case of Apophyllite, Gyrolite, Prehnite and Tacharanite. None of these share zeolites' structural or chemical properties, they just happen to occur frequently with zeolites ... as do Calcite, Quartz, and other non-silicate non-zeolites.
The most prolific producers of zeolites are vesicular basalts (volcanic rocks, also called 'trap rocks'), where zeolites were deposited by water in the vesicles, after the basalt had cooled. The rock is volcanic, but the zeolites formed at low temperatures, often less than 40 C - check Rudy Tschernich's (1992) 'Zeolites of the world' for good descriptions of zeolite formation. The well-known basalt provinces, yielding a wealth of specimens include Iceland, the Faeroe Islands, and Northern Ireland in the Northern Atlantic. The Eifel area in Germany produces a wealth of zeolites, generally with small crystals, but very beautiful. The northwestern USA, chiefly Oregon and Washington, have yielded many beautiful specimens, chiefly found during construction. By far the most aesthetic specimens, and the largest volume come from the Deccan Plateau east of Bombay in the state of Maharashtra, India. These specimens are often labeled 'Poona' or 'Pune' for the main town, where most of the mineral dealers live, even if the true locality can be very far away.
The Deccan Traps cover an area of approximately 470,000 square kilometer in western India, but probably used to cover at least 1.2 million square kilometers after deposition in the late Cretaceous (Wadia, 1966). Most of the Deccan Traps are composed of an Augite basalt that is chemically and mineralogically very uniform. It forms layers up to 3000 meter thickness in the west around Bombay, but dwindles to 30-150 meter near the rim (Wadia, 1966). The basalt is generally poor in vesicles. The reason why we see the abundance of zeolites and other minerals from the Deccan Traps is its immense area and extensive use for road material. Most of the zeolites come from within short distance of either Pune or Nasik, and yet the production from each individual quarry is sporadic at best. Do not expect to see an abundance of druses if you visit the area; there are often weeks or months between finds of any consequence in a given quarry. To add insult to injury, if there ever was a quarry with abundant druses, we would probably never hear of it! The quarries' chief product is road material and druses appreciably depreciate the value of the rock. Consequently, a quarry often encountering druses would be abandoned.
Not only basalt deposits produce zeolites. Other sources include alkaline pegmatites, often as a last stage mineralisation or alteration, and generally quite poor in species diversity. A few ore deposits like the Silver mines in Kongsberg (Norway) and the Harz (Germany) also carry a few zeolites, though the species diversity is low. They too were formed in a late hydrothermal phase of the mineralisation. Zeolites also occur in granites, but again diversity and quality does not match material from basalts - we will, however, emphatically claim they are equally interesting to, perhaps more than, the abundant specimens from basalts. A very rare specimen from a granite, looking so-so, is far more desirable than an attractive piece, of which there are thousands, from a basalt.
Our records indicate that zeolites from a range of deposits occurs on specimens that also carry one or more of the following minerals - please note, most of the 'exotic' species are only found as associates of Analcime: Achantite, €girine, Albite, Andradite var. Melanite, Apatite, Apophyllite, Aragonite, Arfvedsonite, Augite, Babingtonite, Barite, Berborite, Bertrandite, Beryllite, Biotite, Birnessite, Bšhmite, Calcite, Calcium- catapleiite, Cavansite, Celadonite, Chalcopyrite, Chamosite var. Mg-Chamosite, Chiavennite, Chkalovite, Copper, Datolite, Diaspor, Epidote, Epistolite, Eudialyte, Ferrohornblende, Fluorapatite, Fluorite, Forsterite, Galena, Gibbsite, Gyrolite, Helvite, Hematite, Julgoldite, Kalsilite, Kamphaugite-(Y), Kogarkoite, Lueshite, Magnetite, Malachite, Melilite, Microcline, Millerite, Montmorillonite, Muscovite, Nacrite, Nepheline, Neptunite, Okenite, Opal, Orthoclase, Parakeldyshite, Pectolite, Polylithionite, Prehnite, Pumpellyite, Pyrargyrite, Pyrite, Pyrochlore, Pyrophyllite var. Agalmatolite, Quartz, Rhabdophane-(La), Rinkite var. Mosandrite, Saponite, Serandite, Siderazot, Siderite, Silver, Sorensenite, Sphalerite, Steenstrupine-(Ce), Stibnite, Strontium-apatite, Tacharanite, Tennantite, Thaumasite, Titanite, Todorokite, Tremolite var. Byssolite, Tugtupite, Ussingite, Vaalite, Villiaumite, Yofortierite, and Zircon.

Specimen Handling

Zeolites, except Laumontite that dehydrates, are for all practical purposes stable in a normal household environment. They not harmed by light, changes in temperature in the normal comfort range, or known to decompose. Some zeolite specimens - particularly acicular crystals of Natrolite, Scolecite, and Mesolite - are very fragile and should be handled with great care, and preferably touched and moved as little as possible. Zeolites not appreciably soluble in water, but due to their ion-exchange capabilities you may induce slight, invisible chemical changes in the surface layer.

Bibliography


Anthony, John Williams, Richard A. Bideaux, Kenneth W. Bladh & Monte C. Nichols. 1995. Handbook of mineralogy, vols. 2.1 & 2.2
Bancroft, Peter. 1984. Gem & Crystal Treasures.
Birch W.D. 1989. Zeolites of Victoria. The Mineralogical Society of Victoria. Inc. Special Publication 2.
Blackburn, William H. & William H. Dennen. 1997. Encyclopedia of mineral names. Canadian Mineralogist, special publication 1.
Coombs, Douglas S. et al. 1997. Canadian Mineralogist, 35, 1571-1606
Gaines, Richard W., H. Catherine W. Skinner, Eugene E. Foord, Brian Mason, Abraham Rosenzweig & Vandall T. King. 1997. Dana's new mineralogy: the system of mineralogy of James Dwight Dana and Edward Salisbury Dana, 8th ed.
Hintze, Carl (ed.) 1889-1897. Handbuch der Mineralogie, vol. 2.
Mandarino, Joseph A. 1999. The zeolite group. Mineralogical Record, 30(1-2), 5- 6
Noe-Nygaard, Arne. 1966. Mineralogi, 3rd ed.
Ramdohr, Paul & Hugo Strunz. 1980. Klockmann's Lehrbuch der Mineralogie, 16th ed.
Roberts, Willard Lincoln, Thomas J. Campbell & George Robert Rapp jr. 1990. Encyclopedia of Minerals 2nd ed.
Scalisi, Philip & David Cook. 1983. Classic mineral localities of the world: Asia and Australia
Sinkankas, John. 1964. Mineralogy.
Tschernich, Rudy W. 1992. Zeolites of the world.
Wadia, D.N. 1966. The geology of India, 3rd ed. Macmillan, London, xx+536 pp., xix pl.

This page is written and maintaned by Claus Hedegaard