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, Tschrnerite, 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, Bhmite, 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 authored by Claus Hedegaard.
Mineraler