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NATURAL ZEOLITES: WHERE HAVE WE BEEN, WHERE ARE WE GOING ?
FREDERICK A. MUMPTON
Edit
Inc., P.O. Box
591,
Clarkson, New
York 14430,
U.S.A.,
INTRODUCTION
Mr. Chainnan and fellow students of zeolite science. I was pleased to accept Dr. Colella's invitation to speak to you about some of the highlights in the history of
natural zeolites and the role that the International Committee on Natural Zeolites has played over the years. I hope to do just that, but will also try to point out some of the areas that are ripe for future research from both the
academic point of view and from the point of
view of industrial, agricultural, and medical applications, which may increase the wellbeing of mankind. I hope also to have enough time to relay to you some of my con- cerns about the future of natural zeolites and how we need to improve the quality of our research in the years that lie ahead.
WHERE HAVE WE BEEN?
Pre-1950 Studies
I do not want to spend much time chronicling the early history of natural zeolites. We all know about or we can read about this
subject in several review papers, including some of my own, how zeolites were discovered in 1756 by the Swedish mineralogist Cronstedt and how over the years several dozen distinct species have been identified and their crystal structures
determined. We can review how in 1858, Eichhorn showed that these materials can exchange some of their constituent cat- ions for others; how in 1857, Damour demonstrated their hydration-dehydration
properties; and how in 1925, Weigel and Steinhof
separated gas molecules on the basis of size once the water had been removed from the zeolite's internal structure. In 1932, McBain, as many of you know,
termed this phenomenon "molecular sieving", and we use this phrase still today. The fact is, however, that all of this work was carried out on large,
millimeter-centimeter size crystals collected from basalts or trap rocks, in which zeolites
minerals are ubiquitous constituents in vugs and cavities. At that time, zeolite minerals, while attractive to the eye and while adorning
almost every mineral collection, never seemed to occur in sufficient purity or amount in the basalts to be of commercial concern, despite
the growing interest in their adsorption, de-hydration, and cation-exchange capabilities. Barrer's monumental work in London (see, e.g., Barrer, 1938) and Samashima's in
Tokyo (see, e.g., Samashima, 1929; Samashima and Hemmi, 1934; Samashima and Morita, 1935) on zeolite adsorption and molecular sieve phenomena was carried out on chabazite and other zeolite crystals from basalt vugs. Barrer soon realized that the rarity of
chabazite and mordenite, or of any other zeolite for that matter, precluded serious thought of developing industrial processes based on the natural materials. Hence, being a good
chemist, he decided to synthesize them. His successful synthesis of chabazite and
mordenite, the two natural zeolites having the most attractive adsorption and molecular
sieving properties, suggested that the zeolite group of materials did indeed have commercial
potential.
Editors' Note: This review was presented in large part by Dr. F. A. Mumpton, Chairman ofthe International
Committee on Natural Zeolites, as the introductory lecture at the 4th FEZA Euro
workshop on Zeolites held in Ischia, Naples, Italy, prior to Zeolite '97, the 5th
Intemational Conference on the Occurrence, Properties, and Utilization of Natural Zeolites. It also incorporates some of Dr. Mumpton's remarks to the full Conference regarding the future of natural
resrech.
Linde's (Union Carbide) Involvement
In the late 1940s, Linde Division of Union Carbide Corporation in Tonawanda, New York, got into the act. Because Linde's main business at the time was the cryogenic
production of oxygen and nitrogen from air, they were constantly on the lookout for new and different ways of separating these gases from one another. Thus, Linde instituted a program of zeolite synthesis under the direction of Robert M. Milton and Donald W. Breck spe- cifically to produce chabazite for air
separation and other adsorption/molecular sieve applications. Although the first few attempts were total failures (no chabazite was fonned), what was produced was a zeolite, which was not known and which
here to fore had no natural counterpart (Milton, 1959). This zeolite we all know now is the famous Linde zeolite A, which for many adsorption and molecular sieve purposes has properties even more
desirable than those of chabazite. Zeolite A is, of course, a mainstay of the molecular sieve business the world over. [Milton and Breck, along with Edith M. Flanigen, were ultimately able to synthesize chabazite and mordenite and faujasite (synthetic zeolites X and Y) and literally dozens of new
zeolites, having no natural counterparts (see Milton, 1968)].
Thus, in the middle 1950s, as Linde's chemists were busy synthesizing zeolites and developing commercial applications for them, management became increasing worried that natural occurrences of their patented zeolites A and X could exist, thereby
undermining their composition-of-matter patent protection and jeopardizing the several million dollars that had already been spent on the project. Despite assurances from
mineralogical
and mineral-commodity consultants that
mine able deposits of zeolites simply could not be ex- pected, such worries prompted Linde to hire a geologist and a mineralogist to assess
thoroughly the dreaded possibility that natural zeolites could be mined from natural
deposits, a situation that would torpedo their entire synthetic zeolite program. One of the geologists's
first acts was to interview key scientists at the U.S. Geological Survey in Washington, D.c., and Professor Leonard B. Sand at Worcester Polytechnic Institute in Worcester, Massachusetts, with whom many in the natural zeolite field are familiar. At the U.S. Geological Survey, he learned of Coombs' (1954) discovery of 300 feet of laumontite-rich sedimentary rocks in New Zealand, and in
Worcester he discussed with Sand the forthcoming paper by Ames, Sand,
and Goldich on their recent find of high-purity clinoptilolite in tuffaceous sedimentary rocks near Hector, California (Ames et al., 1958). Linde suddenly realized that if laumontite and clinoptilolite occurred in
deposits large enough and pure enough to be mined, there might indeed be a chance of
finding natural deposits of A and X, as well as mine able deposits of chabazite, erionite, and mordenite, the three natural zeolites
possessing adsorption properties similar to those of the synthetics.
At the same time, in late 1957 and early 1958, a literature search was made at Linde regarding the occurrence of natural zeolites in non-basaltic environments. About a dozen such occurrences had indeed been described in the literature, although the articles were not highly publicized. Seven papers describing the occurrence
of heulandite or clinoptilolite in volcanogenic sedimentary rocks of the western United States were published be- tween 1914 and 1936 (e.g., Johannsen, 1915; Bradley, 1928; Kerr, 1931; Bramlette and Posjnak, 1933), to say nothing of the phillipsite discoveries of Murray and Renard in 1891 in pyroclastic-rich deep-sea
sediments. The work of Sudo (1950), who described the abundance of clinoptilolite in the Green Tuff of Japan, the several papers de- scribing "mordenite" in Russian sedimentary rocks, and the key papers by Scherillo (1950) and Sersale (1958), in which the zeolitic
nature of the "tufo giallo napoletano" (Neapoli- tan Yellow Tuft) was uncovered, testify that the discoveries of Coombs and Ames, Sand, and Goldich were not flukes. Although we can look back now and admire the authors of these earlier papers for their recognition of zeolites in such non-basaltic rocks, the geo- logic world paid little attention to these
papers at the time, and for the most part, zeolites were considered by geologists and
mineralogists to be found in the vugs and cavities of basalts. [I am afraid that even today in many universities such is the dogma that is being passed down to students; almost no note is taken of the millions (if not billions) of tons of high-grade zeolitic tuff that have been found in hundreds of sedimentary deposits in more than 40 countries.]
I believe that one of the reasons for this lack of interest by the geological profession was that no one
appreciated that zeolite minerals had commercial potential, aside from Sersale's report of the role of several Italian zeolitic tuffs in poz- zolanic concrete (see Sersale, 1992, 1995). Much of the early exploration for natural
zeolites in the United States was reviewed by Mumpton (1984).
If recognizing the possibility of mine able natural zeolites was not enough by itself to shake Linde's management, in January 1958, they received a sample of what was purported to be erionite from a prospector who said that "I've got a mountain
of it !". Of course, all prospectors say that they've got a mountain
of
it, whether it's gold or diamonds or sand or gravel. In this case, it was true. The
material was definitely erionite and almost pure at that. Although there wasn't quite a
"mountain" of it, there was enough to make decent mining operation. Subsequent examination showed that it came from an extensive
deposite of flat lying zeolitized volcanic tuff near
Rome, Oregon. Shortly afterwards, Linde became aware
of Van Houten's (1964) and Deffeyes' (1958) work on zeolitic tuffs in Wyoming and Nevada, respectively, and
of the occurrence of erionite in both localities. Linde's approach suddenly changed from one that really did not want its geologists to find deposits
of
natural zeolites and thereby undermine its synthetic business to one that
aggressively explored the American west for such deposits, trying to secure the best
deposits before the rest of the geological profession became aware of the situation.
Linde initiated an unpublicized zeolite search (1) to ascertain whether or not one could expect to find natural deposits of
Linde's synthetic A and X (faujasite); (2) to discover mine able deposits of chabazite, erionite, and mordenite, the natural zeolites most competitive with their synthetic
products; and (3) to catalog deposits of all other "sedimentary" zeolites that might be
encountered. Linde soon recognized that clinoptilolite was the number one zeolite in volcanogenic sediments, at least in the United States, but they showed almost no
commercial interest in this zeolite because it did not
possess attractive adsorption properties,
Linde's main concern at the time. The
exploration program lasted three years (1959, 1960, 1961), and by the end of that time, other
petroleum and chemical companies in the United States were also looking for natural zeolites to supplement their own synthetic zeolite programs and to obtain deposits
of clinoptilolite, which had been shown by Ames and coworkers in the 1960s to have extremely desirable cation-exchange properties,
not only for radioactive cesium and strontium, but also for NH in municipal sewage
effluents. As a result of Linde's exploration efforts alone, more than 300 individual occurrences
of zeolite minerals were discovered in volcanogenic sedimentary rocks. The world's supply of the heretofore "rare" zeolite erionite changed from an amount that would fit into a
waste basket to one that could be measured in millions of tons. The first truly sedimentary
deposit of chabazite was discovered (in contrast to the Italian and Germen ash-flow tuffs), and the first authenticated identification of
mordenite in sediments was made (the earlier Russian identifications of this mineral were in error; they were probably dealing with clinoptilolite). During this period, Sand and Regis (1968) identified the world's first
occurrence of ferrierite, another previously thought-to-be rare zeolite, in sedimentary rocks.
World Attention
Now the stage was set. In both the United States and abroad, the chemists were making zeolites in the laboratory and developing uses for them, and the geologists were beginning to realize that certain zeolites were abundant in nature. The geologists were stimulated
because their discoveries appeared to be of more than academic interest they were actually good for something, and the chemists were inspired to think in
terms of low-cost applications using the natural materials. In the next decade or so, more than 1000 separate
occurrence of sedimentary zeolites were discovered in more than 40 countries. Apart from the
industrial potential of these materials, far-seeing geologists began to recognize that
zeolites were key constituents of many tuffaceous rocks and that their occurrence and
distribution might provide insight into the genesis of these rocks.
Pithy Observations
Please note the following aspects of the above narrative that go beyond the "walk down memory lane":
(1) Every investigation should begin with a thorough examination of the literature, and keeping
up with that literature must thereafter be a systematic part of that investigation.
(2) The literature must be examined
regardless of the language in which the papers are written. If it pertains to the subject at hand, the researcher must read it. This advice is
especiallv aimed at colleagues from my own country, who are notorious for reading only papers written in English.
(3) Prior to 1950, the accepted knowledge was that all zeolites occurred in vugs and
cavities in basalts. Today, we know differently, but the old "party line" is still being taught in many schools. Students should listen to their instructors, but they must be cognizant of what others are finding out as well. To
paraphrase Martin Luther, students should accept what is good, but throw out what is bad!
(4) Geologists for years hiked across beds of high-grade zeolites without having any knowledge of their mineralogical
composition or what they were walking on. Those who examined these rocks with a petrographic microscope were often frustrated by the
ultra-fine particle size of the constituent phases, and the rocks were simply referred to as
"altered rhyolites" or "devitrified tuffs". This is inexcusable, because a quick X-ray
diffraction examination of these rocks would have shown their zeolite content immediately. The morale of the tale is that in studying anything, one must use the techniques and instruments that are necessary to answer the questions, not just those methods or apparatuses that are handy or happen to be available. My hat goes off to Richard Hay, Ken Deffeyes, Riccardo Sersale, Toshio Sudo, Leonard Sand, Douglas Coombs, Lloyd Ames, and others who
determined by X-ray diffraction that what appeared to be an ordinary clay sediment was actually 90-100% pure
zeolite.
(5) A special rule for my chemist and agricultural friends if the subject at hand has to do with rocks and/or
minerals, get help from someone who is knowledgeable about such materials. Run, don't walk, to the
nearest geologist or mineralogist!
Zeolite Conferences
Back to the story. Until the early 1970s, things progressed rather slowly-more
deposits were found in more countries, the Japanese use of zeolites in agriculture became known in the west, and large deposits of clinoptilolite
in Hungry, Bulgaria, and Russia.
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