Let's Get It Right: Cavansite or Pentagonite?

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    Let's Get It Right: Cavansite or Pentagonite?John S. WhitePublished online: 08 Jul 2010.

    To cite this article: John S. White (2002) Let's Get It Right: Cavansite or Pentagonite?, Rocks & Minerals, 77:4, 274-275

    To link to this article: http://dx.doi.org/10.1080/00357529.2002.9925646

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  • Lets Get It Right

    Cavansite or Pentagonite? JOHN S. WHITE P.O. Box 332 Stewartstown, Pennsylvania 17363 john@ jwkustos.com

    am sure that most of us who attend major mineral shows I such as those in Denver and Tucson are well aware that ever since the first reports of cavansite coming out of India (Wilke, Schnorrer-Kohler, and Bahle 1989; Wilson 1989), there has been a virtual flood of it in radial spheres of vivid blue, usual- ly on pale-colored stilbite, with many dealers handling vast quantities at various times. Curiously, this sudden abundance did not appear until some twelve years after the occurrence of cavansite in India was first noted by Bill Birch, National Muse- um of Victoria in Australia (Wilson 1977), who actually iden- tified it in 1974. A year earlier, in 1973, Rustam Kothavala also had found in an Indian dealers stock some specimens with bright blue crystals that were subsequently identified as cavansite (Kothavala 1991). For 15 years I searched in vain, wrote Kothavala of his attempts to locate the source of these specimens. The source turned out to be the Wagholi quar- ries about 20 kilometers northeast of the center of Poona (Pune), one of the quarries that he never visit- ed. For a more detailed discussion of cavansite from India the reader is referred to Robert Cooks Con- noisseurs Choice column in the MayIJune 1996 issue of Rocks & Minerals (Cook 1996).

    Cavansite was first discovered with its polymorph pentagonite in Oregon (Staples, Evans, and Lind-

    quarries? The two species have exactly the same chemistry and form under nearly identical conditions.

    Well, this has happened, but with very little fanfare thus far. Pentagonite from India fEst came to my attention at the 2001 Tucson Show. An Indian dealer had some material that he claimed was pentagonite, but I was leery because it looked exactly like cavansite, so I acquired just one piece. When I quizzed him about why he thought it was pentagonite, he told me of another Indian dealer who had verified pentagonite and, in fact, even had some examples of it. So, I hustled over to that

    dealer, whom I knew and trusted. Upon looking at his selection I became convinced that his speci- mens really were pentagonite, and with great excitement I acquired a few from him as well. Once home, I was able to examine them under a microscope; it then became very clear that although those from the second dealer were pentagonite, that from the first was simply cavansite. Even though pentagonite is relative- ly rare with respect to cavansite, I have seen quite a lot of it subse- quently, and I find that it is very easy to recognize.

    The typical habit for Indian cavansite, by far, is as spherical clus- ters of dark. rich blue color. averan-

    Y

    Figure I . Pentagonite crystal cluster on heulandite from the Wagholi quarries, near Poona (Pune), India. G. Edwin Maccubbin Jr. specimen, Jeff Scovil photo.

    ing bemeen 1 and 2 cm in diameter. They are commonly attractively scattered over a contrasting matrix

    say 1973) but only in small quantities of microscopic crystals. An observation made by those authors is of particular interest: Pentagonite, found so far only at Owyhee Dam, seems to have formed at a later stage than cavansite, but their mineral associ- ations and mode of occurrence are nearly identical. Why, then, should one not expect to find pentagonite in the Wagholi

    John S. White, a consulting editor of Rocks & Minerals, operates Kustos, a museudcollector consulting and minerallgem sales business.

    274 ROCKS & MINERALS

    of usually smaller crystals of white or off-white stilbite. The second-most-common habit is as lin- ear clusters of more distinct individual crystals somewhat resembling a pile of logs. In both cases the crystals tend not to have sharp and distinct terminations, and their diaphaneity can best be described as translucent rather than transparent. Both of these habits are beautifully illustrated in photographs in Mineralogical Record (Wilson 1989; Kothavala 1991).

    Pentagonite, by contrast, does not usually form tight spheres of crystals. Instead, it is most often found as flamboyant sprays jutting out unevenly from a common center (fig. 1). Individual crystals tend to exhibit widely varying lengths, and one is

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    Figure 2. Pentagonite cyclic twins. Lower views are clinographic projections in standard orientation; upper views are looking down on the tops of the same crystals. Roman numerals desig- nate the different individual crystals that are joined in the twin. Dotted lines show boundaries between individuals on faces that are composites of coplanar segments from two or more individu- als. A: lspical pentagonite twins, with unequal individuals and incomplete development of the full cyclic twin; this drawing shows three individuals. B: Ideal cyclic twin composed of five individual crystals of equal size. Drawn by R Peter Richards after Staples, Evans, and Lindsay (1973).

    immediately struck by the sharp detail of these crystals and their relative transparency. Because they tend to be quite small, a microscope or a hand lens is helpful in observing some of the features that make these crystals so distinctive. Apart from the lovely development of individual crystals, pentagonite also occurs as twins whereas cavansite does not. In fact, the twin- ning of pentagonite is so common that one can expect to see an example of it on every specimen. The name penragonire was given to this mineral because of its twinning. According to Staples, Evans, and Lindsay (1973), Pentagonite is nearly always twinned by reflection across the prism m (1 lo), which is the composition plane. The prism angle mhm = 72.7 so that multiple twinning leads to groups which closely resemble five- fold symmetry (fig. 2B). On the extreme left of figure 1 a small star shape can be seen, beautifully illustrating the feature for which this great mineral was named. In my experience such cyclic twins are rare. Far more common are two- and three- component twins as illustrated in figure 2A.

    Kothavala (1991) stated that there are several quarries north and west of the village of Wagholi, but the Main quarry pro- duced the best cavansite. I have been told that the pentagonite specimens come from the same quarry as cavansite, but only in one particular part of that quarry. As at the type locality in Oregon, pentagonite appears to have formed under slightly different conditions than cavansite, which is why they are not

    found together in India, at least not yet. Further, pentagonite has crystallized on microsized heulandites instead of stilbite, which forms the matrix of cavansite, another aid in distin- guishing between these two similarly colored minerals. It is obvious, unfortunately, that searching through countless spec- imens in large lots of cavansite is unlikely to result in finding a pentagonite.

    It is difficult to assess how much pentagonite will appear on the market, and, surprisingly, it has not yet created the level of excitement that I expected. I have seen perhaps as many as one hundred specimens thus far, all very small, but then I have seen thousands of cavansites, many very large. Keep your eyes open for pentagonite-you may find some! It is truly worth the effort.

    REFERENCES Cook, R. B. 1996. Cavansite, near Wagholi, Poona district, Maha-

    rashtra, India. Rocks & Minerals 71:180-82. Kothavala, R. Z. 1991. The Wagholi cavansite locality near Poona,

    India. Mineralogical Record 22:415-20. Staples, L. W., H. T. Evans Jr., and J. R. Lindsay. 1973. Cavansite and

    pentagonite, new dimorphous calcium vanadium silicate minerals from Oregon. American Mineralogist 58:405-11.

    Wilke, H. J., G. I. Schnorrer-Kohler, and A. Bahle. 1989. Cavansit aus Indien. Lapis 14:39-41.

    Wilson, W. E. 1977. Whats new in minerals? Mineralogical Record 8:61-62.

    . 1989. Whats new in minerals? Mineralogical Record 20:234-35. 0

    ~

    At its Spring 2002 meeting the Mineralogical Society of America awarded Michael F. Hochella, Jr., of the Virginia Polytechnic Institute and State University, its Dana Medal. The Medal recognizes continued outstanding scientific contributions to the mineralogical sciences through originaI research by an individual in the midst of their career. Dr. Hochella studies the atomic structure and composition of mineral surfaces in reactions with water and gases. Minerals react chemically with their surroundings through their surfaces. Mineral surface science was always recognized as important to understand the mechanics of crystallization and reactions. But the processes could only be studied indirectly or imagined. With todays technology of scanning force microscopy we can actually see the mineral surface structure, defects, and reaction products on an atomic scale. Nano-scale geoscience (one billionth of a meter) has thus far studied low-temperature mineral reactions of weathering, mine tailings, polluted sites, and what can be described as bacteria-mineral reactions. It allows the design of clay-polymer combinations with properties superior to many plastics. It has found that in earthquakes rocks may be able to overcome friction and slide past one another because of nanometer-sized, self-healing cracks on the fault face.

    Mineralogical Society of America 1015 lLYh St NWSte 601

    Washington, DC 20036-5212 USA phone: 202-775-4344 fnx: 202-775-0018

    e-mail: business@minsocam.org website: www.minsocam.org

    Volume 77, JulylAugust 2002 275

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