Green crystals of uvite on quartz with scattered crystals of clear magnesite. Note tiny magnesite crystals perched on large uvite crystal. Length of specimen ~4 cm.
Readers of this little blog know that I am always on the lookout for nice specimens, especially those displaying rather unfamiliar minerals. That is, since I am not a mineralogist I usually know almost nothing about “lots of” minerals; however, I am always up to a challenge of “trying to find out” additional information! So, at the recent RMFMS show in Sandy, Utah, I was intrigued by a specimen labeled “Uvite Tourmaline”. I am somewhat familiar with many Tourmaline Group members such as elbaite, indicolite, rubellite, schorl, and dravite, but uvite? What is the world was that? So, I pulled my handy-dandy mineral guide out of my backpack---but no uvite listed. That little bit of information prodded me to purchase this specimen, and besides, it was a beautiful green crystal perched on some quartz and what appeared to be magnesite. It was a reasonable price, three dollars, so it was wrapped and on the way to Colorado Springs.
I was like a kid in a candy store (or a rockhound in a mineral store) upon returning home and checking out all my “hot buys”. There it was, the uvite! Immediately I checked out my other mineral books and about the only reference located was in Roberts, Rapp, and Weber (1974) who noted that uvite was a hypothetical end member of tourmaline. Now, what does that mean? So, on to the internet with a short stop in Eckels and others (1997) where they stated that “black-brown crystals of tourmaline from Italian Mountain [Gunnison County] were identified as uvite…by XRD [x-ray diffraction]”.
What I found out, after extensive reading, was that uvite is a valid mineral name, sort of, that was redefined by the International Mineralogical Association in 2011 as being two different mineral species. There is a uvite series (two members) with one end member being a hydroxyl-dominant mineral and the other end member being a fluorine-dominant uvite:
The general chemical formula for the series is usually written as: [Ca(Mg3)MgAl5(Si6O18)BO3)3(OH)3(F/OH)]. Note the fluorine or the hydroxyl at the end of the formula. The hydroxyl-dominant form is referred to as uvite (to avoid confusion with the original use of the name) while the latter is called fluor-uvite.
It seems like all/most of the Tourmaline Group minerals have fluorine-dominant analogues, for example fluor-schorl (fluorine-dominant) to go with schorl (hydroxyl-dominant). MinDat (www.mindat.org) furthermore goes on to note that an analysis really needs to be completed before many specimens of uvite (sensu lato) can be assigned to one of the end members. Since I don’t carry an XRD in my back pocket, I could be in trouble when it comes to correct identification!
Uvite is often green in color, as in my specimen, but at times it may be black, brown or even colorless or perhaps white. I suppose, but am guessing here, that the color range may be due to the magnesium? The crystals seem to be short and stubby compared to other tourmaline species, for example schorl and elbaite. In fact, they almost look flattened (along the C-axis) or tabular. Good crystals are complexly terminated and my specimen is transparent, vitreous, and gemmy although others may be translucent. Tourmaline Group minerals are harder than quartz coming in at about 7.5 (Mohs).
The specimen that I acquired came from Brumado (Bom Jesusdos Meiras), Bahia, Brazil. I don’t know much about the local geology; however, magnesite (one of the largest deposits in the world) and talc are mined in the region. Cassedanne and Cassedane (1978) published an article on the Brunado District in the Mineralogical Record; however, I have been unable to locate this vintage article. I do believe the uvite is associated with the magnesite deposits that in turn are located in Precambrian rocks partially subjected to replacement by hydrothermal solutions. Henry and others (2011) made the Brumado District the type locality for uvite (sensu stricto); however, MinDat notes “it is not possible to assign 'uvites' from Brumado to the correct species without a reliable analysis”.
In summary, I listen to music while writing, mostly the “oldies station,” and often remember the words of the world’s greatest rock and roll band, the Rolling Stones: You can't always get what you want, but if you try sometimes, you might find, you get what you need. So, I “got what I need”, a nice crystal of uvite.
Cassedanne, J.P. & Cassedanne, J.O., 1978, Famous Mineral Localities: The Brumado District, Bahia , Brazil: Mineralogical Record v. 9, p. 196-205.
Eckel, E. B., 1997, Minerals of Colorado: Fulcrum Publishing, Golden, Colorado.
Henry, D., Novák, M., Hawthorne, F.C., Ertl, A., Dutrow, B.L., Uher, P. & Pezzotta, F., 2011, Nomenclature of the Tourmaline Supergroup Minerals: American Mineralogists, v. 96, p. 895-913.