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March-April 2016

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Reading Pegmatites—Part 2: What Tourmaline Says

THIS IS THE SECOND OF SEVERAL ARTICLES THAT ARE MEANT TO INFORM MINERAL MINERS, COLLECTORS, AND AMATEUR SCIENTISTS ABOUT THE GEOLOGIC ENVIRONMENTS OF THE MINERALS THEY ACQUIRE AND SEEK. In these articles, the principal environment will be that of granitic pegmatites, from which many of the finest mineral specimens and gem materials are obtained. The minerals chosen will be those that presently have much to say about the pegmatitic environment in which they form. The series continues with tourmaline, the most common of the boron-rich minerals.

Like beryl (London 2015a), tourmaline is well known to collectors for the exquisite forms that its crystals commonly display and for the beautiful colors of its gem-quality varieties (fig. 1). Whereas beryl occurs almost exclusively in association with granitic pegmatites (see London 2008, 2015a), tourmaline is a common accessory mineral in sedimentary, metamorphic, and igneous rocks, wherein its abundance (volumetric proportion) rarely exceeds 1 percent. Tourmaline-rich rocks occur in two special environments, both described here.

Dr. David London is a Stubbman-Drace Presidential Professor, Norman R. Gelphman Professor of Geology, and director of the Electron Microprobe Laboratory at the University of Oklahoma.

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