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July-August 2018

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Reading Pegmatites: Part 4—What Quartz and Feldspars Say

THIS IS THE FOURTH OF SEVERAL ARTICLES THAT ARE MEANT TO INFORM MINERAL MINERS, COLLECTORS, AND AMATEUR SCIENTISTS ABOUT THE GEOLOGIC ENVIRONMENTS OF THE MINERALS THEY COLLECT (previous three: London 2015, 2016, 2017a). In these articles, the environment is that of granitic pegmatites. The minerals chosen are those that presently have much to say about the pegmatitic environment in which they form. The series continues with quartz and the feldspars.

The vast majority of pegmatites are granitic in composition. By definition, that means that quartz and feldspars (including the plagioclase and the alkali feldspar series, and their respective species) comprise greater than or equal to 80 volume percent of the rock. In the common pegmatites, quartz and feldspars constitute nearly 100 percent of the rock, with only minor amounts of micas, garnet, tourmaline, apatite, and so on. At the opposite end of the pegmatite spectrum, the Tanco pegmatite at Bernic Lake, Manitoba, which is one of the most mineralogically diverse igneous rock bodies on Earth (104 mineral species documented), consists of 33 percent quartz, 29 percent sodic plagioclase, and 20 percent K-feldspar (Stilling, Černý, and Vanstone 2006). Together, these constitute 82 percent of the pegmatite, making Tanco truly granitic in composition as well.


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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