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January-February 2016

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Connoisseur's Choice: Carletonite, Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Québec, Canada

If you have trouble keeping up with the changes in mineral nomenclature, you will be able to commiserate with the petrographer. (Petrography is the branch of petrology that concerns the detailed study of specific rocks, usually with the aid of a microscope; etrogenesis is the branch that studies the origin and formation of rocks.) When I took a class in sedimentation (way back in what I jokingly call the Lower Devouring Period), naming some of the sedimentary rocks in lab tests was not for the faint of heart. But my interest in sedimentary rocks was low. In my opinion, they are just the pitiful remains of once-proud igneous and metamorphic rocks. However, the nomenclature for metamorphic and igneous rocks is not exactly easy to pickup either, even for professionals. Between 1931 and 1938, University of Chicago petrologist Albert Johannsen (1871–1962; the mineral johannsenite is named for him) published the definitive (for the time) treatise on igneous rocks and their nomenclature in four volumes! Fortunately, as mineral collectors, most of these igneous rock types need not overly concern us because they are rare and devoid of collectible specimens. One of the exceptions to this broad generalization is the igneous rock type called feldspathoidal syenites.

Paul W. Pohwat, a consulting editor of Rocks & Minerals, is the collection manager (minerals) in the Department of Mineral Sciences at the National Museum of Natural History (Smithsonian Institution).

Dr. Robert B. Cook, an executive editor of Rocks & Minerals, is a professor emeritus in the Department of Geology and Geography at Auburn University. He wrote the Connoisseur's Choice column from its inception in 1993 until 2011—with just a sprinkling of guest columnists—adding up to more than 120 columns to his credit.       

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