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September-October 2015

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Mineral Mysteries: Quartz Cores in Tourmaline Crystals

Many years ago (1971) a paper appeared in the American Mineralogist titled “A Pseudomorphic Quartz-Tourmaline Relationship from Northern Nigeria.” I remember it well because I was troubled by the explanation provided by the author (McCurry 1971). What she described are color-zoned tourmaline crystals from a pegmatite that have cores composed of aggregates of quartz grains. The tourmaline crystals—presumably elbaite, but she did not specify—are brown, but there are remnants that are green; from these McCurry surmised that the tourmaline cores were also green. The green cores, except for these remnants, McCurry writes (p. 1476), appear to have been replaced by quartz, “the replacement being limited by a zone of differing composition.” This assessment was based upon the optical properties of the two zones; the green is suggested to have a 7 weight percent iron content in the form of FeO, the brown a 9 weight percent iron in the form of Fe2O3. McCurry further suggested (p. 1476) that “it seems possible that Fe2+ is more readily removed by hydrothermal solutions than Fe3+, a process perhaps facilitated by the lower percentage of total iron in the green zones.”       

John S. White, a consulting editor of Rocks & Minerals, operates Kustos, a museum/collector consulting business. Now retired, he is the former curator-in-charge of the National Mineral and Gem Collection of the Smithsonian Institution.       

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