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July-August 2018

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Who's Who in Mineral Names: Richard Alexander Fullerton Penrose Jr. (1863–1931)

Penroseite, ideally NiSe2, but often (Ni,Co,Cu)Se2, is the nickel selenide member of the pyrite group. At the type locality, the Virgen de Sunumi mine, Pakajake Canyon, Chayanta Province, Potosí Department, Bolivia, penroseite occurs in hydrothermal veins associated with tiemannite, naumannite, gerdsdorffite, clausthalite, pyrite, and chalcopyrite. Here, it is typically in columnar, metallic, steel-gray crystalline masses. Penroseite is relatively soft (2.5–3.0 on the Mohs scale) and dense with a specific gravity of 6.58–6.74. It was described and named by Samuel G. Gordon in 1925. Due to the analytical techniques available at the time and the specific nature of the acquisition of the type specimen, both the formula and the type locality have been subsequently revised. Some of the penroseite from the type locality appears to be microscopically intergrown with naumannite (Ag2Se) and clausthalite (PbSe). As a result, Gordon's analyses yielded a mineral containing silver and lead in additional to nickel and selenium with lesser amounts of cobalt and copper substituting for nickel. In addition, the type specimen was part of a poorly labeled collection Gordon purchased in nearby Colquechaca. This led to a tentative assignment of the wrong type locality. Eventually, the current type locality was established because the Colquechaca mine and other nearby mines contain no selenium. The Virgen de Sunumi mine is also the type locality for ahlfeldite, mandarinoite, and olsacherite as well as penroseite. The relationship between penroseite and blockite was examined by Bannister and Hey (1937), who determined they were identical and suggested blockite was not valid. A good description of the type locality was published by Redwood in 2003.

Penroseite was named in honor of R. A. F. Penrose Jr., an American mining geologist, investor, and entrepreneur. Penrose's early survey of the Cripple Creek, Colorado, gold deposits for the U.S. Geological Survey was seminal.

 

Dr. Steven C. Chamberlain, a consulting editor of Rocks & Minerals, is an avid collector and researcher specializing in the minerals of New York State.

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