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Named for the type locality in the Fillow quarry in Fairfield County, Connecticut, USA. Additional localities for Fairfieldite include others in the United States including here in the Black Hills of South Dakota in the Tip Top, Bull Moose, Dan Patch, and Bob Ingersoll mines, as well as in Germany, Portugal, Spain, Russia, and a few others. Faifieldite is an uncommon mineral that occurs as an accessory in granite pegmatites.
Ref. Handbook of Mineralogy, Anthony et al (1995) and MSA at http://www.handbookofmineralogy.org/pdfs/fairfieldite.pdf
Named in honor of Dr. Mark N. Feinglos, an American medical researcher and mineral collector who specialized in Tsumeb minerals, where the mineral gets its type locality at the Tsumeb mine in Namibia, and was the first to discover the mineral in his studies. In addition to its type locality, Feinglosite can be found, debatably, at the Christiana mine in Laurion, Greece. It occurs as a secondary mineral in chalcocite cavities from the oxidized zone of polymetallic hydrothermal ore deposits in dolostone at its type occurrence.
Ref. Handbook of Mineralogy, Anthony et al (1995) and MSA at http://www.handbookofmineralogy.org/pdfs/feinglosite.pdf