Search Mineralpedia – A Mineral Photo Database and Identification Guide
Named to honor Austrian Archduke Johann Baptist Josef Fabian Sebastian, the founder of the Landesmuseum in Graz, Austria. Johannite is an uncommon mineral that forms in oxidized, sulfide-bearing uraninite deposits, and usually forms post-mine. It can be found in localities in the Czech Republic, Germany, England, France, Tajikistan, Gabon, the United States, and Canada, among a couple others. Johannite is strongly radioactive. It will decompose in water and leaves a bitter or sour taste.
Ref. Handbook of Mineralogy, Anthony et al (1995) and MSA at http://www.handbookofmineralogy.org/pdfs/johannite.pdf