Bayldonite, (after Mimetite)

Mineral: Bayldonite, Mimetite
Size: 180 mm
Owner: NMNH; Smithsonian Institute
Photographer: Malcolm Southwood

A large and classic example of bayldonite (?) pseudomorphous after hexagonal, prismatic, mimetite crystals (to 30 mm).    While the specimen is labelled as bayldonite it is noted that, on analysis, many similar pseudomorphs have been shown to be arsentsumebite, or mixtures of arsentsumebite and bayldonite, replacing the mimetite. One or two broken crystals show that the replacement is incomplete, with cores of mimetite still clearly present. 

Specimen number R8594, in the collection of the National Museum of Natural History, Smithsonian Institute.   

The specimen was collected from 10 Level, in the first oxidation zone and was in the collection of Friedrich Wilhelm Kegel, director and general manager at Tsumeb between 1922 and 1932. 

Kegel's collection was purchased by the Smithsonian in 1950 (White, 1977).

Image courtesy of the US National Museum of Natural History, Smithsonian Institute.