A lot of black-grey very small single crystals of the extremely rare cobalt arsenide mineral modderite from the the locality in Modder (Modderfontein Mine, Benoni, Ekurhuleni District, Gauteng Province, South Africa). Ex Vandenbroucke Museum collection specimen, from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Modderite
Modderite is an IMA-recognized mineral species in the sulfide class, with the chemical formula CoAs.
It crystallizes in the orthorhombic crystal system (space group Pnam). It is a member of the Modderite structural group.
Key properties: Mohs hardness 4; specific gravity 8.28.
IMA symbol Mod, grandfathered status — known before the IMA’s founding, named in 1923.
Found in heavy mineral concentrates (Witwatersrand, South Africa).
It is found associated with Nickeline, alloclasite, safflorite, glaucodot, cobaltite, pentlandite, pyrrhotite, chalcopyrite (Witwatersrand, South Africa); alloclasite, glaucodot.
Derivation not given; presumably named for the Modderfontein mine, South Africa.