Bluish grey crystal aggregates of the very rare Cu arsenide mineral koutekite. From: Mohawk Mine, Mohawk, Keweenaw County, Michigan, United States of America. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY 4.0
Koutekite
Koutekite is an IMA-recognized mineral species in the sulfide class, with the chemical formula Cu₅As₂.
It crystallizes in the orthorhombic crystal system (space group Ibam).
Key properties: Mohs hardness 4; specific gravity 8.48; Black streak; Metallic. Anisotropism: Strong. R1 –R2: (400) —, (420) —, (440) 45.9–46.4, (460) 44.6–43.4, (480) 43.0–41.4 luster.
IMA symbol Kou, grandfathered status — known before the IMA’s founding, named in 1958.
Found in arsenical copper deposits.
It is found associated with Paxite, arsenic, silver, skutterudite, nickeline, löllingite, chalcocite, algodonite, domeykite, allargentum, kutı́naite, calcite.
The name refers to Jaromir Koutek (1902–1983), Professor of Economic Geology, Charles University, Prague, Czech Republic.