Silvery metallic crystal aggregates of the very rare Cu-Fe-Ag selenide geffroyite from the famous Moctezuma mine in Mexico famous for its Te minerals. Curiously this one is not a telluride but a selenide. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Geffroyite
Geffroyite is an IMA-recognized mineral species in the sulfide class, with the chemical formula (Cu,Fe,Ag)₉Se₈.
It crystallizes in the cubic crystal system (space group Fm3m). It is a member of the Pentlandite structural group.
Key properties: specific gravity 5.39; Metallic. R: (400) —, (420) 19.0, (440) 21.4, (460) 23.5, (480) 25.5, (500) 27.5, (520) 29.1, (540) 30.1, (560) 31.7 luster.
IMA symbol Gef, IMA status: approved, named in 1980.
Occurs as late-stage deposits in veins cutting granite (Chaméane mine, France).
It is found associated with Eskebornite, clausthalite, chaméanite, ankerite (Chaméane mine, France).
The mineral was named to honor Jacques Geffroy (1918–1993), metallurgist for the French Atomic Energy Commission.