Black crystal aggregates of the very rare Ag-Cu selenide mineral eucairite from probably the type locality in Sweden (Skrikerum Mine, Valdemarsvik, Östergötland County, Sweden), eventhough the label states "Kalmas, Sweden". Ex Vandenbroucke Museum collection specimen, from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Eucairite
Eucairite is an IMA-recognized mineral species in the sulfide class, with the chemical formula CuAgSe.
It crystallizes in the tetragonal crystal system (space group Pmmn).
Key properties: Mohs hardness 2.5; specific gravity 7.6–7.8; Shining streak; Metallic luster.
IMA symbol Eca, grandfathered status — known before the IMA’s founding, named in 1818.
Widely distributed in selenium deposits of hydrothermal origin; locally abundant with other selenides.
It is found associated with Berzelianite, weissite, crookesite, clausthalite, umangite, klockmannite, tiemannite, chalcomenite, malachite, calcite.
Its name comes from the Greek for opportunity, because it was discovered shortly after discovery of the element selenium.