Black microcrystals of the extremely rare thallium mineral chalcothallite from the type (Ilímaussaq, Narsaq, Kujalleq, Greenland) and only one of the few known localities, all of them in Greenland. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Chalcothallite
Chalcothallite is an IMA-recognized mineral species in the sulfosalt class, with the chemical formula (Cu,Fe,Ag)₆.₃(Tl,K)₂SbS₄.
It crystallizes in the tetragonal crystal system (space group I4/mmm). It is a member of the Bukovite structural group.
Key properties: Mohs hardness 2–2.5; specific gravity 6.6; Black streak; Metallic luster.
IMA symbol Ccth, IMA status: approved, named in 1966.
Found in ussingite veins cutting poikilitic sodalite syenite.
It is found associated with Galena, vrbaite, cuprostibite, thalcusite, silver, gudmundite, chalcocite, sphalerite, molybdenite, avicennite, chkalovite, epistolite, niobophyllite.
The name refers to its composition.