Crookesite is a selenide mineral composed of copper and selenium with variable thallium and silver. Its chemical formula is reported either as Cu7(Tl,Ag)Se4 or (Cu,Tl,Ag)2Se. It is formed by precipitation from hydrothermal fluids.
Crookesite is an opaque, bluish grey to pink toned brown metallic mineral crystallizing in the tetragonal system. It has a Mohs hardness of 2.5 to 3 and a specific gravity of 6.9.
It was discovered in 1866 in Skrikerum, Sweden and named for Sir William Crookes (1832-1919), the discoverer of the element thallium.