Crookesite is a selenide mineral composed of copper and selenium with variable thallium and silver.
Its chemical formula is reported either as or . It is formed by precipitation from hydrothermal fluids, and contains by mass: 16.3% Tl, 47.3% Cu, 2.9% Ag, and 33.6% Se.
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 (1832âÂÂ1919), the discoverer of the element thallium.