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. ==Name and discovery==