| Crookesite | |
| Category: | Selenide mineral |
| Imasymbol: | Crk[1] |
| Strunz: | 2.BD.50 |
| System: | Tetragonal |
| Class: | Disphenoidal H-M symbol: |
| Symmetry: | I |
| Color: | Lead-gray |
| Habit: | As finely divided, disseminated specks, and as small veinlets. |
| Cleavage: | Good, two at right angles |
| Fracture: | Brittle |
| Mohs: | 2.5–3 |
| Luster: | Metallic |
| Gravity: | 6.90 |
| Diaphaneity: | Opaque |
| References: | [2] [3] |
Crookesite is a selenide mineral composed of copper and selenium with variable thallium and silver.
Its chemical formula is reported either as or .[4] 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.[2]
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.[2]