Crookesite
General
Category	Selenide mineral
Formula (repeating unit)	Cu 7(Tl, Ag)Se 4
IMA symbol	Crk[1]
Strunz classification	2.BD.50
Crystal system	Tetragonal
Crystal class	Disphenoidal (4) H-M symbol: (4)
Space group	I4
Identification
Color	Lead-gray
Crystal habit	As finely divided, disseminated specks, and as small veinlets.
Cleavage	Good, two at right angles
Fracture	Brittle
Mohs scale hardness	2.5–3
Luster	Metallic
Diaphaneity	Opaque
Specific gravity	6.90
References	[2][3]

Crookesite is a selenide mineral composed of copper and selenium with variable thallium and silver.

Characteristics

Its chemical formula is reported either as Cu
₇(Tl, Ag)Se
₄ or (Cu, Tl, Ag)
₂Se.^[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.

Name and discovery

It was discovered in 1866 in Skrikerum, Sweden and named for Sir William Crookes (1832–1919), the discoverer of the element thallium.^[2]