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Category Selenide mineral
(repeating unit)
Strunz classification 02.BD.50
Color Lead-gray
Crystal habit As finely divided, disseminated specks, and as small veinlets.
Crystal system Tetragonal - Disphenoidal
Cleavage Good, two at right angles
Fracture Brittle
Mohs scale hardness 2.5–3
Luster Metallic
Diaphaneity Opaque
Specific gravity 6.90
References [1][2]

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.[3] 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.[1]

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[edit]

It was discovered in 1866 in Skrikerum, Sweden and named for Sir William Crookes (1832–1919), the discoverer of the element thallium.[1]

See also[edit]


  1. ^ a b c Webmineral site
  2. ^ Handbook of Mineralogy
  3. ^ Mindat