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Category Selenide mineral
(repeating unit)
Strunz classification 2.BD.50
Crystal system Tetragonal
Crystal class Disphenoidal (4)
H-M symbol: (4)
Space group I4
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 [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