Metallic crystal aggregates and crusts of the extremely rare crookesite, a copper-thallium-silver selenide, a mineral full of rare elements; in fact it's the only mineral of almost 5000 that contains Ag, Tl and Se at the same time. Type locality mineral. From: Skrikerum Mine, Valdemarsvik, Östergötland, Sweden. — Photo: David Hospital · CC BY-SA 4.0
Crookesite
A rare copper selenide carrying variable thallium and silver — named for scientist William Crookes.
Intro adapted from Wikipedia (CC BY-SA).
Crookesite is an IMA-recognized mineral species in the sulfide class, with the chemical formula Cu₇TlSe₄.
It crystallizes in the tetragonal crystal system (space group I4/m).
Key properties: Mohs hardness 2.5–3; specific gravity 6.9; Metallic. Anisotropism: Weak but distinct, in brownish tones. R1 –R2: (400) 33.6–35.9, (420) 33.3–35.8 luster.
IMA symbol Crk, grandfathered status — known before the IMA’s founding, named in 1867.
Of hydrothermal origin, with other selenides.
It is found associated with Umangite, berzelianite, eucairite, klockmannite, clausthalite, sabatierite, selenian linnaeite, calcite, quartz.
In honor of William Crookes (1832–1919), English chemist who discovered thallium.