Formula: Cu7(Tl,Ag)Se4
Origin of name: After Sir William Crookes (1832-1919), who discovered thallium
Crystal system: Tetragonal
Crystal class: 4/m, 4 or -4
Twinning:
Unit cell: a = 10.435 Å, c = 3.95 Å
Colour: Lead-gray
Diaphaneity: Opaque
Luster: Metallic
Habit: As finely divided, disseminated specks and as small veinlets
Hardness: 2.5 to 3
Specific gravity: 6.90 (measured), 7.443 (calculated)
Cleavage: two at right angles, fairly well developed
Tenacity: Brittle
Streak:
Synonyms/varieties:
Comments: Found with other selenides of hydrothermal origin
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