Formula: Tl2(As,Sb)8S13
Origin of name: For James Gilluly, American geologist
Crystal system: Monoclinic
Crystal class: 2/m
Twinning:
Unit cell:
Colour: Deep red to maroon
Diaphaneity: Translucent
Luster: Near-metallic when tarnished
Habit: Cleavable masses to 2 cm, rare as slender prismatic crystals to 1 mm
Hardness: 2
Specific gravity: 4.02
Cleavage: 1; {001} perfect
Tenacity:
Streak: Brick red
Synonyms/varieties:
Comments: Colour resembles realgar but the two can be distinguished if side by side
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