Chalcothallite

Formula:  Tl₂(Cu,Fe)₆SbS₄

Origin of name:  From composition

Crystal system:  Tetragonal

Crystal class:  4/m 2/m 2/m

Twinning: as fine grid of lamellae parallel to {110} and {1-10}

Unit cell: a = 3.827 Å, c = 34.280 Å

Colour:  Lead-gray to iron-black; tarnishes iridescent

Diaphaneity: Opaque

Luster:  Metallic

Habit:  Lamellar aggregates

Hardness:  2 to 2.5

Specific gravity:  6.60

Cleavage:  3; In three perpendicular directions

Tenacity: Plastic, tectonically deformed

Streak:  Black

Synonyms/varieties:

Comments:  Splits easily into fine flakes or rectangular plates. Found in ussingite veins cutting poikilitic sodalite syenite