Argentopyrite

Formula:  AgFe₂S₃

Origin of name:  From silver (Latin argentum) and physical similarity to pyrite

Crystal system:  Orthorhombic

Crystal class:  2/m 2/m 2/m

Twinning: pseudo-hexagonal twins parallel [100] by interpenetration of {120}, lamellar twinning also present

Unit cell:   a = 6.64 Å, b = 11.46 Å, c = 6.45 Å

Colour:  Steel-grey to tin-white; tarnishes bronze-brown, green

Diaphaneity: Opaque

Luster:  Metallic

Habit:  Minute pseudo-hexagonal crystals

Hardness:  3.5 to 4

Specific gravity:  4.25

Cleavage:  None

Tenacity: Brittle

Streak:

Synonyms/varieties:

Comments:  Dimorphous with sternbergite. As crusts of euhedral crystals formed in hydrothermal veins