Acanthite

Formula:  Ag₂S

Origin of name:  from Greek for thorn because of shape of crystals

Crystal system:  Monoclinic, pseudo-orthorhombic

Crystal class:  2/m

Twinning:  on {-111}, polysynthetic; on {-101}, contact

Unit cell:  a = 4.23 Å, b = 6.93 Å, c = 9.53 Å, b = 125.5°

Colour:  Iron-black

Diaphaneity: Opaque

Luster:  Metallic

Habit:  Crystals short to long prismatic, crystals rare

Hardness:  2 to 2.5

Specific gravity:  7.22

Cleavage:  None

Tenacity:  Sectile

Streak:  Black

Synonyms/varieties: none

Comments:  Dimorphous with argentite, which is stable only above 177°C. Occurs in moderately low sulphide veins, and in zones of secondary enrichment, widespread in silver deposits