Acanthite

Formula: Ag2S

Colour: lead grey, black

Lustre: Metallic

Hardness: 2 – 2½

Specific Gravity: 7.2 – 7.4

Crystal System: Monoclinic

Member of: Acanthite Group

Name: Named in 1855 by Gustav Adolf Kenngott from the Greek άκανθα (“akantha”) meaning “thorn”, in allusion to the thorn-like crystal shape of his material. With the advent of instrumental analytical equipment, it was discovered that the mineral called “argentite” which had isometric crystal shapes was structurally and chemically identical to acanthite.

Type Locality: Jáchymov, Karlovy Vary District, Karlovy Vary Region, Czech Republic

Dimorph of: Argentite

Isostructural with: Aguilarite

Acanthite is the low-temperature modification of silver sulphide. All natural silver sulphide specimens at room temperatures are acanthite. At atmospheric pressure, argentite is stable above 177°C and acanthite is stable below 177°C. The tarnish on manufactured sterling silver objects is chemically the same as acanthite.