Acanthite - Encyclopedia

    Class : Sulfides and sulfosalts
    Subclass : Sulfides
    Crystal System : Monoclinic
    Chemistry : Ag₂S
    Rarity : Quite common

Acanthite is one of the most common silver ores (it contains 87%), present in the hydrothermal veins of lead-zinc-silver and gold, mainly in their cemantation zone. It is usually found in centimetric ranges associated with other silver minerals and different sulphides. Thus, acanthite is associated with pyrite, sphalerite, galena and many other silver minerals (polybasite, pyrargyrite, tetrahedrite); it is also a mineral accompanying gold in epithermal veins, metalliferous emanations from volcanic activity. Its name comes from the Greek acantha (thorn) in connection with the shape of its crystals. It most often results from the transformation of argentite, an unstable mineral below 179°C, of which it retains the massive, arborescent forms, sometimes in cubo-octahedral crystals. It is a lead gray mineral which quickly tarnishes black in air, characterized as a lot of silver sulphides by a high density (7.2) and a low hardness of just over 2. Acanthite is an important silver ore but which rarely forms large clusters. This mineral was called "silver mine" by the designers of the Middle Ages who used its property to write on paper. More discreetly, the tarnish of native silver is due to the formation of a surface film of acanthite.

Main photo : Acanthite from Imiter mine, Tinghir Cercle, Drâa-Tafilalet, Morocco

In France, acanthite is found in microcrystals in the kaolin quarries at Echassières (Allier), at the Le Rhun mine (Côtes-d'Armors), in the Lopérec gold mine (Brittany), in the silver mines of Ste-Marie-aux-Mines (Haut-Rhin), at the mine of Ste-Lucie (Lozère), at Peyrebrune (Tarn), etc...

Acanthite may show polysynthetic twins on (111), but these are not visible to the naked eye. Complete twins by interpenetration of 2 octahedra are extremely rare and form a 6-pointed star

No fake known for this mineral.