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.

Acanthite in the World

Automorphic crystals, which are in fact mostly argentite pseudomorphs (paramorphs), are however rare : the most beautiful samples, cubes and octahedra reaching up to 8 cm, come from Freiberg and Schneeberg (Saxony, Germany), from Jáchymov (Czech Republic), Guanajuato (Mexico), Chañarcillo (Chile) and Monte Narba (Sardinia, Italy). Finally, the epithermal silver deposit of Imiter in Morocco produced octahedra as well as firs of more than 4 cm.

Acanthite in France

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 from Echassières, Allier, France - © G. Niceus
Acanthite from Echassières, Allier, France - © G. Niceus
Acanthite from Neuenberg, Ht-Rhin, France - © T. Brunsperger
Acanthite from Peyrebrune, Tarn, France - © Jean Leconte

Twinning

Acanthite may show polysynthetic twins on (111), but these are not visible to the naked eye.

Fakes and scams

No scam known for this mineral.



Hardness : 2 to 2.5
Density : 7.2 to 7.4
Fracture : Sub-conchoidal 
Streak : Gray




TP : Opaque
RI : -
Birefringence : -
Optical character : -
Pleochroism : None
Fluorescence : None


Solubility : Nitric and hydrochloric acids and ammonia

Magnetism : None
Radioactivity : None