Alloclasite - Encyclopedia

    Class : Sulfides and sulfosalts
    Subclass : Sulfides
    Crystal System : Monoclinic
    Chemistry : (CoFe)AsS
    Rarity : Rare

Alloclasite is a rare to very rare mineral present in high temperature cobalt and arsenic hydrothermal deposits, as well as exceptionally in certain silicified metamorphic rocks. Its name comes from the Greek allos (other) and klastos (to break), in reference to its particular cleavage which allows it to be differentiated from similar minerals such as glaucodot and marcasite. Alloclasite is a dimorph of glaucodot ; it constitutes short to elongated prismatic crystals according to [010], frequently striated, and columnar to radiate aggregates forming hemispheres. It has a strong metallic luster, a pewter-white color and a perfect {101} cleavage. It is a very accessory ore of cobalt.

Alloclasite in the World

Beautiful crystals are known from Oravita (Romania) and Cobalt (Canada). Good specimens also come from Bou Azzer (Morocco), see main photo and right photo © Stephan Wolfsried.

Alloclasite in France

In France, alloclasite is reported at the Clochettes mine, near Briançon (Hautes-Alpes).


No twin known for this mineral species.

Fakes and treatments

No fake reported for this mineral species.

Hardness :  5
Density : 5,95
Fracture : Irregular to sub-conchoidal
Trace : Black

TP : Opaque
RI : Not measurable
Birefringence : None
Optical character : None
Pleochroism : None
Fluorescence : None

Solubility : Nitric acid

Magnetism : None
Radioactivity : None


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