Arsenopyrite is an iron arsenic sulfide (FeAsS). It is a hard (Moh's 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. Arsenopyrite contains about 46% arsenic and is a principle ore of arsenic. Arsenopyrite crystallizes in the monoclinic crystal system and often shows prismatic crystal or columnar forms with striations and twinning common. Arsenopyrite may be referred to in older references as orthorhombic, but has been shown to be monoclinic. The crystal habit, hardness, density, and odor (garlic) when struck are supposedly diagnostic.

Various transition group metals occur in arsenopyrite as subtitutes for iron. A cobalt rich variety is known as danaite. Arsenopyrite also can contain significant amounts of gold.

Arsenopyrite is found in high temperature hydrothermal veins, in pegmatites, and in areas of contact metamorphism or metasomatism.

The arsenopyrite group of minerals includes the following rare minerals:

• Clinosafflorite: (Co,Fe,Ni)AsS
• Gudmundite: FeSbS
• Glaucodot, Danaite, or Alloclasite: (Fe,Co)AsS
• Iridarsenite: (Ir,Ru)AsS
• Osarsite or Ruarsite: (Os,Ru)AsS or (Ru,Os)AsS

References

• Dana's Manual of Mineralogy ISBN 0471032883
• mindat.org
• Minerals.net