Arsenopyrite is an iron arsenic sulfide (FeAsS). It is a hard (Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it produces sulfur. When arsenopyrite is heated, it becomes magnetic, and gives off toxic fumes.

Arsenopyrite contains about 46% arsenic and is a principle ore of arsenic along with orpiment.

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 garlic odor when struck are diagnostic. Arsenopyrite in older literature may be referred to as mispickel, a name of German origin.

Various transition group metals occur in arsenopyrite as subtitutes for iron. A cobalt rich variety is known as danaite, named after James Dana the famous American mineralogist.

Arsenopyrite also can contain significant amounts of gold and consequently is used by geologists as an indicator of potential gold bearing reefs. Many arsenopyrite gold ores are refractory, the gold unable to be easily liberated from the arsenopyrite matrix.

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