Arsenopyrite

Arsenopyrite (IMA symbol: Apy) is an iron arsenic sulfide mineral with the chemical formula FeAsS. It is a hard mineral, rating between 5.5 and 6 on the Mohs scale, and possesses a metallic, opaque luster with a color ranging from steel grey to silver white.

Its relative density is notably high at 6.1. When struck, the mineral emits a distinctive garlic odor, a key diagnostic feature along with its crystal habit, hardness, and density. In older literature, it may be referred to as mispickel, a term of German origin, or as mundic, a Cornish dialect word that can also denote a copper ore or a form of deterioration in concrete made with mine tailings.

Chemically, when arsenopyrite is dissolved in nitric acid, it releases elemental sulfur. Upon heating, it produces vapors of sulfur and arsenic.

With an arsenic content of 46%, it is, together with orpiment, a principal ore of arsenic. When deposits of this mineral are exposed to the atmosphere, they slowly convert into iron arsenates. Unlike iron pyrite, which can contribute to acid mine drainage, arsenopyrite is generally an acid-consuming sulfide mineral.

Arsenopyrite is often associated with significant amounts of gold, making it an indicator of gold-bearing reefs. Many gold ores containing arsenopyrite are refractory, meaning the gold is not easily extracted via cyanide leaching from the mineral matrix. The mineral is found in high-temperature hydrothermal veins, in pegmatites, and in areas of contact metamorphism or metasomatism.

Crystallography

Crystallographically, arsenopyrite belongs to the monoclinic crystal system, though older references may describe it as orthorhombic. It commonly exhibits prismatic or columnar crystal forms with striations and twinning. In its atomic structure, each iron center is bonded to three arsenic atoms and three sulfur atoms.

The material can be described as Fe³⁺ with the diatomic trianion AsS³⁻, though this ionic description is imperfect because the material is semiconducting and the bonds are highly covalent. The connectivity of atoms is more similar to that in marcasite than in pyrite. Various transition metals can substitute for iron, giving rise to rare related minerals such as clinosafflorite, gudmundite, glaucodot, alloclasite, iridarsenite, osarsite, and ruarsite.