Alloclasite [(Co, Fe)AsS] is a sulfosalt mineral. It is a member of the arsenopyrite group. It is the most abundant arsenic mineral and the primary ore of arsenic metal. In addition to being found in deposits that are large enough to be minable, arsenopyrite is widely distributed. It was first described in 1866 as an occurrence in Romania. Its name is derived from Greek for “other” and “to break,” in reference to its distinct cleavage which distinguished it from the similar appearing mineral marcasite.

Physical Properties of Alloclasite

Alloclasite crystallizes in the monoclinic system and typically forms as columnar to radiating acicular prismatic clusters. It is an opaque steel-gray to silver-white, with a metallic luster and a black streak. It is brittle with perfect cleavage, a Mohs hardness of 5 and a specific gravity of 5.91–5.95.

  • Lustre: Metallic
  • Transparency: Opaque
  • Colour: Steel gray to silver
  • Streak: Nearly black
  • Hardness: 5 on Mohs scale
  • Hardness: VHN100=818 – 940 kg/mm2 – Vickers
  • Tenacity: Brittle
  • Fracture: Irregular/Uneven, Sub-Conchoidal
  • Density: 5.95 g/cm3 (Measured) and 6.188 g/cm3 (Calculated)
  • Crystal system: Orthorhombic
  • Crystal Habit: Prismatic, Radial, Aggregates

However, it usually occurs in such small amounts and in such small particle sizes that it is easily overlooked. It is associated with other sulfide minerals in organic-rich sedimentary rocks, metamorphic rocks, and igneous rocks in many parts of the world.


Significant amounts of Alloclasite have been produced from deposits in Germany, England, Bolivia, Japan, Greece, Spain, Sweden, Mexico, and Japan. In North America deposits are located in Ontario, Canada, and in South Dakota, New Jersey, and New Hampshire in the United States.


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