Arsenopyrite

Arsenopyrite
General
Category Sulfide mineral
Formula
(repeating unit)		 FeAsS
Strunz classification 2.EB.20
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group P21/c
Unit cell a = 5.744, b = 5.675
c = 5.785 [Å]; β = 112.3°; Z = 4
Identification
Color Steel grey to silver white
Crystal habit Acicular, off-square prismatic, stubby; striated; also compact, granular, columnar
Twinning Common on {100} and {001}, contact/penetration twinning on {101}
Cleavage 110 (distinct)
Fracture Subconchoidal to rough
Tenacity Brittle
Mohs scale hardness 5.5 - 6
Luster Metallic
Streak Black
Diaphaneity Opaque
Specific gravity 5.9 - 6.2
Optical properties Anisotropism - strong red-violet
Pleochroism Weak, white or bluish tint, faint reddish yellow
Fusibility Yes
Solubility Nitric acid
Other characteristics Garlic odor when struck, greenish tinge when weathered, green staining of wall rocks
References [1][2][3]

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.[1] When dissolved in nitric acid, it releases elemental sulfur. When arsenopyrite is heated, it produces poisonous sulfur and arsenic fumes which can be fatal if inhaled in large quantities. With 46% arsenic content, arsenopyrite, along with orpiment, is a principal ore of arsenic. When deposits of arsenopyrite become exposed to the atmosphere, the mineral will slowly oxidize, converting the arsenopyrite into an iron arsenate, a relatively stable compound. Arsenopyrite is generally an acid consuming sulfide mineral unlike iron pyrite which can lead to acid mine drainage.

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.[4]

Arsenopyrite also can be associated with significant amounts of gold. Consequently, it serves as an indicator of gold bearing reefs. Many arsenopyrite gold ores are refractory, i.e. the gold is not easily cyanide leached from the mineral matrix.

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

Crystallography

Arsenopyrite crystal from the Yaogangxian Mine, Hunan, China (size: 2.7 x 2.0 x 1.7 cm)

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 it has been shown to be monoclinic. In terms of its atomic structure, each Fe center is linked to three As atoms and three S atoms. The material can be described as Fe3+ with the diatomic trianion AsS3−. The connectivity of the atoms is more similar to that in marcasite than pyrite. The ion description is imperfect because the material is semiconducting and the Fe-As and Fe-S bonds are highly covalent.[5]

Microscopic picture of Arsenopyrite

Various transition group metals can substitute for iron in arsenopyrite. The arsenopyrite group includes the following rare minerals:

  • Clinosafflorite: (Co,Fe,Ni)AsS
  • Gudmundite: FeSbS
  • Glaucodot or alloclasite: (Fe,Co)AsS or (Co,Fe)AsS
  • Iridarsenite: (Ir,Ru)AsS
  • Osarsite or ruarsite: (Os,Ru)AsS or (Ru,Os)AsS

See also

References

  1. 1 2 Hurlbut, C. S.; Klein, C., 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
  2. ↑ http://rruff.geo.arizona.edu/doclib/hom/arsenopyrite.pdf Handbook of Mineralogy
  3. ↑ Mindat.org
  4. ↑ Mindat Mispickel
  5. ↑ Vaugn, D. J.; Craig, J. R. Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. ISBN 0-521-21489-0.
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