| Arsenopyrite | |
|---|---|
 | |
| General | |
| Category | Sulfide mineral |
| Formula | FeAsS |
| Strunz classification | 2.EB.20 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (sameH-M symbol) |
| Space group | P21/c |
| Unit cell | a = 5.744, b = 5.675 c = 5.785 [Å]; β = 112.3°; Z = 4 |
| Identification | |
| Colour | 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 |
| Lustre | 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 odour when struck, greenish tinge when weathered, green staining of wall rocks |
| References | [1][2][3] |
Arsenopyrite (IMAsymbol: Apy[4]) is an iron arsenicsulfide (FeAsS). It is a hard (Mohs 5.5–6)[5] metallic, opaque, steel grey to silver white mineral with a relatively highspecific gravity of 6.1.[1]
When dissolved innitric acid, it releases elementalsulfur. When arsenopyrite is heated, it produces sulfur and arsenic vapor. With 46%arsenic content, arsenopyrite, along withorpiment, is a principalore of arsenic. When deposits of arsenopyrite become exposed to the atmosphere, the mineral slowly converts into iron arsenates. Arsenopyrite is generally an acid-consuming sulfide mineral, unlikeiron pyrite which can lead toacid mine drainage.[citation needed]
Thecrystal habit, hardness, density, and garlic odour when struck are diagnostic. Arsenopyrite in older literature may be referred to asmispickel, a name of German origin.[6] It is also sometimes referred to asmundic, a word derived fromCornish dialect and which also refers to a copper ore, as well as a form of deterioration in aggregate concrete made with mine tailings.
Arsenopyrite also can be associated with significant amounts ofgold. Consequently, it serves as an indicator of gold bearing reefs. Many arsenopyrite gold ores arerefractory, i.e. the gold is not easily cyanide leached from the mineral matrix.
Arsenopyrite is found in high temperaturehydrothermal veins, inpegmatites, and in areas ofcontact metamorphism ormetasomatism.
Arsenopyrite crystallizes in themonocliniccrystal system and often shows prismatic crystal or columnar forms withstriations andtwinning common. Arsenopyrite may be referred to in older references asorthorhombic, 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 inmarcasite thanpyrite. The ion description is imperfect because the material is semiconducting and the Fe-As and Fe-S bonds are highlycovalent.[7]
Varioustransition group metals can substitute for iron in arsenopyrite. The arsenopyrite group includes the following rare minerals:
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