arsenopyrite

A mineral having composition FeAsS and crystallizing in the monoclinic system. Crystals have pseudo-orthorhombic symmetry because of twinning. The Mohs hardness is 5.5–6.0, and the specific gravity is 6.0. The luster is metallic and the color silver-white. Arsenopyrite is the most widespread arsenic-bearing mineral. It is commonly found in veins containing gold (Lead, South Dakota; Deloro, Ontario), tin or tungsten minerals (Bolivia; Cornwall, England), or nickel-cobalt-silver minerals (Cobalt, Ontario; Freiberg, Germany). See also Arsenic.

Environment

High- and medium-temperature veins, pegmatites; sometimes disseminated in crystalline (igneous) rocks.

Pseudo-orthorhombic, commonly in distinct crystals, which, like marcasite, are elongated sideways, parallel to a horizontal axis, even appearing prismatic. Often forms solid masses.

Silver-white. Luster metallic; hardness 5Ɖ-6; specific gravity 5.9-6.2; fracture uneven; cleavage prismatic. Brittle.

Iron sulfarsenide (34.3% Fe, 46.0% As, 19.7% S). In veins, often intergrown with loellingite and rammelsbergite.

Arsenic (garlic) smell is noted immediately after fracturing with hammer blow. On charcoal gives white fumes and leaves black magnetic mass or, after long blowpiping, a globule. (Make no platinum wire test without extended roasting to free all arsenic.) Decomposed by nitric acid, leaving spongy sulfur mass. A sulfur test can be obtained by fusing with sodium carbonate, crushing to a powder on a silver disk (find an old dime!), and wetting. Tarnished spot proves sulfur's presence.

Distinguished from the sulfur-free white nickel arsenides by a sulfur test and by negative results in cobalt-nickel tests. Wedge-topped crystals are common enough to be guides to identification.

Arsenopyrite is an ore of arsenic, usually an unwelcome by-product of other mining operations. Good crystals were common in the Freiberg, Germany, nickel-silver mines; in the Cornwall, England, and tin mines of Bolivia, and in the Iname Mine, Aichi Prefecture, Japan. The Panasqueira, Portugal, arsenopyrites are among the best ever found, 12 in. (30 cm). long. Naillike crystals from Pitkaranta, Finland are unique and remarkable. Typical deposits include a Japanese locality (Obira mine, Kyushu I.); Trepca, in the former Yugoslavia; and a generous Zacatecas, Mexico, mine. A massive vein of arsenopyrite once was mined in Edenville, New York.

Found in pegmatites in isolated crystals and associated with garnet and the phosphates. Loellingite sometimes takes the place of arsenopyrite, as at Franklin, New Jersey. A cobaltiferous variety, danaite, in which cobalt may take the place of as much as 9 percent of the iron, is named for J. Freeman Dana of Boston, not for mineralogist James D. Dana.

Arsenopyrite
Arsenopyrite, Panasqueira Mine, Portugal
General
Category	Mineral
Chemical formula	FeAsS
Identification
Color	Steel grey to silver
Crystal habit	Acicular, off-square prismatic, stubby or massive; striated
Crystal system	monoclinic; ?/m ?/m ?/m
Cleavage	110 (distinct)
Fracture	Subconchoidal to rough
Mohs scale hardness	5.5 - 6
Luster	Metallic
Streak	Black
Specific gravity	5.9 - 6.2
Refractive index	Opaque
Pleochroism	None
Fusibility	Yes
Solubility	Nitric acid
Other characteristics	Garlic odour when struck, greenish tinge when weathered, green staining of wall rocks

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 becomes magnetic and gives off toxic fumes. With 46% arsenic content, arsenopyrite, along with orpiment, is a principal ore of arsenic. When deposits of arsenopyrite become exposed to the atmosphere, usually due to mining, the mineral will slowly oxidize, converting the arsenic into oxides that are more soluble in water, leading 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[1].

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 liberated from the mineral matrix.

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

Crystallography

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 AsS^3-. 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.^[2]

Related minerals

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

References

1. ^ Hurlbut, C. S.; Klein, C., 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
2. ^ Vaugn, D. J.; Craig, J. R. Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. ISBN 0521214890.

• mindat.org
• Minerals.net

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