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Arsenopirita
http://es.wikipedia.org/wiki/Arsenopirita


Arsenopyrite
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Arsenopyrite
http://en.wikipedia.org/wiki/Arsenopyrite
Arsenopyrite, Panasqueira Mine, Portugal
General
Category Sulfide mineral
Chemical formula FeAsS
Identification
Color Steel grey to silver white
Crystal habit Acicular, off-square prismatic, stubby; striated; also compact, granular, columnar
Crystal system monoclinic; 2/m, pseudoorthorhombic
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
Pleochroism Weak
Fusibility Yes
Solubility Nitric acid
Other characteristics Garlic odour when struck, greenish tinge when weathered, green staining of wall rocks
References [1][2]

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.