Arsenopyrite
A hard, metallic ore of arsenic and gold indicator.
Arsenopyrite (IMA symbol: Apy) is an iron arsenic sulfide mineral with the formula FeAsS. It is a hard, metallic, opaque, steel grey to silver white mineral, notable for its high relative density of 6.1 and its diagnostic garlic odor when struck. As a principal ore of arsenic, along with orpiment, it is economically significant and also serves as an indicator of gold-bearing reefs due to its frequent association with gold.
- formula
- FeAsS
- hardness
- 5.5–6 (Mohs)
- density
- 6.1
- crystal_system
- monoclinic
- color
- steel grey to silver white
- principal_use
- ore of arsenic
Lore & Background
Arsenopyrite crystallizes in the monoclinic crystal system, often showing prismatic or columnar forms with striations and twinning. Older references may describe it as orthorhombic, but it has been shown to be monoclinic. In its atomic structure, each iron center is linked to three arsenic atoms and three sulfur atoms, and the material can be described as Fe3+ with the diatomic trianion AsS3−, though this is imperfect because the material is semiconducting and the bonds are highly covalent. When dissolved in nitric acid, arsenopyrite releases elemental sulfur. When heated, it produces sulfur and arsenic vapor. Deposits exposed to the atmosphere slowly convert into iron arsenates. Unlike iron pyrite, which can lead to acid mine drainage, arsenopyrite is generally an acid-consuming sulfide mineral. Arsenopyrite in older literature may be referred to as mispickel, a name of German origin, or as mundic, a Cornish dialect word that also refers to a copper ore and to a form of deterioration in aggregate concrete made with mine tailings.
Reader's Guide
Arsenopyrite holds significance as a principal ore of arsenic, with a 46% arsenic content, and as an important indicator of gold-bearing reefs. Many arsenopyrite gold ores are refractory, meaning the gold is not easily cyanide leached from the mineral matrix. The mineral is found in high temperature hydrothermal veins, pegmatites, and areas of contact metamorphism or metasomatism. Its diagnostic properties—crystal habit, hardness, density, and garlic odor when struck—aid in identification. The arsenopyrite group includes rare minerals such as clinosafflorite, gudmundite, glaucodot, iridarsenite, osarsite, and ruarsite, where various transition metals can substitute for iron. Its legacy includes historical names like mispickel and mundic, reflecting its long recognition in mining and mineralogy.
Did You Know?
- Arsenopyrite releases elemental sulfur when dissolved in nitric acid.
- When heated, arsenopyrite produces sulfur and arsenic vapor.
- It is generally an acid-consuming sulfide mineral, unlike iron pyrite.
- Arsenopyrite in older literature may be referred to as mispickel, a name of German origin.
Frequently Asked Questions
Who is Arsenopyrite?
Arsenopyrite is an iron arsenic sulfide mineral with the chemical formula FeAsS, crystallizing in the monoclinic system. It presents as a hard, metallic, opaque stone ranging from steel grey to silver white.
What are Arsenopyrite's powers/role?
It serves as the primary ore from which arsenic is extracted, working alongside orpiment in that capacity. It also acts as a reliable field indicator that gold-bearing reefs are nearby, since the two minerals frequently co-occur in the same deposits.
How does Arsenopyrite's story end?
In industrial processing, the mineral is smelted and broken down into its constituent elements for arsenic recovery. In the field, its Mohs hardness of 5.5–6 and high relative density of 6.1 mean it resists casual weathering but ultimately yields to refining and chemical separation.
Why is Arsenopyrite important?
It is the principal commercial source of arsenic, making it economically vital for a range of industrial applications. Its consistent association with gold deposits also gives prospectors a practical, low-cost tool for locating valuable reefs.
What makes Arsenopyrite's appearance unique?
Striking the mineral releases a distinctive garlic-like odor, which serves as its most diagnostic quick field test. Combined with its metallic luster, high density, and steel-grey to silvery coloration, this smell sets it apart from other sulfide ores.
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