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Arsenopyrite

earth science Maturity 7-9 Next article →

Arsenopyrite is a hard and heavy mineral made of iron, arsenic, and sulfur. Its chemical formula is FeAsS. This mineral is metallic and opaque in appearance. It shows colors ranging from steel grey to silver white. Scientists use the Mohs scale to measure its hardness. Arsenopyrite ranks between 5.5 and 6 on this scale. It also has a high relative density of 6.1.

Arsenopyrite-176706
Arsenopyrite-176706

This density means the mineral feels quite heavy for its size.

This mineral undergoes specific changes when it meets certain substances. If you dissolve arsenopyrite in nitric acid, it releases elemental sulfur. Heating the mineral also causes a reaction. This heat produces both sulfur and arsenic vapor. When arsenopyrite deposits are exposed to the atmosphere, they change over time. The mineral slowly converts into iron arsenates. Unlike iron pyrite, arsenopyrite is an acid-consuming sulfide mineral. This is an important distinction in mining environments.

Arsenopyrite by petrographic microscope NL
Arsenopyrite by petrographic microscope NL

Arsenopyrite has a very specific crystal structure. It belongs to the monoclinic crystal system. This describes how its atoms are organized in space. The mineral often grows in prismatic or columnar forms. These shapes may show striations, which are fine lines on the surface. Twinning is also a common feature in these crystals. In the past, some researchers called it orthorhombic. However, it has since been proven to be monoclinic.

Arsenopyrite-176706
Arsenopyrite-176706

At the atomic level, the structure is quite complex. Each iron center connects to three arsenic atoms. It also connects to three sulfur atoms. The material can be described as Fe3+ with a diatomic trianion AsS3−. Because the mineral is a semiconductor, the ion description is not perfect. This is because the bonds between iron-arsenic and iron-sulfur are highly covalent. The way these atoms connect is more similar to the mineral marcasite than to pyrite.

Arsenopyrite by petrographic microscope NL
Arsenopyrite by petrographic microscope NL

History provides many different names for this mineral. Older scientific literature often refers to it as mispickel. This name has German origins. Another name used is mundic. This term comes from a Cornish dialect. Mundic is also used to describe copper ore. It can also refer to a type of deterioration in concrete made with mine tailings. These names show how different cultures and industries have interacted with the mineral over time.

Arsenopyrite is highly significant as a source of arsenic. It contains 46% arsenic by content. This makes it one of the principal ores of arsenic, along with orpiment. It is also a vital tool for miners. Arsenopyrite is often found associated with large amounts of gold. Because of this, it serves as an indicator of gold-bearing reefs. However, extracting the gold can be difficult. Many arsenopyrite gold ores are refractory. This means the gold is not easily leached from the mineral matrix using cyanide.

Geologists find arsenopyrite in several specific environments. It is found in high-temperature hydrothermal veins. It also appears in pegmatites. You can find it in areas of contact metamorphism or metasomatism. These are environments where heat and chemical changes occur. Other transition group metals can substitute for iron within the mineral. There is also an arsenopyrite group that includes rare minerals. These include clinosafflorite, gudmundite, glaucodot, iridarsenite, and osarsite.

Arsenopyrite-176706
Arsenopyrite-176706
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