Mineral Properties and Uses

Arsenopyrite and milky quartz on limestone from the Smith Vein, Carrock Mine, Caldbeck Fells, Cumberland, Cumbria, England. Specimen is approximately 8.8 x 6.2 x 4.8 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

What is Arsenopyrite?

Arsenopyrite is an iron arsenic sulfide mineral with a chemical composition of FeAsS. It is the most abundant arsenic mineral and the primary ore of arsenic metal. In addition to being found in deposits that are large enough to be minable, arsenopyrite is widely distributed. However, it usually occurs in such small amounts and in such small particle sizes that it is easily overlooked. It is associated with other sulfide minerals in organic-rich sedimentary rocks, metamorphic rocks, and igneous rocks in many parts of the world.

Physical Properties of Arsenopyrite

Chemical ClassificationSulfide
ColorSilvery-white to steel gray
Mohs Hardness5.5 to 6
Specific Gravity5.9 to 6.2
Diagnostic PropertiesHigh specific gravity. A slight odor of garlic might be noticed when arsenopyrite is crushed, broken or scraped across a streak plate. A garlic odor is also released when arsenopyrite is heated to a temperature that causes alteration. The fumes produced by heating can be toxic.
Chemical CompositionIron arsenic sulfide, FeAsS
Crystal SystemMonoclinic
UsesThe primary ore of arsenic metal. Arsenic is toxic to many organisms, and that is its role in insecticides, herbicides, pesticides, chemical weapons and other poisons. Arsenic metal is used to produce specialty alloys. Oxides of arsenic are used to make pigments. Modern organic compounds are displacing arsenic from many of its traditional uses.

Geologic Occurrence

Much of the arsenopyrite that has been mined formed as a high-temperature mineral in hydrothermal veins. It is often mined, together with other metallic minerals, from veins that might contain gold, silver, lead, tungsten, or tin. In these deposits arsenopyrite usually occurs in a granular massive form. It is often intergrown with other sulfide minerals such as chalcopyrite, galena, pyrrhotite, pyrite, and sphalerite; precious metals such as gold and silver; or other minerals such as scheelite, cassiterite, and quartz.

Arsenopyrite has also been mined from sulfide deposits formed by contact metamorphism. It is occasionally found in pegmatites. Well-formed crystals of arsenopyrite are most often found in pegmatites and in carbonate rocks that have been altered by contact metamorphism.

Significant amounts of arsenopyrite have been produced from deposits in Germany, England, Bolivia, Japan, Greece, Spain, Sweden, Mexico, and Japan. In North America deposits are located in Ontario, Canada, and in South Dakota, New Jersey, and New Hampshire in the United States.

Arsenopyrite and Glaucodot

In some deposits, cobalt will substitute for some of the iron in the arsenopyrite crystal structure. This produces a solid solution series between arsenopyrite (FeAsS) and glaucodot ((Co,Fe)AsS).

Arsenopyrite: Massive granular arsenopyrite from Gold Hill, Utah. Specimen is approximately 10 centimeters across.

Weathering of Arsenopyrite

Arsenopyrite is unstable in most environments of Earth's surface. It easily alters from its silver-white or steel-gray color to yield a bronze to brown tarnish. If a geologist strikes the mineral with a hammer to view a fresh surface or does a streak test, an odor of garlic might be detected. This is a characteristic of many arsenic minerals and a clue that arsenopyrite might be present.

Arsenopyrite often oxidizes to form scorodite, a hydrated iron arsenate mineral with a chemical composition of FeAsO4.2H2O. Scorodite often weathers to limonite, an amorphous, hydrated iron oxide of variable composition. In areas where mining has exposed large amounts of sulfide ores, arsenopyrite can be a contributor to acid mine drainage problems.

Arsenopyrite as a Source of Gold

Arsenopyrite sometimes contains gold that is so small that it cannot be detected with a hand lens. This "invisible gold" can sometimes be recovered in economic quantities by crushing the ore, concentrating the heavy fraction, and treating the heavy fraction with cyanide to dissolve the gold.

The "invisible gold" occurs in two forms: 1) tiny particles of elemental gold; and, 2) gold that is chemically bound within the arsenopyrite. Elemental gold exposed by crushing can be removed with the cyanide. The chemically bound gold is more difficult to remove without smelting.

The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Store.

Arsenopyrite By Other Names

The name “arsenopyrite” is a contraction of “arsenical pyrites,” an archaic name used for the mineral. “Mispickel” is another name for arsenopyrite.

Uses of Arsenopyrite and Arsenic

Arsenopyrite is the primary ore of arsenic metal, which is used to produce alloys. It was historically used to harden lead in ammunition, but this use has declined in the past few decades.

Much arsenic is recovered by smelting in the form of arsenious oxide. This is used to produce a variety of insecticides, herbicides, pesticides, and chemical weapons. Arsenic compounds are also used in medicines, as pigments in paints, and to produce color in fireworks and glass.