pyrite

pyrite, a naturally occurring iron disulfide mineral. The name comes from the Greek word pyr, “fire,” because pyrite emits sparks when struck by metal. Pyrite is called fool’s gold; to the novice its colour is deceptively similar to that of a gold nugget. Nodules of pyrite have been found in prehistoric burial mounds, which suggests their use as a means of producing fire. Wheel-lock guns, in which a spring-driven serrated wheel rotated against a piece of pyrite, were used before development of the flintlock. Pure pyrite (FeS₂) contains 46.67 percent iron and 53.33 percent sulfur by weight. Its crystals display isometric symmetry. For detailed physical properties, see sulfide mineral.

Pyrite is widely distributed and forms under extremely varied conditions. For example, it can be produced by magmatic (molten rock) segregation, by hydrothermal solutions, and as stalactitic growth. It occurs as an accessory mineral in igneous rocks, in vein deposits with quartz and sulfide minerals, and in sedimentary rocks, such as shale, coal, and limestone.

Pyrite occurs in large deposits in contact metamorphic rocks. Deposits of copper-bearing pyrite are widely distributed and often of great size. They usually occur in or near the contact of eruptive rocks with schists or slates. Pyrite weathers rapidly to hydrated iron oxide, goethite, or limonite; pseudomorphs of goethite after pyrite are common. This weathering produces a characteristic yellow-brown stain or coating, such as on rusty quartz.

Historically, pyrite was used commercially as a source of sulfur, particularly for the production of sulfuric acid, but today sulfur is largely collected as a by-product of petroleum processing. Because of the availability of much better sources of iron, pyrite is not generally used as an iron ore.

For many years Spain was the largest producer, the large deposits located on the Tinto River being important also for copper. Today Italy and China are the world’s largest producers, followed by Russia and Peru.