Pyrite (FeS2) is known as “Fool’s Gold” due to its metallic luster and brassy yellow hue resembling gold. Despite this, pyrite has distinct characteristics and a rich history of applications, from ancient to modern uses.
Identifying Pyrite
Pyrite can be distinguished from gold by physical properties. Pyrite is harder (6-6.5 Mohs) than gold (2.5-3). Pyrite scratches glass; gold is scratched by a copper penny or knife. On unglazed porcelain, pyrite leaves a greenish-black or brownish-black streak, contrasting with gold’s yellow streak.
Crystal formations also distinguish pyrite. Pyrite often forms well-defined cubic, octahedral, or pyritohedral crystals, sometimes with striations. Gold typically forms irregular shapes, flakes, or nuggets; well-formed crystals are rare. Pyrite is brittle, shattering when struck, while gold is malleable, deforming without breaking.
Historical Applications
Pyrite played significant historical roles, particularly for its spark-producing ability. Its name, from the Greek “pyr” (fire), reflects this; striking it against metal generates sparks. This made it useful in early firearms, notably wheel-lock mechanisms (16th-17th centuries), where pyrite ignited gunpowder against a serrated wheel. Nodules were also used for fire-starting in prehistoric burial mounds.
Pyrite was also a primary historical source of sulfur, essential for sulfuric acid production (historically “oil of vitriol”). Sulfuric acid was crucial for many industrial processes during and after the Industrial Revolution. Pyrite also served as a minor source of iron.
Industrial and Technological Uses
Pyrite remains commercially relevant as a source of sulfur for sulfuric acid production, a fundamental chemical used in fertilizers, detergents, and many other chemicals. Though much sulfur now comes as a byproduct of petroleum and natural gas processing, pyrite contributes to this supply.
Modern technology explores new applications for pyrite. It shows promise in thermoelectric materials, converting heat directly into electrical energy. Pyrite is also researched as a cathode material in certain lithium-ion batteries. Its semiconducting properties, earth abundance, and non-toxicity make it a candidate for low-cost thin-film solar cells. Research indicates pyrite has a suitable band gap and high absorption coefficient, comparable to or exceeding silicon in certain light spectrums, suggesting its future role in sustainable energy.
Decorative and Collectible Appeal
Pyrite’s aesthetic qualities make it popular for decorative and collectible purposes. Its metallic luster and precise crystal formations (cubes, octahedrons, pyritohedrons) appeal to mineral collectors. These well-formed crystals are prized for their geometric beauty.
Pyrite is frequently used in jewelry, often as polished cabochons or beads. It is also the primary mineral in “marcasite jewelry,” popular during the Victorian era. Despite the name, this jewelry features small, faceted pyrite pieces, showcasing its metallic sheen and sparkle.