Marcasite

Marcasite is an iron sulfide, FeS2, and the orthorhombic polymorph of pyrite, sometimes traded historically as "white iron pyrite." On fresh surfaces it shows a pale bronze-yellow to almost tin-white color, distinct from the deeper brassy tone of pyrite, and its gray-black streak (versus pyrite's slightly greenish-black streak) is a key diagnostic. Crystals are typically flattened or lens-shaped, and marcasite is famous among collectors for its distinctive twinning: the classic "cockscomb" habit, formed by repeated twinning on {101}, produces crested, comb-like rosettes, while the related spear-shaped twin known as "sperkise" is especially characteristic of specimens from chalk deposits. Radiating clusters called "marcasite dollars" and stalactitic or reniform masses are also well known, and the mineral commonly forms pseudomorphs after pyrite, gypsum, and fluorite, or alters to goethite and melanterite over time. Geologically, marcasite forms under low-temperature, mildly acidic conditions, either as a primary sedimentary mineral in nodules and concretions or as a hydrothermal and secondary product replacing pyrrhotite and chalcopyrite. The type locality material comes from the chalk cliffs on both sides of the English Channel, at Dover, Kent, and Cap Blanc-Nez, Pas-de-Calais, where sharp individual crystals, sperkise twins, and nodules are classically collected. In Spain, the Reocín mine in Cantabria is renowned for cockscomb crystal groups from complex sulfide ore. North America has produced some of the finest cabinet specimens: the Tri-State district spanning Missouri, Kansas, and Oklahoma — including the Joplin field and Picher field — yielded superb cockscomb marcasite on sphalerite, galena, and chert, while the Sweetwater Mine in Reynolds County, Missouri, is famous for iridescent, reddish-tarnished cockscomb clusters. Marcasite is also a defining mineral of soft-coal deposits in the northwestern Czech Republic, at localities such as Vintířov and Komořany, and occurs in the Upper Mississippi Valley zinc-lead district of Wisconsin, plus notable occurrences at Santa Eulalia, Chihuahua, Mexico, and Llallagua, Bolivia. Collectors seek marcasite for its striking crystal architecture rather than rarity, since well-formed cockscomb and sperkise groups display a sculptural, almost botanical beauty unmatched among sulfides. Because marcasite is notoriously unstable and prone to "pyrite decay" — oxidizing to iron sulfate and sulfuric acid under humid conditions — fine, undamaged old specimens from classic localities are increasingly prized and require low-humidity storage to preserve their luster and crystal integrity for the long term.