marcasite

Marcasite is an iron sulfide mineral with the chemical formula FeS2. It is the orthorhombic polymorph of iron disulfide, distinct from pyrite, which has a cubic crystal structure. Although they share the same chemical composition, marcasite and pyrite have different crystal habits, with marcasite commonly forming tabular or prismatic crystals that can be striated.

Marcasite typically exhibits a metallic luster and a pale brass-yellow to grayish color. It is relatively soft and brittle, with a Mohs hardness around 6. Compared with pyrite, marcasite cleaves more readily and fractures easily, which makes it less durable for some applications. When exposed to air and moisture, marcasite tends to oxidize and weather, sometimes forming sulfuric acid as it deteriorates over time.

Geologic occurrence and formation are associated with low-temperature hydrothermal environments and sedimentary settings. It occurs worldwide, often in association with other iron sulfide minerals in shales, coal beds, and carbonate rocks, and can appear in veins and nodules.

Uses and stability: because of its sensitivity to moisture and air, marcasite is not favored for long-term use as a gemstone. Historically, small crystals were incorporated into jewelry, but the mineral can degrade. Modern pieces that resemble marcasite typically use simulated stones or other iron sulfides; real marcasite is less common in contemporary jewelry but remains of interest in mineralogy and collectibility.

Notes: marcasite is recognized as the orthorhombic form of FeS2, differentiated from the more common pyrite by its crystal structure, habit, and stability.