ironsulfide

Iron sulfide is a term for compounds consisting of iron and sulfur. The principal natural iron sulfides are troilite (FeS), iron disulfide (FeS2), which occurs as two polymorphs known as pyrite and marcasite, and the iron-deficient solid solution pyrrhotite (Fe1−xS), where x varies with composition and ordering.

Pyrite and marcasite share the same chemical formula but differ in crystal structure; pyrite is cubic whereas marcasite is orthorhombic. Troilite, FeS, is a distinct mineral phase. Pyrrhotite exhibits a range of iron deficiency and often shows magnetic properties that depend on its exact composition. These minerals range from relatively pure sulfides to mixed iron sulfide phases in natural settings.

Natural occurrence varies by form. Pyrite is one of the most common sulfide minerals and is found in a wide range of rock types, including sedimentary, metamorphic, and hydrothermal deposits. Marcasite is less common and tends to form under particular, often low-temperature conditions. Pyrrhotite is common in mafic and ultramafic rocks and in some ore deposits. Troilite is relatively rare on Earth but is abundant in meteorites and certain lunar and planetary materials.

Chemical behavior and environmental relevance. FeS and FeS2 are relatively insoluble, but they react with acids to release hydrogen sulfide. Oxidation of pyrite and pyrrhotite can generate sulfuric acid, contributing to acid rock drainage in mining environments. In industrial contexts, sulfide minerals influence ore processing and can affect the properties of steels and other iron-based materials.

Geological and industrial significance. Iron sulfide minerals serve as indicators of geological processes and ore potential, and their oxidation products have important environmental implications.