pyrrhotite

Pyrrhotite is an iron sulfide mineral with the approximate formula Fe1−xS, where x ranges up to about 0.125. Its iron content is non-stoichiometric, and the mineral occurs in several crystal forms, notably monoclinic and hexagonal polytypes. The magnetic properties of pyrrhotite vary with composition, and some forms are distinctly magnetic.

Pyrrhotite commonly occurs in mafic to ultramafic igneous rocks, as well as in hydrothermal veins and contact metamorphic deposits. It is often found with other sulfide minerals such as pyrite, chalcopyrite, pentlandite, and sphalerite, and it can be a characteristic component of sulfide-rich magmatic ore systems. In meteorites, iron sulfide is represented by troilite (FeS), while pyrrhotite-like non-stoichiometric phases are also known.

In terms of physical properties, pyrrhotite typically has a metallic luster and a brass-brown to bronze color, with a hardness of about 3–4 on the Mohs scale. It has a dark gray to brownish streak, poor cleavage, and a brittle fracture. Many samples exhibit magnetism due to their iron content, and the mineral has a specific gravity near 4.5 g/cm³.

Economic and geological significance lies mainly in its role as an indicator of sulfide-rich ore systems rather than as a primary iron ore. Pyrrhotite can complicate ore processing because of its variable composition and magnetic behavior, and it is used by geologists to trace magmatic sulfide assemblages.

Weathering of pyrrhotite can produce oxides and sulfates, and in some contexts pyrrhotite-bearing aggregates in concrete can undergo expansion when exposed to moisture, potentially causing structural cracking. The name derives from the Greek pyrrhos, meaning fiery or red-brown, reflecting its color and luster.