Fe7S8

Fe7S8 is an iron sulfide mineral with the chemical formula Fe7S8. It is a nonstoichiometric member of the pyrrhotite group (Fe1−xS) representing one iron-rich composition in which iron vacancies reduce the iron content relative to FeS. The formula corresponds to x = 0.125, making Fe7S8 one of the more iron-rich end-members in the pyrrhotite series.

Fe7S8 crystallizes in a hexagonal lattice related to the NiAs-type structure, with iron vacancies arranged in ordered or semi-ordered patterns depending on the polytype. It has a metallic luster, a bronze to gray color, is opaque, and typically has a hardness around 3–4 on the Mohs scale. It is magnetic at room temperature, a common property of pyrrhotite minerals.

Fe7S8 forms in sulfide-rich magmatic systems under reducing conditions and is found in ultramafic and mafic rocks as an accessory phase. It occurs together with troilite (FeS) and pentlandite ((Fe,Ni)9S8) and is also found in meteorites and other planetary materials where pyrrhotite is present.

Although not a major ore, Fe7S8 and the pyrrhotite group are important for understanding magmatic differentiation and planetary processes. They can contribute to magnetic anomalies in rocks and influence the long-term behavior of sulfide-rich rock masses. In civil engineering, sulfide minerals such as pyrrhotite can oxidize and expand, potentially causing cracking in concrete, a durability concern in susceptible aggregates.