Fe2S2

Fe2S2 is the chemical formula used to denote a compound consisting of iron and sulfur in a 1:1 ratio. In standard chemical practice, this composition is equivalent to FeS, iron(II) sulfide, because the two iron atoms and two sulfur atoms can be reduced to a one-to-one ratio. Consequently, Fe2S2 does not correspond to a widely recognized discrete phase in mineralogy or solid-state chemistry; most literature treats the material as iron sulfide (FeS).

In mineral form, the stable iron sulfide known is troilite, which has the composition FeS and a hexagonal NiAs-type structure. If a hypothetical pure Fe2S2 phase existed, its crystal structure and properties would align with those of FeS, since the empirical formula reduces to FeS.

Iron sulfides are common in meteorites and sulfide-rich ore deposits. Troilite occurs as inclusions in iron meteorites and as a minor phase in terrestrial sulfide systems. In many contexts, industrially relevant iron sulfide materials are prepared by direct reaction of iron with sulfur, but a separate Fe2S2 compound is not typically isolated.

Nonstoichiometry is common in iron sulfide systems. Formulations such as Fe1−xS describe a range of compositions with iron vacancies, leading to multiple phases (the pyrrhotite-Fe1−xS series). These nonstoichiometric relationships explain why Fe2S2 is rarely observed as a distinct, pure phase.

Related topics include FeS, FeS2, troilite, and pyrrhotite.