Iron sulfide group of minerals whose composition ranges widely between its end members pyrrhotite (Fe7S8) whose crystal structure is monoclinic, and troilite (FeS) whose crystal structure is hexagonal. Its general formula is Fe1−xS (where x = 0 to 0.17). The troilite phase is found mainly in meteorites and in the sulfide melts produced by the meteoritic impact like those found around the Sudbury impact structure1. The magnetic properties of pyrrhotite are due to the iron vacancies in the crystal structure. The greater the number of Fe vacancies, the stronger the magnetic susceptibility. This deficiency results from the substitution 3Fe2+ ↔ 2Fe3+ + o (where o is an iron vacancy). Since troilite has no vacancies, it is not magnetically susceptible while monoclinic pyrrhotite is the most magnetically susceptible.

When referring to pyrrhotite, meteoriticists most often mean Fe-deficient (x > 0), and when they say troilite, they mean stoichiometric (x = 0 so that Fe2+ + S2- combine to form FeS). If an analysis is not provided, meteoriticists are usually just assuming the mineral is what is normal for that meteorite type. Note that troilite  may be referred to as 2C pyrrhotite, while Fe7S8 may be referred to as 4C pyrrhotite in some literature.

Pyrrhotite has been found as an accessory mineral in some carbonaceous chondrites (TBD)2, R chondrites (TBD)3, ordinary chondrites (Soko-Banja (LL4))4, shergottites (Tissint)5, nakhlites (Nakhla, and Governador Valadares)6 , irons (Bocaiuva)7 and within troilite nodules in many other iron meteorites, and other meteorites.