Pyrrhotite, with a variable iron content, is an unusual mineral of iron sulfide: Fe (1-x) S (x = 0 to 0.2). Troilite is the name given to the endmember, FeS. Pyrrhotite is referred to as magnetic pyrite since its color has similarity with that of Pyrite, but with a weak magnetic inclination that goes down with decreased iron content. Troilite is not magnetic in the least.
Chemical Formula Fe1-xS (x = 0 to 0.2
Derivation of Name
It gets its name from Greek pyrrhos, meaning flame-colored.
Pyrrhotite is quite a common trace component of especially norites that are mafic igneous rocks found associated with sulfides like chalcopyrite and pentlandite and others in layered intrusions as segregation deposits. It forms an important part of the intrusion of Sudbury where it is found as masses linked with nickel and copper mineralization. You can also see them in pegmatites in zones of metamorphic contacts. Pyrrhotite is generally found along with Pyrite, Magnetite and Marcasite. It has no specific applications.
Pyrrhotite possesses some strange properties. First, it has an odd formula. The amount of sulfur varies by about 20% or 50-55 atoms of sulfur per 50 atoms of iron. Or is it iron that fluctuates? Really, the difference is the same, thus giving it a strange formula of Fe1-xS. Then, it has 2 symmetries. Though it should show the presence of two minerals, in case of pyrrhotite, mineralogists decided to have an exception. When sulfur content is low and the formula is nearer to the correct formula, FeS, the structure becomes hexagonal. But as sulfur content goes high, its structure becomes monoclinic. Visibly 2 different symmetries with 2 different formulae and thus two different minerals come into being, but, crystals of natural Pyrrhotite contain both the phases within the same crystal. As a conformist, you could consider a Pyrrhotite crystal as a collection of 2 minerals, but the majority of mineralogists prefers treating it as one. Moreover, Pyrrhotite has magnetic properties, though weak.
Its color variation is from bronze to dark brown