Pyrrhotite is an unusual iron sulfide mineral with a variable iron content: Fe(1-x)S (x = 0 to 0.2). The FeS endmember is known as troilite. Also called magnetic pyrite because the color is similar to pyrite and it is weakly magnetic, the magnetism increases as the iron content decreases.
Pyrrhotite is odd also because it has two crystal symmetries. When pyrrhotite is high in iron and the formula is closer to true FeS the structure is hexagonal. But, when it is low in iron, the structure is monoclinic. Both symmetries occur together is the same specimen.
The name is derived from Greek pyrrhos, flame-colored.
Pyrrhotite is a rather common trace constituent of igneous rocks. It occurrs as segregation deposits from mafic igneous rocks associated with pentlandite, chalcopyrite and other sulfides. It also occurrs in pegmatites and in contact metamorphic zones. The troilite endmember is found in meteorites.
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