Pyrrhotite with pentlandite from the Sudbury Impact Structure in Ontario, Canada. (field of view 8.35 cm across)

Pyrrhotite is imperfect iron monosulfide (Fe(1-x)S). The atomic structure of pyrrhotite has holes due to an insufficient number of iron atoms, cf. sulfur atoms. Iron monosulfide is a common, but minor, component of many meteorites, but it lacks the atomic-scale “holes” of pyrrhotite, and is called troilite (FeS).

Pyrrhotite is superficially like pyrite in appearance and chemistry, but they are different minerals. Pyrrhotite has a metallic luster, a brownish-brassy or bronzish color, a black streak, no cleavage, and is magnetic. What’s particularly distinctive about pyrrhotite is that it is variably magnetic. The holes in the atomic structure gives pyrrhotite its magnetism. But, there's variation in the number of missing iron atoms from sample to sample, so pyrrhotite ends up having variable magnetism. More holes results in stronger magnetism. Few holes results in weaker magnetism.

Shown above is a polymineralic massive sulfide rock from Canada's Sudbury Complex that is dominated by pyrrhotite but also has significant pentlandite and magnetite. The Sudbury Complex (Sudbury Basin) in southeastern Canada has intrigued geologists for decades, and not just due to the tremendous economic value of the area’s mineral deposits. Sudbury is one of the largest preserved impact structures on Earth. The impact occurred ~1.85 billion years ago, during the late Paleoproterozoic. The Sudbury Impact Structure is no longer circular or subcircular in shape, however - it's been compessed into a stretched-egg shape from an ancient continental collision event.

This massive sulfide specimen consists of brassy gray-brown pyrrhotite (Fe(1-x)S - imperfect iron monosulfide) with brighter brassy-colored patches of pentlandite ((Ni,Fe)9S8 - nickel iron sulfide), plus a network of grayish to black patches of magnetite (Fe3O4 - iron oxide).

Geologic Context & Age: massive sulfide, 800 Orebody at contact of the Copper Cliff offset dike (quartz diorite) & McKim Formation deltaic metapelites (upper Elliot Group, lower Huronian Supergroup, lower Paleoproterozoic, 2.45 b.y.), sulfide mineralization was syn-impact or early post- Impact, 1.85 Ga

Locality: 800 Orebody, South Mine (Copper Cliff South Mine), near Sudbury, southeastern Ontario, southeastern Canada

Photo gallery of pyrrhotite: