Formula: Fe_1-xS. Also given as Fe_1-x (x = 0 to 0.17). The various polytypes known have slightly different stoichiometries.

Species: Sulfides

Colour: Bronze brown, bronze red, or dark brown

Lustre: Metallic

Hardness: 3½ – 4

Specific Gravity: 4.58 – 4.65

Crystal System: Monoclinic

Member of: Pyrrhotite Group

Name: Named in 1847 by Ours-Pierre-Armand Petit-Dufrénoy from Greek πνρρός “pyrrhos”, flame-colored.

Pyrrhotite is found with pentlandite in basic igneous rocks, veins, and metamorphic rocks. It is also often found with pyrite, marcasite, and magnetite. It has varying magnetic powers, depending on the number of Fe vacancies in the crystal structure. A related species with no vacancies (and therefore non-magnetic), is called troilite and has been found in meteorites and rarely terrestrially. Visually similar to smythite and troilite.