[7][page needed] Materials with the NiAs structure often are non-stoichiometric because they lack up to 1/8th fraction of the metal ions, creating vacancies.

[10] The ferromagnetism which is widely observed in pyrrhotite is therefore attributed to the presence of relatively large concentrations of iron vacancies (up to 20%) in the crystal structure.

Unlike other common brassy-colored sulfide minerals, pyrrhotite is typically magnetic (varies inversely with iron content).

[14] Diagnostic characteristics in hand sample include: brassy/bronze color with a grey/black streak, tabular or hexagonal crystals which show iridescence, subconchoidal fracture, metallic luster, and magnetic.

[19] The following optical properties[20] are representative of polished/puck sections using ore microscopy: Pyrrhotite typically appears as anhedral, granular aggregates and is cream-pink to brownish in color.

[20] Pyrrhotite will not display twinning or internal reflections, and its strong anisotropy from yellow to greenish-gray or grayish-blue is characteristic.

[20] Diagnostic characteristics in polished section include: anhedral aggregates, cream-pink to brown in color and strong anisotropy.

It is an important constituent of the Sudbury intrusion (1.85 Ga old meteorite impact crater in Ontario, Canada) where it occurs in masses associated with copper and nickel mineralisation.

[6] Pyrite also decomposes into pyrrhotite in hot reductive technogenic environments, such as blast furnaces[22] and direct coal liquefaction (in which it is an important catalyst).

[6] Black smokers release high sulfur concentrations onto the sea floor, and when the surrounding rocks are metamorphosed, pyrrhotite can crystallize.

[6][24][25][26] In the eastern United States, pyrrhotite occurs in highly metamorphosed rock that forms a belt along the Appalachian Mountains.

[6] The following are some of the locations worldwide where pyrrhotite has been reported during mining:[15] Note: This was a quarry producing crushed rock aggregate for use in construction Named in 1847 by Ours-Pierre-Armand Petit-Dufrénoy.

[42][43] The iron sulfide it contains can naturally react with oxygen and water, and over time pyrrhotite breaks down into sulfuric acid and secondary minerals like ettringite, thaumasite and gypsum.

[38][6] These secondary products occupy a larger volume than pyrrhotite, which expands and cracks the concrete leading to home foundation or block failure.

[44][45] Iron is seldom extracted from pyrrhotite due to a complicated metallurgical process[44] It is mined primarily because it is associated with pentlandite, a sulfide mineral that can contain significant amounts of nickel and cobalt.