Pyrrhotit

Pyrrhotite is an iron sulfide mineral with the formula Fe1−xS, where x denotes iron deficiency due to vacancies in the iron sublattice. The composition ranges from nearly stoichiometric FeS to Fe0.875S, and the nonstoichiometry leads to a family of crystal structures and polytypes.

Crystallography: Pyrrhotite exists in several polytypes, including hexagonal (NiAs-type) structures for Fe-rich compositions and monoclinic structures for more iron-poor variants. The arrangement of iron vacancies influences the unit cell and physical properties, giving rise to different magnetic and structural characteristics.

Physical properties: It has a metallic luster and a bronze to steel-gray color, with a hardness of about 3.5 to 4 on the Mohs scale and a dark gray to black streak. It is ferrimagnetic and often strongly magnetic at room temperature, a feature that makes it useful in magnetic studies of rocks and sulfide ore deposits.

Occurrence and associations: Pyrrhotite forms in magmatic and metamorphic rocks, especially in mafic to ultramafic rocks, and is a common constituent of nickel–copper sulfide ore deposits where it occurs with pentlandite, chalcopyrite, and pyrite. It is also found in meteorites and lunar samples, highlighting its widespread occurrence in both terrestrial and extraterrestrial contexts.

Significance and hazards: It is not a major iron ore due to its variable composition. Pyrrhotite serves as an indicator mineral for sulfide systems and aids in ore exploration. In civil engineering, oxidation of pyrrhotite-containing aggregates can cause expansion and cracking in concrete, a problem observed in some regions.