What is Pyrrhotite?
Pyrrhotite is a magnetic iron sulfide whose bronze colour and variable iron content make it a familiar mineral of mafic and hydrothermal ore systems.
Mineralogy and crystal structure
Reference formula: Fe1-xS. Pyrrhotite has vacancies in its iron sites and several polytypes, so the ideal formula and symmetry vary with composition. Magnetic response is useful but not by itself diagnostic. Pyrrhotite is classified with sulfides and has a hexagonal crystal system. Typical hardness is 3.5–4.5 on the Mohs scale and specific gravity is approximately 4.5–4.8.
Colour, lustre and habit
Typical colours include bronze, brown, black, grey. The usual hexagonal, massive, granular habit and metallic, sub-metallic lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Pyrrhotite forms in or is associated with magmatic sulfide deposits, hydrothermal veins and metamorphosed iron-sulfide assemblages. Important documented localities include Canada, Russia, Norway, South Africa, Finland and the United States.
A magnetic sulfide with changing symmetry
Pyrrhotite makes crystal chemistry visible: tiny changes in iron vacancies alter polytype, magnetism and the way a bronze sulfide records its geological history.
Collecting and care
Keep dry, avoid strong magnets near sensitive equipment and protect metallic surfaces from oxidation.
