Ferrimagnetic Structure of 3C Pyrrhotite (Fe₇S₈) From Neutron Diffraction

Abstract Pyrrhotite is a paleomagnetically important magnetic mineral in many geological settings. It forms numerous polytypes with different stacking patterns of the NiAs structure along the crystallographic c axis to produce different vacancy‐ordered superstructures. Monoclinic 4C pyrrhotite (Fe 7 S 8 ) is the best‐known ferrimagnetic member of the pyrrhotite family; the magnetic properties of other pyrrhotite polytypes remain largely unknown. Recent discovery of the importance of magnetism in 3C pyrrhotite (Fe 7 S 8 ) in methanic sedimentary environments makes it important to establish its magnetic structure and magnetic properties. We present powder neutron diffraction results at low and high temperatures, which enable determination of the magnetic structure of 3C compared to 4C pyrrhotite. We find that 3C pyrrhotite is a collinear ferrimagnet with a saturation magnetization of 2.714(3) μ B at 300 K, which is less than the 3.048(3) μ B determined for our measured 4C pyrrhotite sample. Our analyses indicate iron deficiency in both studied samples, which likely reduces the expected net magnetization compared to the respective fully vacancy ordered cases. The studied 3C pyrrhotite is thermally unstable above 390 K. Demonstration of the ferrimagnetism of 3C pyrrhotite, which has contrasting magnetic properties to 4C pyrrhotite, has important implications for interpreting sedimentary magnetic signals. This work indicates a need to document more fully the magnetic properties of pyrrhotite polytypes, including 3C pyrrhotite with variable vacancy ordering.