SUMMARY Ferrimagnetic, monoclinic 4C pyrrhotite (Fe7S8) is the only iron sulphide with high relevance for palaeomagnetism and rock magnetism that can be identified in materials by its characteristic low-temperature anomaly. Despite natural many magnetic studies over last decades, physics crystallography behind this anomaly, also denoted Besnus transition, a matter of debate. In study, we analys...