The Low-Temperature Besnus Magnetic Transition: Signals Due to Monoclinic and Hexagonal Pyrrhotite

The low-temperature magnetic properties of the many pyrrhotite varieties have not been studied extensively. Monoclinic pyrrhotite (Fe₇S₈) goes through the Besnus transition at ~30–34 K, which is used widely to diagnose its presence in bulk samples. Other pyrrhotite polytypes are assumed to be antiferromagnetic, although it has been suggested occasionally that some may also have remanence-carrying capabilities. Here we compare the magnetic properties of monoclinic (4M) and hexagonal (3T) pyrrhotite at low temperatures. The 4M pyrrhotite records a Besnus transition consistently. Despite not recording a Besnus transition, 3T pyrrhotite has a magnetic remanence at room temperature and has distinctive room- and low-temperature magnetic properties that cannot be explained by known or unidentified impurities (with abundances <0.1%). Some 3T-pyrrhotite samples have exceptionally high (>700 mT) and stable coercivities below 50 K. The importance of this mineral in fossil or active gas hydrate and methane venting environments makes it important to develop a more detailed understanding of its occurrences and magnetic properties.