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Licensed Unlicensed Requires Authentication Published by De Gruyter September 28, 2018

Changes in physical properties of 4C pyrrhotite (Fe7S8) across the 32 K Besnus transition

Michael W.R. Volk, Eric McCalla, Bryan Voigt, Michael Manno, Chris Leighton and Joshua M. Feinberg
From the journal American Mineralogist


Pyrrhotite, Fe7S8, is a common sulfide mineral in the Earth’s crust and mantle, as well as in a range of meteorites and is of interest to a wide variety of disciplines including economic geology, geophysics, and material science. The 4C variety of pyrrhotite shows a dramatic change in magnetic properties at T ≈ 30 K, known as the Besnus transition. Although this transition is frequently used to detect pyrrhotite in geologic samples, the underlying mechanism driving the transition has not yet been identified. This study presents a high-resolution view of the changes in heat capacity, magnetic, and electronic properties of a natural single crystal of nearly pure, monoclinic 4C pyrrhotite across the Besnus transition. Contrary to previous studies, all of these properties show clear evidence of the Besnus transition, specific heat, in particular, revealing a clear transition at 32 K, apparently of second-order nature. Small-angle neutron scattering data are also presented, demonstrating an unusual change in short-range magnetic scattering at the transition. Furthermore, a magnetic field dependence of the transition temperature can be seen in both induced magnetization and electrical resistivity. These new observations help narrow the possible nature of the phase transition, clearly showing that interactions between intergrown coexisting 4C and 5C superstructures, as suggested in some literature, are not necessary for the Besnus transition. In fact, the changes seen here in both the specific heat and the electronic transport properties are considerably larger than those seen in samples with intergrown superstructures. To further constrain the mechanism underlying the Besnus transition, we identify five separate potential models and evaluate them within the context of existing observations, thereby proposing experimental approaches that may help resolve ongoing ambiguities.


We thank Anette von der Handt for acquiring electron microprobe data and Jeanette Voelz for help in measuring powder X-ray diffraction data. Furthermore, we thank Rupert Hochleitner and the Bavarian Mineralogical State Collection for providing the sample material and for their kind support. E.M., B.V, M.M., and C.L. acknowledge support from the DOE through the UMN Center for Quantum Materials under DE-FG02-06ER46275 and DE-SC-0016371. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron facilities used in this work; the assistance of John Barker is gratefully acknowledged in that regard. The Institute for Rock Magnetism is a U.S. National Multiuser Facility supported through the NSF-EAR Instrumentation and Sciences Program and by funding from the University of Minnesota. This is IRM publication (no. 1710).

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Received: 2018-02-16
Accepted: 2018-06-11
Published Online: 2018-09-28
Published in Print: 2018-10-25

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