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Changes in physical properties of 4C pyrrhotite (Fe (sub 7) S (sub 8) ) across the 32 K Besnus transition

Michael W. R. Volk, Eric McCalla, Bryan Voigt, Michael Manno, Chris Leighton and Joshua M. Feinberg
Changes in physical properties of 4C pyrrhotite (Fe (sub 7) S (sub 8) ) across the 32 K Besnus transition
American Mineralogist (October 2018) 103 (10): 1674-1689


Pyrrhotite, Fe (sub 7) S (sub 8) , 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 nearly equal 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.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 103
Serial Issue: 10
Title: Changes in physical properties of 4C pyrrhotite (Fe (sub 7) S (sub 8) ) across the 32 K Besnus transition
Affiliation: University of Minnesota, Department of Earth Sciences, Minneapolis, MN, United States
Pages: 1674-1689
Published: 201810
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 63
Accession Number: 2019-007193
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: McGill University, CAN, Canada
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201906
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