Fundamental magnetic properties of Greigite (Fe3S4)
University of Southampton, School of Ocean and Earth Science,
Over the last twenty years, greigite (Fe3S4), an authigenic magnetic iron sulphide mineral, has been increasingly identified in sulphate-reducing marine and lacustrine sedimentary systems. Its presence can significantly affect palaeomagnetic and environmental magnetic records. Understanding the recording characteristics of any magnetic mineral requires that its fundamental magnetic properties are known. However, unlike magnetite (Fe3O4) and other common terrestrial magnetic minerals, the fundamental magnetic properties of greigite are poorly known. The metastability of greigite and difficulties in producing pure greigite samples make it difficult to characterize its magnetic properties. In this study, pure synthetic greigite samples were produced using a new hydrothermal method. Sample purity has been confirmed by X-ray diffraction, neutron diffraction, Mössbauer spectroscopy and elemental analyses. Based on this suite of highly pure synthetic greigite samples, as well as a range of natural greigite samples collected from widely distributed localities, extensive analyses have documented some of the fundamental magnetic properties of greigite. The saturation magnetization (Ms) of greigite was determined to be 59 Am2kg-1 (equivalent to 3.13 µB/formula unit (f.u.)) from magnetic measurements and 3.03 µB/f.u. from neutron scattering. Neutron scattering, Mössbauer spectroscopy and X-ray magnetic circular dichroism were used to probe the magnetic structure of greigite. Results confirm that greigite has a collinear ferrimagnetic structure with antiferromagnetic coupling between the tetrahedral and the octahedral sites, but with lower magnetic moments of Fe ions for the two sublattices compared to magnetite. The low magnetic moment in greigite compared to magnetite is probably caused by an increased degree of covalency between iron and sulphur compared to oxygen ligands, and/or by greater delocalization of the 3d electrons. By measuring the low-temperature Ms of pure greigite samples and based on the Bloch spin wave expansion, the spin wave stiffness of greigite was determined to be ~193 meV·Å2, with a corresponding exchange constant JAB of ~1.03 meV. High-temperature magnetic analyses on a stable natural greigite sample from Italy indicate that the Curie temperature of greigite most exceed 350°C. However, thermal alteration of greigite at elevated temperatures precludes determination of its Curie temperature. Production of large-grained synthetic greigite samples enables an extension of our knowledge of the magnetic properties of greigite to include the pseudo-single-domain/multi-domain region, which now provides coverage of the complete domain state range. Low-temperature magnetic analyses indicate that greigite has strong domain-state dependence of magnetic properties, although greigite has no low-temperature magnetic transitions. These fundamental studies provide important new magnetic data for greigite that will benefit future efforts to model magnetizations of greigite in a wide range of palaeomagnetic and environmental magnetic contexts.
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