Magnetic Properties of Sedimentary Smythite (Fe9s11)

Smythite (Fe9S11) is an occasionally reported magnetic iron sulfide mineral that occurs in varied geological settings and co‐occurs commonly with other magnetic iron sulfide minerals. Determining the magnetic properties of smythite is important for understanding its geological distribution and paleomagnetic and environmental magnetic significance. We report sedimentary smythite occurrences from three locations in Taiwan (one terrestrial and two marine), which suggest that smythite forms in methanic diagenetic environments into which sulfide has been reintroduced. We report the magnetic properties of our purest smythite sample, which also contains greigite, and compare them with those of other magnetic iron sulfide minerals. The magnetization of smythite is controlled by multiaxial anisotropy, with magnetic easy axes that lie within the crystallographic basal plane. Smythite is magnetically stable with no low‐temperature magnetic transition. Hysteresis and coercivity properties of stable single domain smythite are similar to those of greigite at and below room temperature. The magnetic properties of smythite at elevated temperatures are dominated by thermal alteration, which precludes Curie temperature determination. In contrast to greigite and like pyrrhotite polytypes, smythite crystals occur as hexagonal plates. This morphological contrast facilitates discrimination of smythite from greigite in electron microscope observations, but it does not assist discrimination from pyrrhotite. Similar magnetic and morphological properties between smythite and other magnetic iron sulfides means that diagnostic mineralogical analyses (e.g., X‐ray diffraction) are needed to identify these minerals.