Magnetic Properties
The ideal FeS lattice, such as that of troilite, is non-magnetic. The ferromagnetism which is widely observed in pyrrhotite is therefore attributed to the presence of relatively large concentrations of iron vacancies (up to 20%) in the crystal structure. Vacancies lower the crystal symmetry. Therefore, monoclinic forms of pyrrhotite are in general more defect-rich than the more symmetrical hexagonal forms, and thus are more magnetic. Upon heating to 320 °C, pyrrhotite loses its magnetism, but also starts decomposing to magnetite. The saturation magnetization of pyrrhotite is 0.12 tesla.
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