Magnetic properties of synthetic analogues of pyrrhotite ore in the grain size range 1-24 m

Abstract

Iron and sulphur in proportions appropriate to Fe7S8 were reacted in evacuated quartz tubes for 24 hr at 500 °C. Quenching produces predominantly hexagonal pyrrhotite, whereas further prolonged annealing at 250 °C produces monoclinic pyrrhotite. We mainly report the properties of material cooled from 500 °C at 10 °C min-1, which is predominantly monoclinic but probably contains hexagonal pyrrhotite together with pyrite, FeS2. This material, which we refer to as ‘non-ideal’ pyrrhotite, may be a better analogue of pyrrhotite ore than ideal synthetic monoclinic pyrrhotite. The samples were characterized by X-ray, Mössbauer effect and thermomagnetic analyses. The slow-cooled non-ideal monoclinic pyrrhotite exhibits thermal stability intermediate between monoclinic and hexagonal pyrrhotites. One striking observation is that the deviation from ideal monoclinic pyrrhotite increases with decreasing particle size. Particles about 1 µm in size have a low saturation magnetization value of about 6 Am2 kg-1, indicating a lower concentration of ferrimagnetic monoclinic pyrrhotite (saturation magnetization 18 Am2 kg-1) than particles of about 6 µm and above (saturation magnetization 12 Am2 kg-1). On the other hand, magnetization process parameters-coercive force, ratio of saturation remanence to saturation magnetization, coercive force of remanence, median destructive and inductive fields, the magnetic susceptibility-follow well-behaved power law dependences on particle size, similar to monoclinic pyrrhotite. Magnetic domain patterns in the slow-cooled non-ideal pyrrhotite are similar in type and style to those observed in synthetic monoclinic pyrrhotite and in natural pyrrhotite particles in rocks. Dynamic observations of domain walls in changing external alternating and direct magnetic fields are consistent with macroscopic observations of magnetization change with field. The critical size for the monodomain/multidomain transition in synthetic monoclinic pyrrhotite, at about 1 µm, is essentially the same as that previously determined for natural pyrrhotite grains in rocks. The observations suggest that the domain walls lie normal to the c-axis of the structure, consistent with the known magnetocrystalline anisotropy symmetry of monoclinic pyrrhotite. The type and style of the magnetic microstructures of hexagonal pyrrhotite are quite different from those of monoclinic pyrrhotite. The domain patterns are on a much smaller scale, wavy, and sometimes resemble those of a hexagonal material such as magnetoplumbite or cobalt.