Magnetic phase relations of pyrrhotite

A series of iron sulphide compositions containing between 50 and 46.0 atomic percent iron has been synthesized and the products of the syntheses annealed at 701, 350, 300 and 144°C. The charges were examined in polished sections by X-ray powder photography, electron microprobe analysis and Mossbauer spectroscopy, and the thermomagnetic properties were examined over a 25-350°C temperature range. The thermomagnetic data were used to construct a series of magnetic phase diagrams. Detailed information from these phase diagrams, together with information from the Mössbauer and X-ray studies, suggest that a series of discrete stoichiometric or near stoichiometric phases may occur at low temperatures. The FeS, Fe₇S₈ and Fe₉S₁₀ compositions appear to be such phases, and Fe₁₀S₁₁ and Fe₁₁S₁₂ may be further examples. These phases represent different vacancy ordering patterns and explain the frequent occurrence of two-phase intergrowths in natural pyrrhotites. The magnetic phase diagrams are required for the thermomagnetic analysis of finely intergrown natural pyrrhotites and also provide information relevant to the magnetic characteristics of pyrrhotite-containing ore bodies.