Superparamagnetic Fe₃O₄ particles formed by oxidation of pyrite heated in an anoxic atmosphere

As a follow-up to previous gas analysis experiments in which pyrite was heated to 681 K in an anoxic (oxygen starved) atmosphere, the first oxidation product, FeSO₄, was studied as a bulk material. No decomposition of FeSO₄ to Fe₃O₄ was observed in the temperature range studied. The lack of decomposition of bulk FeSO₄ to Fe₃O₄ suggests that FeS₂ oxidizes directly to Fe₃O₄, or that FeSO₄, FeS₂ and O₂ react together to form Fe₃O₄. Magnetic susceptibility and magnetization measurements, along with magnetic hysteresis curves, show that small particles of Fe₃O₄ form on the pyrite surface, rather than a continuous layer of bulk Fe₃O₄. A working model describing the oxidation steps is presented.