The intermediate products in the oxidation of pyrite were investigated by measurement of the intensity of magnetization, thermomagnetic analysis and X-ray diffraction. In the course of the oxidation, the intensity of magnetization of the sample increases, reaches a maximum, and then decreases. The maximum value of the sample oxidized at low partial pressures of oxygen was found to be much higher than that oxidized at high partial pressures of oxygen. FeS₁₊ₓ and Fe₃O₄ were the main products of oxidation at low partial pressures of oxygen, and Fe₃O₄ changed into Fe₂O₃ immediately before the end of the oxidation. On the other hand, Fe₂O₃ was formed in addition to Fe₃O₄ from the early stages of the oxidation at high partial pressures of oxygen. The FeS₁₊ₓ formed in the intermediate stage was ferromagnetic, and its composition was near the upper limit of the solubility of sulphur in FeS₁₊ₓ. Samples taken from the roasting hearths of the Herreshoff furnace were investigated by measurement of the intensity of magnetization and thermomagnetic analysis. Pyrite decomposes into FeS₁₊ₓ in the upper hearths and, in the middle hearths, FeS₁₊ₓ is oxidized mainly to Fe₃O₄, and finally, Fe₃O₄ changes into Fe₂O₃ in the lower hearths. The intensity of magnetization reaches the maximum in the middle hearths of the furnace. The formation of a large amount of Fe₃O₄ in the middle hearths indicates that the partial pressure of oxygen in the layer of the charge in the furnace was remarkably low
