According to the report by Hong and Fegley [34], no hematite (Fe2O3) but only pyrrhotite was observed within the temperature range of 400–520 ◦C. It is reported that sulfur appears in the pyrrhotite produced at a lower temperature (440–500 ◦C) [35].
Pulik et al. [7] found that the reaction (2) should be considered in thermal decomposition process without oxygen for the inner portion of pyrite particles. Therefore, it is concluded that the mass loss at 400–480 ◦C is caused by the oxidation of the sample particle surface and the decomposition of the interior for the pyrite particle without oxygen, in accordance with Eq. (1) and (2). The thermal process at 530–570 ◦C is attributed to the evolution of oxidized sulfur stemmed from the last step the decomposed of the pyrite particle without oxygen, Eq. (3). The last step at 563 ◦C is assigned to the oxidation of the pyrrhotite. It is reported by Jorgensen and Moyle
[36] that hematite (Fe2O3) is the solid end product of the reaction in this temperature range. They further concluded that small amounts of pyrrhotite formed as thin layers of intermediate reaction product but in amounts which are small in comparison with
the amount of hematite. Thus Eq. (4) is a reasonable candidate for this process. Therefore, the thermal decomposition of pyrite which started at 430 ◦C was completed at 590 ◦C. It is indicated that the chemical substance SO2 is present in the thermal decomposition for the sample, and this will be further proved by the following IR
results. It is observed that the mass loss at 563 ◦C is higher than that at 495 ◦C. This is due to the existence of the impurity kaolinite in the sample. The dehydroxylation of kaolinite occurs at this temperature range, with the onset of the transformation to metakaolin. The process can be mostly described by the reaction