Pyrite (FeS2) is one of the major contributors to slagging during coal utilization procedure; however, the evolution of FeS2 is a complex process under different conditions. The transformation behaviors of FeS2 in CO2 under different experimental conditions were studied by characterizing the solid and gas products. Mossbauer spectroscopy was used to identify the Fe-containing phase. The results showed that the FeS2 begins to decompose at 560 degrees C, and all decomposition processes are endothermic in CO2 atmosphere. When the temperature is <700 degrees C, pyrrhotite (FeSx, 1 <= x <= 2) is the only solid product during FeS2 thermal decomposition in CO2 atmospheres. However, at the reactor temperature >= 700 degrees C, magnetite (Fe3O4) and FeSx are the primary solid products. The FeO-FeS (i.e., iron oxy-sulfide droplets) molten system begins to appear at a reactor temperature of 950 degrees C, and the eutectic FeO-FeS phase becomes the major phase at the temperatures above 1000 degrees C. High CO2 concentrations may promote the FeS2 to form Fe3O4, CO and SO2. The solid products (e.g., Fe3O4) may be further oxidized to form hematite (Fe2O3) after the FeS2 was heated for a longer time; this would be, mainly due to CO2 reacting with FeS2 thermal decomposition product Fe3O4. A transformation pathway of FeS2 decomposition in CO2 atmosphere was thus defined. (C) 2015 Elsevier B.V. All rights reserved.