X-ray diffraction (XRD) was performed to analyze the mineral matter in coal ash from 1100 to 1500°C, with an interval of 50°C. Relative Intensity Ratio (RIR) method was used to calculate the content of each crystalline in coal ash. The amount of mullite increases with elevated temperature but that of SiO2 decreases. The transformation of mullite and SiO2 accords well with the binary phase diagram of SiO2–Al2O3. At high temperature, the amorphous mineral matters, mainly, aluminosilicates melt above ash softening temperature. During CO2 gasification at high temperature, the carbon conversion is hindered by the aluminosilicates melts, which cover the surface and block the pores of coal particles. Because of the short and ordered sequence of melts, FT-IR analysis was taken for the melts in coal ash after gasification. The structure alteration of aluminosilicates influences the surface tension of melts, which determines the interaction between coal particles and aluminosilicates.