Mineral matter transformation is a specific property of coal which is crucial to the suitability during combustion or gasification. Three low-temperature ashes (LTA) were prepared by incinerating three coals at low temperature (<200 degrees C) in an electronically excited oxygen plasma which is considered as the most efficient method to remove the organic matter of coal. High-temperature X-ray diffraction (HT-XRD) was applied to analyze the in situ mineral transformation of LTA during the heating process with quantitative determination using the reference intensity ratio method. The fusion characteristics of the ashes were studied by ash fusion temperature test. Thermogravimetric and differential scanning calorimetry was used to acquire both the mass and heat signal from which we can analyze different reaction phases. The results show that weight loss rate reaches most in the temperature range 400-800 degrees C during which kaolinite decomposes and then calcite later. After 800 degrees C, different matters recrystallize forming new phases. Quartz remains stable until higher than 1100 degrees C and melted along with the new formed minerals such as anorthite and diopside under 1300 degrees C, while the existence of Al2O3 can result in high flow temperature. The mineral transformation is also calculated by FactSage and compared with the results from HT-XRD. The calculations of FactSage failed to give exact predictions at high temperature when there are new minerals formed during the heating process.