Studies on transformations of minerals in Baragolai, Ledo, and Tipong coals of the Makum coalfield of the northeastern region of India on heat treatment in the presence of air at 350 and 850 degrees C have been carried out using XRD, FTIR, DTA, TGA, and DTG. XRD patterns of the above three coals show that the samples contain both amorphous and crystalline phases. The coal samples exhibit a number of peaks. The most dominant peaks in all the coal samples are due to quartz. The coal samples also exhibit very small peaks assigned to calcite, gypsum, pyrite, marcasite, and chlorite. The XRD patterns of the coal samples are found to change on heating in the presence of air until ash is prepared at 350 degrees C. These samples show some new peaks assigned to kaolinite, illite, and feldspar. These are present in raw coals too but could not be detected due to the presence of these minerals in small amounts in comparison to that of the amorphous material. The large amount of amorphous carbonaceous material possibly envelops the minerals which slowly get oxidized on heating in the presence of air at 350 degrees C resulting in an increase in the concentrations of the minerals kaolinite, illite, and feldspar. The XRD patterns of the 350 degrees C heated ash sample exhibit peaks due to quartz, calcite, and marcasite. Thus, these are not affected by heat treatment of coal in the presence of air at 350 degrees C. There was no peak present due to gypsum because the water of crystallization was lost. Some of the peaks observed in the raw coal and the ash produced at 350 degrees C were found to disappear in the ash prepared at 850 degrees C. The XRD patterns of ash prepared at 850 degrees C exhibit some new peaks due to iron oxide which might have been formed as a product of thermal transformation reactions of some phases present in the coal. The sample did not show peaks due to pyrite, marcasite, and calcite. Pyrite undergoes oxidation at 370 degrees C and marcasite at 430-480 degrees C forming iron oxide and sulfur dioxide. Calcite decomposes in the range of 675-750 degrees C yielding calcium oxide and carbon dioxide. The 850 degrees C heated ash sample also did not exhibit peaks due to kaolinite and illite. Kaolinite transforms into metakaolin at 550-600 degrees C, and illite undergoes dehydroxylation at 450-550 degrees C. Quartz is the dominant phase in the coal and its ash prepared at 350 and 850 degrees C, i.e., no change. This is expected as R-quartz is the best known most stable crystalline form of silica, and it transforms to another crystalline form, cristobalite, through an amorphous transition phase when heated above 1470 degrees C for a considerable time. The FTIR spectra of the raw coal samples, ash obtained at 350 degrees C, and ash prepared at 850 degrees C were recorded and compared. The general characteristics of the FTIR spectra of all the coal samples are almost similar. On comparison of the spectra, it was observed that on heating the coal samples in the presence of air all the stretching, bending bands due to coaly matter functional groups disappear. The FTIR peaks due to presence of different functional groups of minerals support the findings of XRD summarized above. Similarly the TGA, DTA, and DTG results support our XRD findings too as stated above.