Coal fires burning around the world over many years have been responsible for the loss of natural resources and also for negative environmental and human-health impacts. Study of self-burning coal wastes based on organic petrology, mineralogy and geochemistry allows the evaluation of factors responsible for the combustion process, and can also be used to assess the changes that are taking place in thermally affected materials. The main goal of this study is to characterize the materials from the Arroyo Galladas, Arroyo Mourin and Fabero coal waste piles, which resulted from coal mining in the El Bierzo Coalfield (NW Spain). Samples of coal, and of unburned and burned or burning coal waste material, were studied by optical microscopy and X-ray powder diffraction (XRD). The results demonstrate that some of the organic matter has preserved its characteristics, but other organic material shows signs of thermal alteration such as cracks, devolatilization vacuoles, dark reaction rims, plasticized edges, and increased or decreased vitrinite reflectance. The resistance of the unaltered organic matter to thermal effects is attributed to the coalification process previously undergone by these coals. The mineralogical composition of the samples indicates that newly formed minerals are present in the burned or burning material, including mullite, cristobalite, cordierite, hematite, jarosite, sanidine, anorthite, sulfur, pyrite, rozenite, coquimbite, tschermigite, boussingaultite and amorphous material. The formation of these minerals is attributed to combustion at maximum temperatures of at least 1100 degrees C in the Fabero coal waste pile and lower temperatures in the Arroyo Galladas coal waste pile, and to interaction of gases released by combustion with the solid particles, the waters and the atmosphere in and around the waste piles. (C) 2015 Elsevier B.V. All rights reserved.