## 2 Coal-Related Fires--A Geoenvironmental Problem  
Both coal mining and coal consumption have unquestionable benefits for the economic and social segments of many countries, despite their harmful impacts on the environment and human health [10]. The environmental impacts related with coal mining may include alterations in landscape and land use; soil erosion; increased noise generation; production of solid wastes; pollution of the air, surface and groundwater, soils and sediments; land instability, subsidence; coal-related fires; and effects on local biodiversity [10,11].  
Coal-related fires are a worldwide phenomenon that has been extensively investigated in recent years [12, 13, 14, 15, 16]. These occurrences are also an environmental concern and have recently been studied in Europe [17, 18, 19, 20, 21], specifically on the Iberian peninsula [8, 22, 23, 24, 25, 26, 27, 28, 29, 30]. In this context, coal fires are related to spontaneous combustion or ignition caused by extrinsic factors of coal and coal-rich materials that lead to the self-combustion process over variable periods of time (frequently over decades). The self-heatingpropensity of coal and coal-rich materials due to the oxidation of sulfides promotes the ignition. The investigation of coal-related fires occurring all over the world has been the subject of several studies that focused on the economic losses of coal resources, the environmental impacts associated with the combustion process, and monitoring and fire-fighting possibilities [31].  
The self-combustion of coal may occur in unmined coal seams, during coal mining operations, handling, and transportation, storage and in coal mining waste deposits, being responsible for significant losses of natural energy resources [10]. The geological materials most frequently involved in coal-related fires are the coal in seams and the host rocks, the whole coal in storage piles, and the rejected and overburden material resulted from coal mining and beneficiation (containing a mixture of fragments of coal and host rocks). The self-combustion process of coal and coal-rich materials is responsible for the mobilization of large quantities of pollutants caused by the volatilization of elements present in coal and associated mineral matter. The environmental impacts caused by coal-related fires includes the emission of harmful gases and particulate matter into the atmosphere, mobilization and leaching of hazardous elements into soil and water, and the formation of solid byproducts (coal fire gas minerals) [10; 32; 33; 34; 35]. As a consequence, the pollution of air, soils, sediments, surface and groundwater, and effects on human health and biodiversity occur. The uncontrolled release of these pollutants, namely greenhouse gases, volatile organic compounds, particulate matter, and toxic elements, represents a risk to the environment and to human health. The risk is even more severe when the process occurs near populated areas, since some of the pollutants are known to be toxic and human carcinogens [33].  
The comprehensive characterization of the affected geological materials, including petrological, mineralogical, and geochemical studies, permits the evaluation of factors responsible for the self-heating, spontaneous combustion, and self-combustion processes, the identification of the changes induced in the affected materials, and the gathering of insights about the associated potential environmental impacts, allowing for a definition of timely corrective measures to minimize the negative impacts. The characterization of the self-combustion byproducts (gases and coal fire gas minerals) is also essential to address the environmental issues and effects on human health and ecosystems.