The importance of minerals in coal as the hosts of chemical elements: A review

Coal is a complex geologic material composed mainly of organic matter and mineral matter, the latter including minerals, poorly crystalline mineraloids, and elements associated with non-mineral inorganics. Among mineral matter, minerals play the most significant role in affecting the utilization of coal, although, in low rank coals, the non-mineral elements may also be significant. Minerals in coal are often regarded as a nuisance being responsible for most of the problems arising during coal utilization, but the minerals are also seen as a potentially valuable source of critical metals and may also, in some cases, have a beneficial effect in coal gasification and liquefaction. With a few exceptions, minerals are the major hosts of the vast majority of elements present in coal. In this review paper, we list > 200 minerals that have been identified in coal and its low temperature ash, although the validity of some of these minerals has not been confirmed. Base on chemical compositions, minerals found in coal can be classified into silicate, sulfide and selenide, phosphate, carbonate, sulfate, oxide and hydroxide, and others. On the basis of their abundance, they can be classified into common, uncommon, and rare. Elements associated with silicates are largely benign, but many of those associated with sulfides and selenides are toxic to the environment and human health (e.g., S, As, Hg, Tl, Se, and Pb). Critical elements, e.g., rare earth elements and Y, Ga, and Al, are mostly associated with clays, phosphate, and carbonate minerals. There are many unusual mineral phases, such as native W, Au, Ag, and various Pt phases, which may have economic and geochemical significance in coal. Although the modes of mineral occurrence of a number of elements have been widely investigated, there are some elements whose associations, and, in particular, association mechanism with minerals are, to a degree, uncertain or even largely unknown and deserve further attention.