Finkelman, et al., reported in 2019 that 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.
The authors concluded that minerals are the most important components of inorganic matter in coal and, in most cases, play the most significant role in affecting the utilization of coal. Minerals are also the major hosts of the vast majority of toxic, benign, and critical elements present in coal. Although a number of elements and their hosting minerals in coal have been widely investigated, some issues require further investigation to evaluate more fully the modes of occurrence of elements in coal:
• The associations of some elements with minerals are uncertain or even largely unknown (for examples but not limited to Be, Bi, Br, Co, I, In, Mo, Nb, Ta, Te, W, and V) and deserve further investigation using integrated approaches as mentioned in the text above.
• The association mechanism of many elements with minerals are also unknown, e.g., conditions of toxic elements (As, Hg, Tl) substitute for major ions of sulfides in coal.
• Quantitative analysis of elements associated with specific minerals rather than with generic terms such as ‘clays’, ‘silicates’, and ‘carbonates’ needs new technologies for more fully understanding modes of occurrence of elements.
• As mentioned in Section 3.8, a number of interesting and rare phases have been found in coal confirming that additional detailed mineralogical investigations of coal are entirely justified.
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