The past decade has witnessed great strides in the accumulation of information about the origin, chemical and physical structure, and composition of coal. These advances were made possible in part by the development of new analytical techniques, and in part by cooperative research between geologists, chemists, physicists, and biologists.Based on a review of the literature, the consensus is that coal was formed predominantly from lignin. The potential contribution from cellulose or it degradation products is, however, difficult to evaluate. Consideration of sources of energy for the metamorphic processes that convert plant residues into high-rank coals leads to the conclusion that neither bacteria, hydrostatic head, nor localized high temperatures were the geologically active agencies. The application of shear force may provide the energy necessary, but the effectiveness of low-grade thermal activation over long periods of time cannot be overlooked.The relative importance of aromatic and aliphatic structures, and of the various oxygen-bearing functional groups in coal is now understood in considerable detail. Application of such techniques as adsorption, infrared absorption, and X-ray diffraction has enabled the development of a model for the physical structure of coal through which its colloidal characteristics can be reasonably well explained. The conclusion that coal contains a high concentration of free radicals also helps to explain its color and reactivity.The concentrations of germanium, uranium, organic sulfur, phosphorus, vanadium, nickel, chromium, beryllium, and rare gases associated with coal have been studied in less detail. For several elements, notably uranium and germanium, attempts have been made to evolve the geochemical principles underlying the mode of concentration of the elements by coals, and the mechanisms by which they are retained.

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