Previous attempts to determine the mode of occurrence of the trace elements in coal have been largely indirect. Results of the most commonly used appproach, sink-float separation, is often contradictory. Evidence obtained from this study indicate that results from sink-float separations are susceptible to gross misinterpretations. In order to directly determine the mode of occurrence of the trace elements in coal, a tehcnique was developed using the scanning electron microscope (SEM) with an energy dispersive (EDX) detector. This analytical system allows the detection and analysis of in-situ, micron-sized minerals in polished blocks of coal. In addition, mineralogical data were obtained from individual particles extracted from the low-temperature ash of the coal. These techniques were applied in an in-depth study of the Waynesburg and Upper Freeport coals, both bituminous coals from the Appalachian Basin. In addition, brief studies were conducted on about 80 coals representing every rank and type, and every major coal basin in the United States plus about 20 coals from worldwide locations. The results indicate that many trace elements in coal can occur quantitatively in micron-sized accessory mineral grains scattered throughout the organic matrix (macerals). For example, Zn and Cd occur predominantly in the mineral sphalerite; Cu in chalcopyrite; Zr and Hf in zircons; the REE, Y, and Th in monazite and xenotime. In Appalachain Basin coals, lead selenides are dominant over lead sulfides, whereas outside the Basin, lead sulfides are far in excess over the selenides. The majority of the lead in coal, however, may be substituting in barium-bearing minerals. Some elements, such as As and Hg, occur in solid solution with pyrite. The difference in the mode of occurrence between As and Hg and the other chalcophile elements is reflected in their behavior during sink-float separation of the coal; those elements forming micron-size minerals within the macerals are concentrated in the lighter Sp. G. fractions; whereas, those elements associated with pyrite are concentrated in the heavier Sp. G. fractions. Organic associations constitute a major mode of occurrence for several trace elements. Although Ti-bearing minerals are common in many coals, no more than about 50 weight percent of the Ti can be accounted for in this manner. The remainder is probably bound to the organics.
