ASSOCIATION OF COAL MACERALS, SULFUR, SULFUR SPECIES AND THE IRON DISULFIDE MINERALS IN 3 COLUMNS OF THE PITTSBURGH COAL

This study of the Pittsburgh coal supports the premise that the amount of sulfur contained in coal, especially that represented by the iron disulphide minerals, is largely the product of the same swamp chemistry that determines the relative abundances of the various coal macerals. Through its control of the microbial degradation of plant debris and peat, the swamp water pH determines the relative abundances of the vitrinite and exinite macerals. At swamp water pH values above 4.5, increased microbial degradation of the plant and peat materials would decrease the abundance of pre-vitrinitic materials, increase the pre-exinitic materials, produce disulphide ion via the microbial reduction of sulfate ion which would subsequently precipitate as iron disulphide minerals and result in an increase in mineral matter in the pre-coal peat. The subsequently produced coal would be more exinite rich, high in mineral matter and sulfur, especially pyritic sulfur. Within the Pittsburgh coal, the layers at the top and bottom of the coal bed and above and below the middle parting reflect non-ideal conditions of organic preservation. Because of the common chemical control for the production of exinites and iron disulphide minerals, the abundance of exinite and sulfur exhibit a strong statistical association within the Pittsburgh coal. Below pH 4.5, reduced microbial activity results in the preferential accumulation of the pre-vitrinitic woody tissues and the subsequent suppression of the relative abundance of pre-exinitic spores, pollen and cuticles. The same low-pH conditions solubalize the metals, especially iron, suppress the bacterial reduction of the sulfate ion to the disulphide ion and as a result inhibit the formation of the iron disulphide minerals. A coal produced under conditions of low pH would be bright, the sulfur content would be low and dominantly organic and because of the high degree of preservation of the plant debris, the mineral matter in the coal would be relatively low. Within the Pittsburgh coal, the general decrease in mineral matter and sulfur and increase in vitrinite content downward from the tops of both benches reflects the more ideal conditions for organic preservation prevalent within the original swamp. The degradation of plant debris and peat via dissolved oxygen in percolating groundwater or rainwater will result in an enrichment in the pre-exinitic materials but will inhibit the formation of the iron disulphide minerals. Some exinite-rich coal layers are therefore low in sulfur and dominated by organic sulfur. The amount of sulfur contained within partings will depend upon their mode of origin. If the partings formed by the extreme microbial degradation of the peat, they will usually be enriched in sulfur as illustrated by the high sulfur content of the middle parting. On the other hand, if they formed by the slow oxidation of the peat or are of detrital origin, the sulfur content may be low.