INORGANIC NITROGEN IN ANTHRACITE FROM EASTERN PENNSYLVANIA, USA

Up to 20% of the total nitrogen content of anthracite-rank coal from eastern Pennsylvania occurs in authigenic NH4-rich illite. This enrichment is due to the concurrence of two independent reactions in the coal and surrounding organic-rich shale during late-stage organic maturation and diagenesis (T > 200-degrees-C): (1) the release of N from organic matter by devolatilization reactions; and (2) the formation of illite from kaolinite. Our estimates of inorganically bound N are conservative because minerals in fractures, which can be volumetrically important, were removed prior to analysis. Inorganic N estimates using X-ray diffraction analysis are similar to those obtained by elemental analysis of combusted material. The occurrence of inorganically bound N in organic-rich rocks has important implications for geochemical characterizations of high-rank coal (N content and isotopic composition) and laboratory investigations of the kinetics of N loss from organic material during thermal maturation. In addition, because NH4-rich illite authigenesis is temporally linked to coalification, the radiometric age of this mineral can theoretically be used to determine the absolute age of coalification in this region. For detailed characterizations of high-rank coal, we recommend that mineral matter be isolated by low-temperature ashing and analyzed by X-ray diffraction or C-H-N elemental analysis for inorganically bound N.