Metamorphic transformations of nitrogen functionalities: Stabilization of organic nitrogen in anthracite and its effect on δ15N parameter

Abstract

Abstract The coal metamorphism, as it evolves the microstructure of the coals, would also have significant influences on the structural transformations and isotopic discriminations of the primary chemical moieties. The present study aims to illustrate the nitrogen functionalities present within the chemical framework of the bituminous coal samples from the Raniganj and the Jharia Basins and anthracite A samples of the fold-thrust belts of the Sikkim Himalayas as well as to document the metamorphic transformations of these nitrogenous moieties with the advent in the degree of the coalification. The stable carbon (−24.08 to −21.67‰ for bituminous coal samples and −23.91 to −21.39‰ for anthracite A samples) and nitrogen isotopes (1.64–2.81‰ for the bituminous coal samples; 1.07–3.44‰ for the anthracite A samples) do not show any significant variation with the vitrinite reflectance and, therefore, correspond to the terrestrial higher plant sources of the organic matter. Further, the weak shifts of stable nitrogen isotope values in the anthracite A samples may be attributed to the preservation of the organic nitrogen in the thermally stable aromatic and heterocyclic structures. The total organic carbon to total nitrogen (TOC/TN) ratio may represent the vascular organic matter input into the mire in the case of bituminous coal samples (33.49–43.69), while for the anthracite A samples, this ratio may be an indication of the preservation of the organic nitrogen in the stable chemical framework within the microstructure. The primary alteration of the labile nitrogen-containing groups including the pyrrolic structures would have been initiated by the aqueous fluid entering into the reactive chemical framework during hydrothermal metamorphism. During these processes, most of the nitrogen atoms would have been quaternarized, which is evidenced by strong correlation (r = 0.96) between the relative area ratio of the graphitic to pyrrolic nitrogen and the mean random vitrinite reflectance. Moreover, the intensity and relative area ratio of the pyridinic to pyrrolic nitrogen are strongly correlated with the mean random vitrinite reflectance (r = 0.92 and 0.89, respectively) suggesting entanglement of the nitrogen atoms within the pyridinic forms as well with increasing metamorphic temperature. These structural rearrangements would have intensified the cyclazine structures and preserved the pyridinic forms in the anthracite A samples. Due to the increase in aromaticity and consequent increase in surface hydrophobicity and decrease in the interlayer spacing, the fluid would have lost its mobile phase and could not invade the relatively inert aromatic clusters. Thus, the organic nitrogen atoms were preserved within the thermochemically stable functionalities without showing any substantial isotopic variation. The scientific contribution of this present investigation, hence, lies in depicting the transformation of organic nitrogen among the pyrrolic, pyridinic and the cyclazine moieties with insignificant shifts in stable nitrogen isotope during coalification.