Mapping the chemistry of resinite, funginite and associated vitrinite in coal with micro‐FTIR

Abstract

Micro-FTIR mapping is a powerful tool for nondestructive, in situ chemical characterization of coal macerals at high resolution. In this study, the chemistry of resinite, funginite and associated vitrinite is characterized via reflectance micro-FTIR for Cenozoic high volatile C bituminous coals from Colombia. In comparison with the micro-FTIR spectra of vitrinite and inertinite, the corresponding spectra of liptinite macerals in the same coals are characterized by stronger aliphatic CH(x) absorbance at 3000-2800 and 1460-1450 cm⁻¹, but less intense aromatic C=C ring stretching vibration and aromatic CH(x) out-of-plane deformation at 700-900 cm⁻¹. The aliphatic components in resinite have the longest carbon chains and are least branched, bestowing the highest hydrocarbon generation potential on resinite among the three macerals studied. In contrast, funginite exhibits the strongest aromatic character, the highest aromaticity, the lowest 'A' factor values and the lowest C=O/C=C ratios among the three maceral groups. Vitrinite generally displays intermediate chemical characteristics. Reflectance micro-FTIR mapping of coal samples further confirms the aliphatic character of resinite and the aromatic nature of funginite. In addition, chemical mapping of resinite and adjacent vitrinite shows that vitrinite immediately adjacent to resinite displays higher aliphatic CH(x) stretching intensity than more distant vitrinite, suggesting that chemical components from resinite can diffuse over short distances into adjacent vitrinite, specifically causing hydrogen enrichment. It needs to be pointed out, however, that the region of influence is localized and limited to a narrow zone, whose extent likely depends on resinite's properties, such as its size and aliphatic material content. This way, the chemical map of resinite and associated vitrinite provides direct evidence of the intermaceral effects occurring during the peat forming stage or during later coalification. No influence of funginite (primarily fungal spores and sclerotia) on the chemistry of adjacent vitrinite has been demonstrated, which is likely due to the highly aromatic structure of this type of funginite.