Petrolographic characteristics of graphitic carbon in coal-based graphite: Implications for the applicability of reflectance

Coal-based graphite deposits developed in Lutang, Hunan Province, China, provide an opportunity to investigate the petrographic characteristics of graphitic carbon. Optical microscopy and microphotometry were employed to observe the morphology, microstructure, occurrence and reflectance of graphitic carbon and thus classify its type. The results indicated that the evolutionary stage of the coal-based graphite reached the semi-graphite to graphite phase, and five types of graphitic carbon, namely, matrix graphite, granular graphite, needle graphite, flake graphite and pyrolytic carbon, were identified. The maximum reflectances of different types of graphitic carbon vary, indicating a differential evolutionary stage. However, the bireflectance progressively increases through the sequence of needle graphite, pyrolytic carbon and flake graphite, reflecting enhanced structural ordering of basic structural units. In terms of their particle size and reflectance characteristics, granular graphite, needle graphite, pyrolytic carbon, and flake graphite exhibit phanerocrystalline structures, whereas matrix graphite belongs to the cryptocrystalline graphite category. Three primary mechanisms for the origin and formation of graphitic carbon were proposed: in situ solid-phase transformation, liquid-phase crystallization, and vapor-phase deposition. Flake graphite serves as the ideal material for measuring reflectance in coal-based graphite, with its reflectance value acting as an indicator of evolutionary maturity.