Differential graphitization of organic matter in coal: Some new understandings from reflectance evolution of meta-anthracite macerals

Abstract Middle Permian coal from the Yongan Coalfield, Fujian Province, China, with a maximum vitrinite reflectance (VRmax) of 5.33%–8.95%, indicating a typical meta-anthracite stage, enables the study of differential graphitization of macerals. The composition and optical properties of macerals were analyzed using optical (under non-polarized light, polarized light with gypsum test plate (1λ), and cross-polarized light) and scanning electron microscopy (SEM). The reflectance anisotropy and reflectance-indicating surface (RIS) were also analyzed to investigate the differential graphitization of macerals. The results show that although the morphology and structure of the original macerals were mostly preserved, the path of maceral evolution was differentiated. With increasing VRmax, the minimum reflectance of inertinite increased, while that of vitrinite and liptinite inverted from increasing to decreasing at approximately 8.0% and 7.5% VRmax, respectively. The reflectance anisotropy parameters of macerals changed accordingly, further enhancing the optical differentiation between macerals. Individually, VRmax does not adequately represent the evolution of liptinite and inertinite. The three-dimensional RIS ellipsoid revealed that liptinite and vitrinite have a biaxial negative optical character with strong optical anisotropy, while inertinite has a biaxial neutral to positive optical character with optical isotropy. A gradual unidirectional path of optical evolution from liptinite to graphite was observed, indicating that the evolution of macerals from coal to graphite followed a monotonic function model. Based on optical anisotropy, and considering the maximum maceral reflectance (XRmax) of 8.0% as the demarcation point and approximately 10.0% XRmax as the indicator of graphitization, the late coalification of macerals was resolved into two stages: pre-graphitization and quasi-graphitization. The conversion of coal to graphite largely depends on the transformation potential of the individual macerals, rather than on the total organic matter in coal. The graphitization of macerals in the Yongan coal samples was significantly differentiated; most of the liptinite was transformed into graphite and vitrinite was converted to semi-graphite, while all the inertinite remained in the anthracite stage. Biaxial negative macerals, therefore, presented a more prominent graphitization tendency than biaxial positive macerals.