Geochemical and petrographic analysis of graphitized coals from Central Hunan, China

Microcrystalline graphite can form from coal naturally as a result of igneous intrusion, but the mechanism of alteration from anthracite to graphite remains elusive. The occurrence of microcrystalline graphite in altered coal seams is not common globally; however, several microcrystalline graphite occurrences that are associated with igneous intrusions are currently being mined commercially in China. In this study, petrographic, geochemical, and X-ray diffraction analyses of a series of Carboniferous coals with different levels of graphitization were used to study transformations in coal structure and chemical composition evolution during natural graphitization. The graphitized coals were collected from mines located at varying distances from a large (similar to 130 km(2) in extent) Indosinian-period granite intrusion; the coals in this region have been highly altered by this intrusion. Mean random reflectance of the samples increases from 4.36% to 8.23% approaching the intrusion, but decreases to 4.58% in the most graphitized samples. Vitrinite and inertinite become difficult to distinguish under white light with increased coal rank, and newly formed components including pyrolytic carbon, needle graphite, and flake graphite are seen in the most graphitized samples. The pyrolytic carbon accumulated from a vapor phase that was likely generated during intrusion. The needle and flake graphite occur as fracture and void fills and probably represent graphitization of a mobile phase that migrated through the coal. Timing of generation and the source of the mobile phase is unclear, but predates graphitization and could have been associated with the intrusion event. Microcrystalline graphite, with a characteristic texture consisting of fine granular particles, is the dominant component in the most graphitized samples. Reflectance of microcrystalline graphite using standard coal procedures does not accurately reflect its rank. Approaching the intrusion, structural parameters show a progressive change, especially in highly graphitized coals. Samples closest to the intrusion have transformed into graphite (as confirmed by X-ray diffraction and geochemical data). A plot of volatile matter (VM) versus RT suggests that the maturation pathway for graphitized coals differs from that of coals that have undergone normal burial maturation. Elemental changes in graphitized coals also differ from coals that have been intruded by smaller-scale sills and dikes. This is probably due to the extreme level of metamorphism associated with the large-scale intrusion that graphitized these coals.