Optical characteristics of macerals and formation path of graphitic components in coal⁃based graphite in the Yongan coalfield，Southwest Fujian

As an important non⁃metallic mineral， graphite is a basic raw ingredient of carbon materials. Coal⁃ based graphite is one of cryptocrystalline graphite formed by coal metamorphism， which is widely used in the produc⁃ tion of new carbon materials. To study the optical characteristics of macerals and the formation path of graphitic macer⁃ als in the coal⁃based graphite， several coal⁃based graphite samples in the Yong’an Coalfield in southwest Fujian Prov⁃ ince was selected. Using an optical microscope with different observation modes and a scanning electron microscope， the macerals and its optical properties of the coal⁃based graphite were characterized. The graphitization behavior and formation mode of graphitic macerals were explored as well. The results showed that the macerals of coal⁃ based graphite were dominated by meta⁃vitrinite with a proportion of 86.15%-97.24%， followed by meta⁃inertinite and meta⁃liptinite. Anisotropic body and graphite crystallite were newly formed components with a proportion of 0.68%-7.32%. Under the observation of reflected single⁃polarized light combined with rotating platform， the sub⁃macerals meta⁃vitrinite showed a pleochroism and strong optical anisotropy， along with significant differences a⁃ mong macerals. The plant tissue structure was clear under a cross⁃polarized light， which was an effective tool to distin⁃ guish the meta⁃vitrinite. The meta⁃inertinite was mainly composed of meta⁃fusinite， with a weak optical anisotropy un⁃ der a reflected single⁃polarized light， and it was almost black under a cross⁃polarized light observation， exhibiting an optical ‘inertia’ feature. The meta⁃sporinite， meta⁃cutinite and meta⁃resinite of meta⁃liptinite presented clear mor⁃ phologies， and its pleochroism and optical anisotropy were stronger than those of meta⁃vitrinite. Flow⁃like anisotropic bodies were attached to the original macerals’ surface in a lens⁃like or strip⁃like shape. Graphite crystallite occurred in two modes， e.g.， micro⁃crack filling and cell cavity filling. The former was common， showing a flake， needle⁃like， and mosaic morphology as a cluster aggregate. Using the orthogonal polarized light plus a gypsum plate， the graph⁃ ite crystallite showed an interference color of the first order yellow and second order blue， indicating a different ar⁃ rangement orientation of graphite crystallites. The graphite crystallites with line diameters of 10-50 μm in the isochro⁃ matic zone were the most common one. Under a scanning electron microscopy and an electron probe analysis， the graphite crystallites occurred mostly as irregular flakes with flow⁃ and bubble⁃like structures， showing the features of pure carbon. The hydrogen⁃rich meta⁃vitrinite and meta⁃liptinite were the main carbon sources of the newly formed components in the coal⁃based graphite， and there were three formation paths of graphitization macerals. The first was an exsudatinite filled at the boundary of the original maceral particles， which was similar to that of asphalt coke. The second was the anisotropic body formed by thermal softening， melting， and cooling crystallization of a meta⁃liptinite. The third was the carbon⁃containing gas formed by the pyrolysis of active macerals producing graph⁃ ite crystallites in an open space by the vapor deposition. The formation and distribution of coal⁃based graphite were re⁃ lated to tectonics and igneous activities. The combined magmatic heat and emplacement pressure were the main geolog⁃ ical factors of the formation of coal⁃based graphite in southwest Fujian Province.