Quantifying the Structural Transitions of Chinese Coal to Coal-Derived Natural Graphite by XRD, Raman Spectroscopy, and HRTEM Image Analyses

The macromolecular evolution of coal in proximity to intrusion zones provides a means of exploring the coal to coal-derived natural graphite (CDNG) transitions. Here, samples with different metamorphic grades from anthracite to CDNG were examined based on X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) image analyses. The anthracite aromatic layers were limited with the average layers in a stack being a 10, with a height and width of a 4 and a 5 nm, respectively. The CDNG had higher crystallinity with the number of layers in a stack being a 120, with a height and width of a 40 and a 70 nm, respectively. The D1/G area ratios (Raman) decreased from 6.5 to 0.3 (from anthracite to graphite). The short fringe length (0.3-1.14 nm) distributions account for >90% of anthracite, while meta-anthracite and semi-graphite had lower contributions of 88.5 and 81.5%. The fringe orientation value (the frequency of the total fringe length within the most prominent 15° bin) increased from 22.1 (anthracite) to 29.4% (meta-anthracite). For semi-graphite, the orientation was high reaching 72.5%. The high-grade CDNG had essentially 100% of fringes oriented within 15°. The fringe curvature was high in the initial anthracite (90.9% of the fringes) but this decreased to 85.5% in the anthracite of higher metamorphic grade. In meta-anthracite, the curvature decreased to 80.1% as the fringe length further increased. The fringe curvature became less frequent (51.5%) in the highly ordered CDNG. The structural evolution from anthracite to meta-anthracite followed the general coalification characteristics: Fringes are increasingly longer, oriented, and linear but limited to small stacks. An extensive molecular amalgamation occurs, forming fringe curvature and dislocations in the meta-anthracite to semi-graphite transitions. With graphitization, the curvature and the dislocations decrease forming the well-ordered graphite.