Classification and carbon structural transformation from anthracite to natural coaly graphite by XRD, Raman spectroscopy, and HRTEM

Low-weight components of coal macromolecule were subjected to pyrolysis and condensation when magmatic rock intruded into coal measure, eventually, the residual condensed aromatic components can transform into microcrystalline graphite (coaly graphite). To study the structural transformation from anthracite to natural coaly graphite, ten samples with different graphitization degrees from Xinhua and Lutang of Hunan Province, China were characterized by organic geochemical analysis, X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The geochemical parameters (proximate and ultimate analyses) and structural features (XRD, Raman, and HRTEM) of the series naturally graphitized coals exhibit a progressive change as the samples' locations closing to the intrusion. The series naturally graphitized coal samples were classified into four categories, including anthracite, meta-anthracite, semi-graphite, and coaly graphite. But, single parameter cannot classify the series metamorphosed coals well, multi parameters including ash free-basis volatile matter, petrographic features, and carbon structural parameters (based on XRD and Raman spectroscopy) should be considered, additionally, the lattice fringe change observed under HRTEM from anthracite to coaly graphite can verify for the classification. The relatively lower metamorphic grade samples (anthracite and meta-anthracite) have small crystalline sizes, prominent disorders, and amorphous carbon structure, whereas the crystallite structure of highest grade samples (coaly graphite) is three-dimensional crystalline order (testified by XRD and HRTEM), indicating a totally structural transformation from amorphous carbon of anthracite to highly ordered crystalline carbon of coaly graphite in the course of natural graphitization. The carbon structural evolution of coal under natural graphitization process will probably be helpful for synthetic graphite using coal to replace the expensive petroleum coke in the future.