Structural characteristics and evolution of meta-anthracite to coaly graphite: A quantitative investigation using X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy

Meta-anthracite, semi-graphite, and coaly graphite samples were collected from mines of Yongan coalfield, Fujian Province, China, and their structures were analyzed through XRD, Raman spectroscopy, and HRTEM to trace the evolution of coal into graphite during natural graphitization. The results showed that a sharp division and prominent evolution jump existed in the evolution process. Meta-anthracite samples contain tiny crystallites, structural defects between basic structural units (BSUs), and the alignment of lattice fringes shows a regional ordering. In the semi-graphite samples, the crystallite size increased, and the ordering of lattice fringes was enhanced; however, the content of the structural defects was simultaneously reduced. In the coaly graphite samples, the crystalline size increased significantly, and a nearly perfect alignment of lattice fringes was generated. Lateral connection and vertical stacking of BSUs occurred in the structural evolution process, as did the increase in crystallite size and space recording, and poor coal structure transition to form a long-range and well-ordered graphite structure. Consistent with Rmax as proposed by ICCP, both of the parameters derived from XRD, Raman spectroscopy and HRTEM show a nonlinear change, which was able to characterize and discriminate the evolution process of graphitization. The results will benefit the development and rational utilization of coal undergoing different levels of natural graphitization.