The structural evolution and mutation of graphite derived from coal under the influence of natural igneous plutonic intrusion

Abstract

Graphite can be formed from coal under the influence of a pluton intrusion. However, the mechanism by which polyaromatic rings of disordered structure extend laterally and stack vertically during graphitization from coal to graphite remains unknown. Therefore, proximate and ultimate analysis, X-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared spectrometer (FTIR), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy were used to characterize a series of naturally graphitized coals collected from Lower Carboniferous strata in Xinhua county, Hunan province, China. The parameters derived from various analyses such as d002 and full width at half maximum (002) from XRD, ID/IG and I2D/ID + G from Raman spectroscopy, the proportion of sp2 and sp3 + Cvacancy from XPS, and the Aπ1 and Aπ2 from FTIR regularly change, indicating that the ordered graphite increased along with the removal of heteroatoms when approaching the pluton and the small condensed sp2–sp3 hybridized aromatic rings gradually transform to sp2 hybridized aromatic rings during graphitization. Furthermore, some parameters, such as ID/IG, La, Lc, and the proportion of sp2, changed dramatically in the semi-graphite stage, indicating that structural mutation of graphitized coal happens at the semi-graphite stage and the growth of Lc occurs slower than that of La during graphitization. The structural evolution can be divided into three stages: a. basic structural unit growth stage (<10 layers); b. fast growth stage of two-dimensional multilayer “ribbon-shaped” graphene (10–20 layers); c. three-dimensional-ordered microcrystal graphite growth stage (20–50 layers). The carbon-free radicals at layer edges and the conjugated π–π interaction force between graphite layers may be crucial in the stitching and stacking of polyaromatic rings.