Effect of pressure on the evolution of vitrinite graphitized mesophases: An experimental study on anthracite under high temperature and pressure

Abstract

In this study, the influence of pressure on the evolution of the intermediate phase of graphitization of vitrinite was determined. Using samples of Taixi anthracite from the Rujigo Coal Mine (Ordos Basin, China), experiments were conducted in a piston cylinder press at a constant temperature of 1073 K and pressures from 500 MPa to 1 GPa. Raman spectroscopy, high-resolution transmission electron microscopy, selective electron diffraction, and electron loss energy spectra were used to characterise the microcrystalline morphology, structural order, and atomic valence states of the samples. Under confined conditions, pressure significantly promoted the graphitization process; however, the effect of different pressures on such process in vitrinite varied. Low pressure (<700 MPa) promoted an ordered growth of aromatic rings; meanwhile, a medium pressure (700–800 MPa) destroyed the microporous structure of vitrinite, leading to a less-ordered carbon layer and a sp3 metastable carbon structure resembling nano-diamond. Notably, high pressure (>900 MPa) promoted the condensation and directionalisation of the carbon layer of the vitrinite by destroying the microporous structure, resulting in a rapid growth of microcrystalline graphite. This suggests a transition to the key stage of anthracite graphitization, which may be an important feature in the formation of intermediate phases. In fact, pressure has been shown to exert an important role in the rupture of microporous structures in vitrinite, whose mechanism may be related to the sp3 metastable carbon formed between the basic structural units in different high-pressure environments. These findings provide insight into the formation mechanism of coal graphite, bringing new understanding of the transition from multi-faceted nanocrystals to graphitization in an orogenic metamorphic zone (e.g., in high-pressure schist and hornblende).