Occurrence and origin of graphite microcrystal in meta-anthracite

Microcrystalline graphite occurs widely in meta-anthracite, to analyse the occurrence characteristics and genesis of graphite microcrystal, by methods of optical microscope, scanning electron microscope and transmission electron microscope, the optical properties, material composition and crystal structure of graphite microcrystal in Yong'an coal field of Fujian Province were characterized, and their genesis was preliminarily discussed. The results show that the graphite microcrystal in coal occurs mainly in micro cracks and cavities, and most of them have fibrous microstructure similar to petroleum coke, with bubble membrane structure. The maximum reflectivity of the microcrystal ranges from 9.29% to 10.83% and significantly stronger than the original components in coal. Using the orthogonal polarized light plus gypsum test plate, the microcrystal shows the interference color of primary yellow and secondary blue as well as the mosaic, regional and fibrous microstructure, with obvious local orientation. Under scanning electron microscopy and electron probe microanalysis, the microcrystal was mostly scaly, with flowing and gas pore structures, and showed the characteristics of pure carbon. Under high-resolution TEM, the straight and directional lattice stripes of the microcrystal were observed, and the selected area electron diffraction shows a typical lattice ring and spot of graphite. It was preliminary considered that the carbon of the graphite microcrystal in coal came from the highly molten meta-exinite and hydrogen-rich meta-vitrinite, and the heat source was the Yanshanian granite intruding into the periphery of the coal field and the bottom of the basin. The molten carbonaceous materials flowed into the pore and fissure space, in which the impurity elements were removed at high temperature, and then formed the graphite microcrystal with significant directivity under the strong magma emplacement pressure. At the same time, it was also possible that the microcrystalline was formed by the transformation of exsudatinite in coal for the study area.