Characterization of structure of kaolinite in tectonically deformed coal: evidence of mechanochemistry

Coal and gas outbursts are one of the most serious disasters in coal mines and are closely related to the distribution of tectonically deformed coal. To explore the environment of formation and the mechanochemical genetics of tectonically deformed coal and investigate the influence of tectonic stress on the crystal structure parameters of kaolinite in coal, this paper identifies whether the occurrence of the advanced evolutionary phenomenon of tectonically deformed coal is dominated by thermal metamorphic temperatures or mechanochemical stress. Three pairs of unaltered coal and tectonically deformed coal samples were collected from three different coal seams at the No. 8 coal mine in Ping Dingshan Henan Province in China. After low-temperature ashing treatment, SEM, XRD and FTIR were used to characterize the kaolinite crystal morphology, crystallinity parameter (Hinckley index, HI), structural defects, and other characteristics. The results show that compared with the unaltered coal, the crystal morphology of kaolinite in tectonically deformed coal is more disordered, the crystal plane spacings, d((020)), d((1-10)), and d((11-1)), increase, the crystallinity parameter HI decrease from 1.018, 1.104, 1.001 (more than 1, ordered) to 0.882, 0.784, 0.978 (less than 1, disordered), and the strength of the characteristic absorption band decreases, the differences in morphology and structural defects are mainly caused by the destruction of bonds, such as -OH, Al-O-Si, Al-O-Al, and Si-O-Si. Tectonic stress promotes the mechanochemical evolution of kaolinite in coal, resulting in lattice dislocations, an increase in crystal plane spacing, an increase in crystal defects and a decrease in the ordering degree. The experimental phenomenon of kaolinite converting to montmorillonite (microcrystalline kaolinite) and the mechanochemical evolution result of the crystallinity parameter decrease reflect that the crystal structure of kaolinite in tectonically deformed coal has not evolved to be ordered with the aid of temperature, which confirms that stress plays a dominant role in the evolution of advanced phenomenon of tectonically deformed coal structures.