Chemical structure and genesis of fusain of Yimin Formation in Hailar Basin

As one of the four macroscopic coal rock components, fusain contains abundant information on Palaeoclimatology, Palaeoecology. This article takes the fusain components of lignite from No. 16 coal seam of Yimin Formation in the Yimin open-pit coal mine and Dayan open-pit coal mine in Hailar Basin as examples. The molecular structure of fusain was quantitatively characterized by the test results of electron microscopy, NMR (Nuclear Magnetic Resonance), EPR (Electron Paramagnetic Resonance), and the genesis of fusain was discussed in combination with the observation of micromorphology. The results indicate that there is a significant difference in the molecular structure between fusain and coal itself. The proportion of aromatic carbon in fusain is about 75%, and the content of fatty carbon is low, indicating a high degree of aromatization. The formation characteristics of fusain fire burning are significant, with significant morphological differences. The cell walls are thin and mostly white, preserving clear plant rings and cell structures, indicating rapid carbonization and burial processes. In addition, the degree of carbonization is controlled by multiple geological factors. EPR studies showed that the free radical concentration of the fusain samples indicated that the carbonization temperature should be between 300 ℃ and 400 ℃, mainly due to fire burning. The morphology and NMR analysis further supported the inference of fire burning causes. The reflectance analysis of inertinite shows that the carbonization temperature is basically the same as that inferred based on the free radical concentration, and the reflectance of fusinite in fusain is significantly different due to the influence of oxygen concentration, burning time and spatial location, which reflects the non-uniformity of the degree of fire burning and reveals the complexity of fire burning in geological history. By comparing and analyzing the results of δ¹³C value tests in the study area, combined with coal facies and paleogeographic features of the coal accumulation period, the ancient environmental characteristics of the hot climate and frequent wildfires during the early Cretaceous Yimin Formation coal period in the Hailar Basin are further confirmed.