Anisotropic and Heterogeneity of Coal Measures Graphite
Micro-Crystalline Structure by Raman Spectroscopy
LI Huan-tong1, 2, ZOU Xiao-yan3*, XIA Yan4, ZHANG Wei-guo1
1. College of Geology and Environment, Xi'an University of Science and Technology, Xi'an 710054, China
2. Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education, Wuhan 430074, China
3. College of Urban, Rural Planning and Architectural Engineering，Shangluo University, Shangluo 726000, China
4. Coal Geology Survey of Ningxia Hui Autonomous Region, Yinchuan 750011, China
Abstract：Coal is very sensitive to geological environmental conditions such as temperature and pressure. Driven by tectonic stress, coal's “graphite crystallite” structure grows in orientation, and the physical chemistry and structure show anisotropy. X-ray diffraction and Raman spectroscopy characterized the samples with different degrees of graphitization. The results show that the coal seam slides along the layer under high temperatures and shear stress. The structural deformation makes the graphite crystallites rotate and preferentially oriented, increasing the stacking height along the c-axis direction. The anthracite stage is divided by the stacking degree Lc≤5 nm, which is a turbulent layer structure with random orientation or irregular arrangement of carbon layers, optically isotropic; when Lc≥30 nm, it is regarded as a sign of the formation of a perfect graphite structure, and the optical anisotropy is significant. It belongs to the transition state structure (semi-graphite stage) with an imperfect graphitization structure between 10~20 nm. The 1 350 cm-1 band (D1) and 1 620 cm-1 band (D2) in the Raman spectra of SXL100 and SXL130 samples in the graphite stage are obvious. Still, the full width at half maximum of the D1 to G peak (ID1/IG, R1), and the intensity ratio of D2 to G peak (ID2/IG, R3) in the Raman spectra of the graphite edge plane are significantly higher than those of the preferred orientation plane, indicating that the Raman parameters such as intensity ratio depend on the orientation of the edge plane of coal-based graphite. The intensity of the D1 peak depends on the degree of defect or disorder of the sample. The D2 peak of the edge plane has asymmetric characteristics, and the bimodal structure is significant. The D′1 peak changes with the D2 peak, which also shows the spectral behavior of the edge plane defect. The stages of anthracite (R1≥1.0), semi-graphite (1.0>R1≥0.5), and graphite (R1<0.5) were divided by the defect density or order degree index R1 of coal measures graphite, and the uniformity of coal measures graphite and the proportion and distribution of components with different graphitization degrees were evaluated. It was found that the proportion of advanced evolution to semi-graphite structure in the metamorphic anthracite CM130N sample was 3.52%, the proportion of anthracite structure in the semi-graphite BC210 sample was 46.40%, and the SXL130 sample was graphitized as a whole. However, the proportion of anthracite structure was still 3.84%, and there were still defects in the preferred orientation plane and edge plane structure. The established method has more advantages in distinguishing anthracite and semi-graphite. When the incident direction of the Raman laser is constant, R1 and R3 parameters can be used to explore the Raman spectral characteristics of coal measures graphite preferred orientation plane affected by tectonic stress and to evaluate the heterogeneity of graphitization and the orientation of graphite crystallites.
