Carbon microspheres in natural coke: Optical microscopic characteristics and their origin and implications

Carbon microspheres (CMs) in natural coke are formed when coal seams undergo magma-induced rapid thermal metamorphism. Analyzing the origin of the optical microscopic characteristics of CMs assists in gaining deep insights into the rapid thermal carbonization and metamorphism of coals. This study examined the natural coke samples rich in CMs from Shitai Coal Mine in Huaibei, Anhui Province. Using experiments like high-resolution reflected polarized light microscopy and scanning electron microscopy (SEM), this study characterized the particle sizes, distribution, optical microscopic characteristics, and ultramicrofabrics of CMs in natural coke, revealing the origin and thermal evolution implications of their optical anisotropy. The results indicate that the natural coke far from the rock masses is primarily composed of coarse-grained CMs with sizes ranging from 10 to 100 μm, and even above 150 μm in some cases. In contrast, the natural coke adjacent to rock masses predominantly comprises fine-grained CMs with sizes between 1 and 10 μm. Additionally, the particle sizes of CMs are even below 1 μm in veined coke within rock masses. The analytical results indicate that coarse-grained CMs and carbon semispheres exhibit cross and wavy extinction, respectively, which are the optical characterization of radial uniaxial indicatrix when being projected onto the tangent plane. Fine-grained CMs manifest cross-hyperbolic extinction, which is the rotational effect generated when radial biaxial indicatrix inside are projected onto the sections. Hence, with a reduction in the distance from rock masses and an increase in vitrinite reflectance, indicatrix of coke gradually transitions from regular spheres (optically isotropic substances) toward biaxial ellipsoids (uniaxial crystals) and triaxial ellipsoids (biaxial crystals). During the transition, the coarse-grained CMs with cross-wavy extinction are the product of the thermal devolatilization of coals, whereas the fine-grained CMs with cross-hyperbolic extinction are the sign of coal lamination formed by thermal polycondensation. The optical microscopic characteristics of CMs in natural coke, stemming from the thermal carbonization of coal seams, essentially represent the optical characterization of indicatrix subjected to thermal deformation. Therefore, these characteristics can be employed to effectively assess the thermal metamorphism degree of magma-altered coal coke.