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 in-sights 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 polar-ized 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 evolu-tion implications of their optical anisotropy.The results indicate that the natural coke far from the rock masses is primar-ily 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 re-flectance,indicatrix of coke gradually transitions from regular spheres(optically isotropic substances)toward biaxial el-lipsoids(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 char-acteristics of CMs in natural coke,stemming from the thermal carbonization of coal seams,essentially represent the op-tical 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.
carbon microsphere(CM)cross extinctionoptical anisotropyoptical indicatrixnatural cokeShitai Coal Mine in Huaibei,Anhui Province
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