天然焦内炭微球显微光学特征、成因及其意义

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

天然焦内炭微球是煤层遭受岩浆速热变质的产物,其显微光学特征成因解析有助于深入认识煤速热碳化和热变质。采集安徽淮北石台煤矿富含炭微球的天然焦样品,利用高分辨率反射偏光显微镜和扫描电镜等实验手段,表征了天然焦中炭微球的粒度、分布、显微光学和超微组构特征,以揭示其光学各向异性的成因和热演化意义。结果显示,远离岩体天然焦中炭微球以粗粒为主,粒径10~100μm,少数可达150μm以上;紧邻岩体天然焦炭微球以细粒为主,粒径1~10μm,在岩体内脉状焦内甚至低至1μm以下。分析显示,粗粒炭微球和炭半球分别发育“十字”消光和“波状”消光,是其内部放射状一轴晶光率体在切面内投影的光学表征;细粒炭微球发育“十字-双曲线”消光,是其内部放射排布的二轴晶光率体切片投影的旋转效应。由此可见,趋近岩体,伴随着镜质体反射率增加,煤焦光率体逐渐由规则球形(光性均质体),向二轴椭球体(一轴晶)和三轴椭球体(二轴晶)转变。在该过程中,“十字”消光和“波状”消光的粗粒炭微球是煤热脱挥发分的产物,而“十字-双曲线”消光的细粒炭微球是煤热缩聚而片层化的标志。天然焦中炭微球的显微光学特征是煤层趋热碳化的结果,本质上是其光率体受热变形的光学表征,可有效评价岩浆蚀变煤焦的热变质程度。

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 100μm,and even above 150μm in some cases.In contrast,the natural coke adjacent to rock masses predominantly comprises fine-grained CMs with sizes between 1 and 10μm.Additionally,the particle sizes of CMs are even below 1μm 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.