Optical properties of high-temperature heat-treated vitrinites

Three vitrinites (C = 80.0, 87.9 wt%, daf and 94.2 wt%, dmmf) have been heat-treated over the temperature range 1000–2400 °C in the presence of argon. The variation of morphological and optical properties of the heat-treated samples is studied systematically using reflected-light microscopy. An optically isotropic, highly reflecting material is formed on the surface of anisotropic particles of two of the vitrinites (C, 80 and 94.2 wt%) during semi-graphitization (1000–1400 °C), which decreases in extent with increasing temperature and is absent at the end of this stage. The development and subsequent disappearance of the isotropic material is probably due to the evolution of volatile matter and subsequent deposition of pyrolytic carbon during semi-graphitization. The continuous increase of bireflectance of these two vitrinite residues, with heat-treatment temperature, indicates progressive improvement in the ordering of the molecular structure. The reflectance of the heat-treated anthracitic vitrinite (C, 94.2 wt%), increases continuously whereas the reflectance of the heat-treated low-rank vitrinite (C, 80 wt% decreases slightly from a maximum during semi-graphitization. The third vitrinite (C, 87.9 wt%) does not develop an optically isotropic highly reflecting material on particle surfaces during semi-graphitization. However, this vitrinite softens during carbonization (25–1000 °C) producing a residue with a mosaictype anisotropic microstructure. The condition of polished surfaces prepared from residues of this vitrinite from the semi-graphitization to crystallization stages (1000–2300 °C) deteriorates, values of bireflectance and reflectance decrease continuously with increasing temperature. This appears to result from a reduction in the surface hardness of particles of this vitrinite residue resulting on transformation from two-dimensional ordering to three-dimensional graphitic ordering.