Microstructural evolution of carbonaceous material during graphitization in the Gyoja-yama contact aureole: HRTEM, XRD and Raman spectroscopic study

Graphitization of carbonaceous material (CM) in the Gyoja-yama contact aureole, northwest of Kyoto, Japan, was examined using high-resolution transmission electron microscopy (HRTEM), X-Ray Diffractometry (XRD), and Micro-Raman spectroscopy. In this region, the CM in the sedimentary rocks has been transformed from poorly crystalline to well-crystallized graphite. This conversion is evident from its morphological transformation of the grain structure to filament like structure and finally into hexagonal plate structure. These microstructural evolutions closely resemble the graphitization process that is usually observed in regional metamorphism, as reported in previous HRTEM observations. Micro-Raman and XRD results indicate that the graphitization parameters are also comparable to each other, where Lc(002) and La(110) values increase during graphitization in contact metamorphism. However, the d(002) values have lower correlation coefficients with R2 compared to other parameters. In addition, the crystal size of fully ordered graphite with a d-spacing of <= 3.36 angstrom in contact metamorphic rocks is approximately 100 angstrom larger than that in regional metamorphic rocks. These features indicate that information of d(002) parameter includes not only the peak metamorphic temperature condition, but also several other factors such as deformation, confining pressure, impurities of fluid activities and catalytic effect, which may be the key point to fully understand the evolution of CM to graphite.