Micro-Raman Spectroscopy of Microscopically Distinguishable Components of Naturally Graphitized Coals from Central Hunan Province, China

Micro-Raman spectroscopic analysis of microscopically distinguishable components in a series of high-rank coals (anthracite to graphitized coal) adjacent to a granitic pluton was used to assess the structural evolution of coal during natural graphitization. Microscopically identifiable components were differentiated into six groups: vitrinite; inertinite; microcrystalline graphite with a fine, granular texture and a low reflectance; pyrolytic carbon with layering normal to particle edges; and needle graphite and flake graphite, both of which are similar to commercial synthetic graphite. Approaching the intrusion, Raman spectra exhibit a distinctly different evolution for vitrinite and microcrystalline graphite: the D1 band of the first-order Raman spectrum becomes narrower and more intense for vitrinite, whereas the D1 band intensity decreases for the granular, microcrystalline graphite. A plot of full width at half-maximum for the D1 band versus R1 (intensity ratio of the D1 to the G band) indicates that structural evolution of vitrinite occurs during carbonization, whereas that of the microcrystalline graphite components occurs during graphitization. The increase in the intensity of the 2D1 band and the appearance of the 2450 cm–1 band in the second-order Raman spectrum for the microcrystalline graphite components also suggest that they have reached the graphitization stage. Structural heterogeneity in the metamorphosed coals initially decreases with increased coal rank, but then increases when fine granular particles (microcrystalline graphite) are seen in the highly graphitized coals. The structural heterogeneity of the most graphitized coals increases due to the formation of new components (pyrolytic carbon, and needle and flake graphite). Insights on the structural features and evolution of natural graphitized coals at a maceral scale presented here may be important in future applications, including the production of synthetic graphite from coal and coaly microcrystalline graphite