Correlation between optical, chemical and micro-structural parameters of high-rank coals and graphite

Abstract

Abstract In order to identify the parameters that best characterize the chemical and structural evolution of organic matter during coalification, the relationships between optical, chemical and micro-structural parameters in high-rank coals and natural graphite were studied. The samples include anthracites from Penarroya–Belmez–Espiel Basin (Spain), Douro Basin (Portugal), and Alto Chicama Basin (Peru); and natural graphite from Canada, Mozambique, and Austria. Correlations between the following optical parameters were assessed: vitrinite random reflectance (Rr), Reflectance Indicating Surfaces (RIS) axis (RMAX, RINT and RMIN), and RIS parameters (Ram, Rev and Rst), as well as Bw and AI anisotropy parameters. Furthermore, the chemical parameters used were chosen according to their significant variation in coals, namely volatile matter, carbon, and hydrogen contents calculated in dry ash free basis (VMdaf, Cdaf, Hdaf), as well as the H/C atomic ratio. Structural organization was characterized by micro-Raman spectroscopy and XRD. Raman parameters used were the full width at half maximum (FWHM) and position of G and D1 bands on the first-order Raman spectrum, and the ID1/IG intensity area ratio. The selected XRD parameters were interlayer spacing d002, and crystallite sizes La and Lc. Results show that: (i) RMAX RIS axis seems to correlate best with chemical and micro-structural parameters; (ii) for the majority of studied samples, Hdaf and H/C atomic ratio are the only chemical parameters with significant correlations with RMAX; (iii) the FWHM of the G band of Raman spectrum shows good linear correlation with the XRD parameter d002; and, (iv) structural organization of carbon materials, as measured by trends in their optical and crystalline parameters, is influenced by their hydrogen content (daf basis) and therefore by the H/C atomic ratio.