Elemental composition of coal macerals in relation to vitrinite reflectance, Gunnedah Basin, Australia, as determined by electron microprobe analysis

Abstract

The elemental composition of macerals in high-volatile bituminous coals from the Gunnedah Basin, New South Wales, Australia, has been analysed by light-element electron microprobe techniques. The results have been evaluated in relation to maximum vitrinite reflectance trends in vertical section, including the effects of marine influence and igneous intrusions on the coal-bearing sequence. Mean maximum vitrinite (telocollinite and desmocollinite) reflectance for the analysed samples ranges from 0.63% to 0.99%, and to 2.2% for coal affected by igneous intrusions. The carbon content of the vitrinite, as determined by electron microprobe, ranges from 79.74% to 86.07%, and up to 89.06% for the heat-affected coal studied. A simple relationship cannot be established between the reflectance of the vitrinite and its carbon content over this range of coal samples. Vitrinite in marine influenced coals (with suppressed reflectance) appears to have a slightly higher carbon content than vitrinite in isorank coals without marine influence. The increase in carbon may be due to incorporation of carbon and sulphur–rich lipoid material into the vitrinite component. Notwithstanding these differences, the carbon content of vitrinite (telocollinite) seems to be affected only slightly by the depositional changes that cause suppression of vitrinite reflectance in marine-influenced coals. Telocollinite carbon content may therefore be a useful alternative to vitrinite reflectance as a rank indicator in maturation studies. The carbon content of whole-coal samples, determined by ultimate analysis, shows in vertical section a trend intermediate between the carbon of the vitrinite determined by electron microprobe and that of the associated inertinite macerals. Whole-coal analysis data in this range depend on the relative proportions of the different macerals present, as well as the rank of the coal concerned. Coals affected by igneous intrusion show a different relationship between carbon and oxygen of vitrinite (telocollinite), relative to coals where the rank is determined by depth of burial alone. The difference in C–O relationships suggests that the short-term heating associated with intrusions produces chemical changes in macerals that are not paralleled in more normal rank advance.