OCCURRENCE OF INORGANIC ELEMENTS IN MACERALS OF LOWER-RANK COALS AS DETERMINED BY ELECTRON MICROPROBE TECHNIQUES

摘要

In-situ elemental analysis of the individual macerals in lignites and sub-bituminous coals from several Australian and Indonesian deposits by electron microprobe techniques typically shows the presence of small but significant proportions of Ca, Al and in some cases Fe; these elements do not appear to be related to separate crystalline mineral phases inside the organic material. In cases where comparative data are available, the proportion of these inorganic elements in the macerals is consistent with the concentrations of the same elements released from the coals by ion exchange and selective leaching processes, and they are regarded as representing the inherent non-mineral inorganics commonly reported in more traditional geochemical analysis of lower-rank coal samples. In some cases the elements are more abundant in the vitrinites of the coals than in other maceral groups, but in other cases, despite differences in carbon, oxygen and sulphur contents, the concentration of these inorganic elements is similar in all maceral groups present. With the exception of an unusual coal with a high organic sulphur content from the Cranky Corner Basin of New South Wales, these elements are not found in temporally or geographically equivalent coals of higher rank, where the carbon content of the vitrinite is above around 75 percent. This applies even in cases, such as the Bukit Assam deposit in Indonesia, where igneous intrusive effects have locally increased the rank of several otherwise low-rank coal beds. The Cranky Corner Basin material has a similar carbon content to other seams of equivalent age in the adjoining Sydney Basin, but has both a lower vitrinite reflectance and a significant concentration of Ca and Al in the maceral components. Although the bulk chemical composition of this particular coal has changed as expected with rank advance, the chemical structure of the macerals, especially the vitrinite, seems to have remained similar to that associated with a lower-rank deposit, affecting both the reflectance value and the concentration of non-mineral inorganic components.