The minerals in a range of Colombian coal samples, selected to represent the principal coal-mining regions, have been investigated using low-temperature ashing and quantitative X-ray diffraction techniques, and the results compared to information from chemical analysis and Mossbauer spectroscopy studies. Kaolinite, illite, interstratified illite-smectite and quartz are the dominant mineral matter components in almost all of the coal samples, with small but significant proportions of pyrite (up to around 15% of the mineral matter) being present in around half of the samples studied. Other minerals present in particular samples include chlorite, muscovite, paragonite, ankerite, dolomite, calcite and siderite, along with phosphate minerals apatite and goyazite. Jarosite and coquimbite are also noted in some of the pyrite-bearing coals, probably representing oxidation products of the pyrite developed with exposure or storage. Bassanite is present in the low-temperature ash of some coals, and in one sample occurs along with significant concentrations of hexahydrite, alunogen, tschermigite and paraluminite, all of which were probably derived from interaction of inorganic elements in the organic matter with organic sulphur during maceral oxidation. Quantitative analysis of the minerals in the low-temperature ash of the coal samples using Rietveld-based X-ray diffraction techniques provides results that are consistent, at least for the most abundant elements, with the chemical composition of the coal ash as determined by standard analysis procedures. The evaluation of mineral-rich concentrates by Mossbauer spectroscopy also provides results consistent with the XRD data, especially as regards pyrite and some of the sulphate components. Optical microscopy indicates that much of the pyrite, and in some cases also the carbonates, occurs as anhedral crystals, finely disseminated in the maceral components, as aggregates of crystals (including pyrite framboids), as fine-grained crystals replacing liptinite, and as fracture fillings. Some sub-angular to rounded quartz grains are also present, possibly of detrital origin, but most other minerals, including the clay minerals, occur mainly as aggregates of intimately admixed mineral and maceral constituents.
Lopez, I. C., Ward, C. R., Dai, S., & Chou, C-L. (2008). Composition and mode of occurrence of mineral matter in some Colombian coals; TSOP 2006; papers from the 23rd annual meeting of TSOP. In International Journal of Coal Geology (Vol. 73, pp. 3--18). Elsevier. https://doi.org/10.1016/j.coal.2007.03.005