Modes of occurrence of minerals in the Carboniferous coals from the Wuda Coalfield, northern China: with an emphasis on apatite formation

The mineralogical compositions of the Nos. 9 and 13 coals, which are medium-volatile bituminous coals in rank, from the Wuda Coalfield at the northwestern margin of the Ordos Basin in northern China, were investigated by optical microscopy, field emission-scanning electron microscopy in conjunction with energy-dispersive X-ray spectrometry (SEM-EDX), and X-ray powder diffraction techniques. The minerals in the Wuda coals are mainly represented by quartz, kaolinite, illite, pyrite, marcasite, apatite, dolomite, and ankerite, with trace amounts of anatase, calcite, boehmite, jarosite, gibbsite, anhydrite, and bassanite in some samples. The rod-like pyritized bacteria have been identified with SEM-EDX in Wuda coals. Moreover, the slightly reducing and alkaline environment in the original peat swamp favored bacterial action and propagation. The average concentrations of P2O5 in the Nos. 9 and 13 coals are 0.47 and 0.18 %, respectively. Phosphorus is not uniformly distributed within the Wuda coal seam. The maximum content of apatite in Wuda coals in certain horizon can reach up to 91.4 % (on an organic matter-free basis), corresponding to the fluorine and P2O5 concentrations of 2803 μg/g and 5.96 %. The high proportion of fluorine and P2O5 in the Wuda coals is mainly due to the authigenic apatite. The phosphorus in Wuda coals was probably derived mainly from phospho-proteins in the organic matter of the original peat deposits.