In situ techniques for classifying apatite in coal

Calcium phosphate apatite, Ca5 (PO4)3(OH,F,Cl), is a common accessory mineral in coal seams. Although minor, its role in hosting phosphorus (P) and fluorine (F) can reduce the economic viability of coking and thermal coal deposits. Apatite in coal is commonly <50 μm and may have several modes of occurrence. The modes of occurrence investigated as part of this study include apatite infilling small (i.e. less than 30 μm) cell lumens of fusinite and semifusinite macerals (pore-apatite); apatite infilling fractures (fracture-apatite); apatite occurring as discrete crystals in organic layers or in lithic lenses or bands (detrital-apatite); and apatite nodules or encrustations on silicate minerals (encrusting-apatite). However, the small size of the apatite and its intermixed relationship with the coal and other mineral matter precludes separating the apatite from the coal for analysis and has implications for commercial beneficiation. Therefore, this study documents the trialling of different in situ techniques for evaluating the halogens, specifically F, within these different apatite micro-crystals in polished mounts. In general, standards-based, fully quantitative analysis of major and minor elements (including F, Cl) was accomplished using electron probe microanalysis (EPMA). This in conjunction with electron backscatter diffraction was used to evaluate the measured halogen contents for the different apatites and confirmed that all of the apatites are technically fluorapatite with F contents >1 wt%. However, the small crystallite size of the encrusting-apatites and the pits on the polished surface of the detrital-apatites intermixed with the coal layers hindered determining the halogens by this method. Although, all of the apatites in the coals were able to be analysed by infrared spectroscopy, both the combined kaolinite and apatite spectra, and the noise in the spectra at the lower wavelengths hindered evaluating the F in the apatites using this method. Despite these limitations, the findings of this study show that EPMA is probably the most suitable analytical technique for determining the halogen contents of the different modes of apatites in situ in coal pellets. Future work should focus on determining whether the modes of occurrence reflect paragenesis.