Aluminum-bearing nano-sized minerals in vitrain band and its implications for modes of occurrence of Al in Carboniferous-Permian coals from the Hedong Coalfield, northern China

As one of the elements having significant affinity for organic matter in low-rank coals, aluminum (Al) can form strong complexes with organic matter. Measurable concentration of Al has been detected in ‘clean’ macerals of coal, especially vitrinite. This raises interesting questions on the modes of occurrence of Al in coal and the specific way of Al associated with organic matter. In this study, the mineral characteristics, content of major-element oxides, and modes of occurrence of Al in Carboniferous-Permian coals from the Hedong Coalfield are investigated. Our study shows that Al in coal occurs primarily as aluminosilicate minerals such as kaolinite and illite. This is supported by the strong correlation between Al2O3 and SiO2 concentrations and the bulk of Al being removed by HF during sequential leaching. The proportion of Al bound by maceral as a non-mineral element (NME) is low (< 6%), as suggested by the mass ratio of Al2O3 in the demineralized vitrain band to that of in the bulk coal. This is in accord with the sequential leaching results, which shows that the proportion of Al in the HNO3 leachate and the residue is less than 20%. Transmission electron microscopy (TEM) combined with fast Fourier transform (FFT) and energy-dispersive spectroscopy (EDS) have found that aluminosilicate minerals are the most ubiquitous nanoscale particles in the vitrain band. Nano-sized andalusite, pyrophyllite, donbassite, and kyanite, as well as some amorphous Al-bearing nanomaterials are identified. These results indicate that some of the Al, which is assumed as being in an organic association, may be truly associated with minerals. These Al-bearing nanoparticles and amorphous nanomaterials observed in the studied vitrain band may be of detrital origin, or from Al complexes concentrated in vitrinite.