Elements and phosphorus minerals in the middle Jurassic inertinite-rich coals of the Muli Coalfield on the Tibetan Plateau

The content, modes of occurrence, and origin of elements and phosphorus minerals in the Jurassic coals of the Muli Coalfield, on the Tibetan Plateau, were investigated using optical microscopy, field emission-scanning electron microscopy in conjunction with energy-dispersive X-ray spectrometry, X-ray powder diffraction, X-ray fluorescence spectrometry, and inductively coupled plasma mass spectrometry. The Muli coals (L1 and L2 Coals) are of high volatile A bituminous to medium volatile bituminous rank, characterized by low-sulfur contents, and are generally dominated by the inertinite-group macerals (predominantly semifusinite and fusinite). The L1 Coal contains abundant apatite (7.9% on average, on an organic-matter-free basis) and alumino-phosphate minerals of goyazite–gorceixite–crandallite group (4.9% on average), and, accordingly, the concentrations of elements F (253 μg/g on average), P (2349 μg/g), Sr (526 μg/g), and Ba (790 μg/g) are elevated as compared with common world hard coals. The deposition of phosphorous-bearing minerals in the Muli coals was not derived from volcanic input; penetration of Ca- and Al-rich solutions, release of phosphorus from organic matter during plant decay, and an appropriate sedimentary environment (such as low pH, low water table for peat, and oxidizing conditions) were critical factors in deposition of the phosphorus minerals.