Mineralogical compositions of Late Permian coals from the Yueliangtian mine, western Guizhou, China: Comparison to coals from eastern Yunnan, with an emphasis on the origin of the minerals

Abstract

This paper reports the mineralogical characteristics of a complete coal seam in the Late Permian Longtan Formation from the Yueliangtian mine, western Guizhou, southwestern China. The minerals in the coal were identified using optical microscopy, scanning electron microscopy with an energy dispersive X-ray spectrometer (SEM–EDS), and low temperature ashing plus X-ray diffraction (LTA+XRD). The results show that quartz and kaolinite are the major components of the coal mineral matter; followed by pyrite, marcasite, anatase, and calcite; and in some cases, mixed-layer illite/smectite, chamosite, illite, ankerite, apatite, and anorthite. The highly-elevated concentration of quartz in the coal is mostly derived from siliceous solutions produced by weathering of the Emeishan basalt. However, quartz discrete particles with large size (>100μm) and rounded shape probably resulted from the input of felsic rock materials. The distribution and modes of occurrence of kaolinite, mixed-layer illite/smectite, illite, and anatase reflect a terrigenous origin. In comparison with the coals from eastern Yunnan, the Yueliangtian coals have lower proportion of authigenic chamosite, the formation of which was probably related to the injection of seawater. Pyrite occurring as individual particles or clustered framboids was most likely influenced by seawater during the stages of peat accumulation or early diagenesis, while fracture-filling pyrite probably had an epigenetic origin. Additionally, an intra-seam volcanic ash-derived tonstein layer, which contains sharp-edged quartz, vermicular kaolinite, and well-developed zircon crystals, has been identified in the coal of the present study.