Features of the inorganic components of a Middle Jurassic coal from the Haiwan mine, Shenfu mining area, Ordos Basin, northern China: Emphasis on origins of sub-micron and micron-sized quartz

Fine quartz particles released by coal mining and combustion can be detrimental to human health, potentially causing permanent lung damage and even death. Therefore, research on the particle size distribution, modes of occurrence, and formation mechanisms of quartz in coal is essential for guiding safe and efficient coal mining as well as the safe utilization of coal resources. In this study, multiple analytical methods, including X-ray fluorescence, inductively-coupled-plasma − mass spectrometry, quantitative X-ray diffraction, optical microscopy, and scanning electron microscopy, were employed to determine the geochemical and mineralogical features of coal from the Haiwan mine located in the Ordos Basin, China. The Haiwan coal is a bituminous coal with low ash, low sulfur, and high volatile content. The inorganic constituents in the coals were derived from intermediate and felsic rocks of the Yinshan Orogenic Belt, as well as from multiple stages of authigenesis and fluid injections. Notably, quartz constitutes a high proportion of the low-temperature ashes of the coal, with an average proportion of 42.8 %. Authigenic quartz primarily occurs in the form of aggregates of sub-micron to micron-sized particles, typically closely coexisting with kaolinite and boehmite. This mineral assemblage resulted from precipitation of Si-rich solutions leached from the lithologies of the provenance area, along with the dissolution of terrigenous detrital feldspars and desiliconization of kaolinite. Most of the authigenic quartz particles (<10 µm) with sharp edges in the coals, which are small enough to easily enter the respiratory tract, might do harm to lung tissue. Therefore, although the contents of SiO₂ (4.5 %) and quartz (5.41 %) are low on a whole coal basis, prolonged inhalation of these quartz particles may still pose health risks to humans, especially from the combustion products (i.e., coal ash) which contain much higher proportions of quartz.