Mineralogical and geochemical compositions of the early permian coal in the Huixiang Mine, Dengfeng Coalfield, Henan, China: Occurrence modes and origins of NH4-illite

Abstract

In this study, we investigated the mineralogy and geochemistry of the Pennsylvanian No. 21 coal seam at the Huixiang Mine in the Dengfeng Coalfield, North China by employing a range of analytical techniques, including electron probe micro-analysis, X-ray fluorescence spectrometry, inductively coupled plasma mass spectrometry, and X-ray powder diffraction. The No. 21 coal in Huixiang Mine is classified as semi-anthracite and is characterized by a low total sulfur and ash contents. The predominant minerals identified in the Huixiang coal are NH4-illite, kaolinite, calcite, and chlorite, alongside trace amounts of Al-hydroxide minerals, zircon, and pyrite. Coalification in the Huixiang Mine has been primarily influenced by the heat generated by gravity-gliding tectonic movements since the Yanshan period, and the presence of hydrothermal fluids. During the coalification process, organic nitrogen undergoes thermal ammoniation, releasing NH4+ ions that subsequently react with kaolinite, leading to the formation of NH4-illite. Concurrently, when kaolinite is converted into NH4-illite, the released Si and excess Al may provided the necessary material for the formation of Al-hydroxide minerals and chamosite. The coal in the Huixiang Mine is slightly enriched with Li, Zr, Nb, Ta, and Th, rendering it a promising candidate for potential industrial recovery. The strong correlation of these elements with ash content suggested that their presence was largely influenced by terrestrial material input, attributable to the source materials originating in the North Qinling Orogenic Belt.