Enrichment mechanisms of lithium in the No. 6 coal seam from the Guanbanwusu Mine, Inner Mongolia, China: Explanations based on Li isotope values and density functional theory calculations

Abstract High concentration of Li, associated with coal, has been found in the Guanbanwusu coal Mine, Inner Mongolia, China. Earlier studies suggested that the lithium came from the Yinshan Paleoland during the peat-forming stage. The results of Li isotope determinations and calculations based on density functional theory (DFT) indicated that the lithium in the peat was released into the pore water and adsorbed by kaolinite edge surfaces (octahedral layers and/or tetrahedral layers) in the subsequent diagenesis and coalification. Alternatively, the Li of the partings, the floor, and the roof of the No. 6 coal seam also likely originated from the underlying bauxite of the Benxi Formation. Specifically, we determined the Li isotopic fractionations produced in chelation by small carboxylate (Δ7/6Liorganolithium-aq = −0.35‰), in adsorption onto kaolinite edge surfaces (Δ7/6Lioctahedral layer-aq = 4.36‰, Δ7/6Litetrahedral layer-aq = −4.53‰), and during incorporation into the lattices (Δ7/6LiAl2-aq = −8.2‰ and Δ7/6LiAl1-aq = −11.9‰). Combining these results with the δ7Li of 3.37 ± 0.13‰ for the moyite of the Yinshan Paleoland, we deduced a theoretical δ7Li range for the No. 6 coal seam and the partings, 2.85–7.38‰, exactly covering the determined δ7Li values of the coal and the partings. However, only the partings plotted in the theoretical δ7Li range of 1.13–5.66‰ assuming the bauxites (with δ7Li 1.30 ± 0.03‰) as the lithium source. During weathering and deposition, conversion of structurally incorporated Li within the grain boundaries of polycrystalline minerals of the bauxites to adsorbed Li was inferred to explain the Li isotope excursion in the latter case.