Li concentration and isotopes in kaolinite and organic of coals determined by secondary ion mass spectrometry

Abstract

Lithium (Li) enrichment in coal has attracted significant attention due to its economic potential and complex geochemical behavior during coal formation. This study investigates the concentration and isotopic composition of Li in kaolinite and organic matter from three high-Li coals from North China using Secondary Ion Mass Spectrometry (SIMS), complemented by TESCAN Integrated Mineral Analyzer (TIMA), Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS), Fourier Transform-Infrared Spectroscopy (FTIR), and Confocal Raman Microscopy (Raman). Results reveal that kaolinite is the primary host of Li, with concentrations exceeding 1516 μg/g. Lithium isotopic compositions (δ⁷Li) in kaolinite from high-rank coals exhibit narrow ranges, suggesting Li isotopic homogenization as a result of the elevated paleotemperature. In contrast, δ⁷Li values in lower-rank coals show greater variability, influenced by pore fluids and depth-dependent fractionation processes. Organic material, characterized by lower lithium concentrations (mostly <100 μg/g) and highly variable δ⁷Li values, exhibits a heavier lithium isotopic composition in high-rank coal compared to δ⁷Li values in low-rank coal. These findings highlight the role of kaolinite in this isotopic fractionation. and emphasize the role of kaolinite as a major Li carrier. This study provides critical insights into Li enrichment mechanisms and isotopic fractionation during coalification, contributing to a better understanding of Li sources and mobility in coal-bearing strata.