Atomic-scale visualization and quantification of lithium in lepidolite by AC-TEM-EELS: Implications for pegmatite genesis and advancing lithium extraction techniques

Abstract The small ionic radius of lithium ions (Li+ = 0.6 Å) poses a significant challenge in visualizing and quantifying their presence within the lepidolite lattice. Nevertheless, the use of spherical aberration-corrected transmission electron microscopy (AC-TEM) in conjunction with electron energy loss spectroscopy (EELS) offers an innovative approach to surmount this challenge. In this investigation, we conducted a comprehensive analysis of lepidolite samples obtained from lithium-rich pegmatites in the Shaliuquan region of the Quanji Massif, situated at the northwestern periphery of the Qinghai-Tibetan Plateau in northwest China. Utilizing AC-TEM in tandem with EELS, we captured sub-angstrom images revealing the presence and spatial distribution of lithium atoms within the lepidolite lattice. Additionally, we quantified their concentrations and determined the formula for lithium-rich mica. This study solves a long-standing problem regarding the accurate determination of lithium concentration and showcases the viability of AC-TEM-EELS for both visualizing and quantifying lithium within the lepidolite lattice. Our finding facilitates the differentiation of cation ordering, polytypes, and lepidolite’s chemical compositions, which is helpful for advancing lithium extraction techniques and constraining the forming process of pegmatites.