Host minerals of lithium in Jiujialu Formation Li–rich claystones in South China, and implications for the genesis

Abstract

Lithium is a critical metal, and recent discoveries have identified significant lithium enrichment in the Late Paleozoic Jiujialu Formation (C₁jj) of southwestern China. However, the specific mineral hosts of lithium remain a topic of debate. This study examines two representative sections of the Jiujialu Formation, utilizing major and trace element analysis, δ⁷Li, SEM-EDS, and XRD techniques. However, previous TOF-SIMS studies have shown that lithium is not enriched in K-rich illite. Furthermore, ion exchange experiments indicate that approximately 6 % of lithium can be exchanged by Na⁺. This finding, combined with δ⁷Li values, XRD patterns, and SEM–EDS data, supports further interpretation. In conjunction with a review of previous research on the formation and transformation of clay minerals, this study suggests that lithium is predominantly hosted in the smectite crystal lattice of illite-smectite mixed layers. It proposes a two-stage diagenetic process for the formation of lithium-rich claystones: the first stage involves the formation of lithium-rich smectite, and the second stage entails the transformation of this smectite into illite and chlorite during diagenesis. In the first stage, weathering of argillaceous dolomite created an alkaline, stagnant, and closed sedimentary environment that facilitated the formation of lithium-rich smectite. In the second stage, lithium-rich smectite, as a precursor, undergoes illitization and chloritization. While illitization leads to the formation of stable illite-smectite mixed layers, chloritization of smectite is less likely to form stable chlorite-smectite mixed layers. Consequently, claystones with higher illite content tend to have higher lithium concentrations. Furthermore, complete illitization and chloritization result in the formation of low-lithium claystones. This study resolves discrepancies in previous research and provides a new perspective on the relationship between the diagenetic processes of lithium-rich claystones and the variations in lithium content.