Smectite as the lithium-rich mineral precursor: Key to lithium enrichment in claystone from Central Yunnan, Southwest China

Abstract

Investigating Li-rich claystone associated with bauxite series holds significant potential for establishing a new type of sedimentary Li resource. Recent advancements in mineralogical and geochemical studies have provided foundational findings, yet the specific Li-bearing mineral remains controversial, impeding a comprehensive understanding of the Li enrichment processes. In this study, Li-rich claystone from the Early Permian Daoshitou Formation in the Xiaoshiqiao area in Central Yunnan, Southwest China, is examined to explore the mineralogical composition, Li distribution, and formation mechanism of Li-rich mineral. In-situ analyses reveal that cookeite (chlorite group) is the primary Li-rich mineral, characterized by an Al/Si atomic ratio in the range of 1.3 to 2 and the interplanar spacing (d₀₀₂) of approximately 14 Å. Other Li-bearing clay minerals, including kaolinite, Al-rich chlorite, and chamosite, have significantly lower Li contents compared to cookeite. HRTEM analysis identifies ∼24 Å layers and interstratified 14–10 Å layers within cookeite grains, indicating that cookeite transformed from primary smectite, and corrensite acting as an intermediate product during burial diagenesis. Notably, this study initially investigates the Li distribution in ooids within oolitic claystone, showing that Li is enriched in cookeite within ooids, whereas Al-rich chlorite in the surrounding matrix is Li-poor. This suggests that Li in cookeite is primarily inherited from the smectite precursor, rather than introduced through subsequent hydrothermal activity. This study concludes that Li-rich smectite precursors controlled the pre-enrichment of Li, and the transformation of smectite to cookeite during burial diagenesis stabilized the initially exchangeable Li.