Constraining lithium-clay equilibria in sedimentary environments using a new thermodynamic dataset

Lithium-rich formation brines from sedimentary basins are emerging as key unconventional resources in response to the growing global demand for lithium. This study integrates geochemical data from diverse settings, including the Smackover and Edwards Formations (Gulf Coast, USA), the Alberta Basin (Canada), and Salsomaggiore (Northern Apennine, Italy), to investigate the role of diagenetic processes and clay mineral equilibria on lithium mobility and retention. A new thermodynamic dataset was developed for lithium-bearing clay minerals and jadarite, allowing the construction of activity diagrams, calculation of saturation indices, and modeling. Activity diagrams indicate progressive brine evolution from kaolinite to montmorillonite, and toward Mg-rich saponite/chlorite assemblages, consistent with advanced diagenetic stages and lithium uptake into octahedral sites. The transition from equilibrium with smectites to chlorite-like phases reflects increasing temperature and prolonged water-rock interactions. A hyperalkaline paleo-fluid in equilibrium with jadarite and associated phases was also modeled, indicating that lithium concentrations in the Jadar Basin may have reached levels comparable to those currently observed in the Salar de Atacama. These findings underscore the dual role of clay minerals as buffers and potential sources for lithium in sedimentary systems, providing new insights for exploration and geochemical modeling of lithium-rich formation brines.