Multistep incongruent hectorite dissolution controls lithium release

Clays such as hectorite are major geological sinks for lithium, yet the mechanisms controlling its release during acid dissolution remain poorly constrained. Hectorite from the McDermitt Caldera (Nevada–Oregon, USA) represents one of the most significant clay-hosted Li deposits. Here we combine size separation, mineralogical characterization, and pH- and acid-dependent dissolution experiments with kinetic modeling to elucidate Li release from colloidal hectorite. The colloidal fractions (<1 μm) host nearly 99% of the total lithium, whereas coarser fractions are dominated by quartz, feldspar, and carbonates. The separated (fluo)hectorite exhibits a formula of (Na 0·043 K 0·018 Ca 0.090 ) (Li 0·329 Mg 2·538 Fe 0·107 Mn 0.005 ) (Si 3·908 Al 0.092 )O 10 (OH) 1·0 F 1.0 with an apparent solubility constant of log K = −5.7. Dissolution experiments using five acids (H 2 SO 4 , HCl, HNO 3 , H 3 PO 4 , and citric acid; 0.02-2 M) reveal a multistep, incongruent release sequence of Ca → Mn → Fe → Al → (Si, Li, Mg, F), reflecting preferential dissolution of isomorphic substitutions. A two-step kinetic model captures this behavior, showing that Li and Mg release from octahedral sites is rate-limiting and that dissolution rates increase with increasing proton activity. A minimum of ∼16.3 protons added per Li released can be achieved by optimizing acid concentration according to reaction stoichiometry. These findings define the structural controls and rate laws governing Li release and provide a mechanistic foundation for efficient and sustainable extraction from clay-hosted lithium resources.