Experimental constraints on Li isotope fractionation during clay formation

Knowledge of the lithium (Li) isotope fractionation factor during clay mineral formation is a key parameter for Earth system models. This study refines our understanding of isotope fractionation during clay formation with essential implications for the interpretation of field data and the global geochemical cycle of Li. We synthesised Mg-rich layer silicates (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. Bulk solid samples were treated with ammonium chloride to remove exchangeable Li in order to distinguish the Li isotopic fractionation between these sites and structural (octahedral) sites. Bulk solids, residual solids and exchangeable solutions were all enriched in 6Li compared to the initial solution. On average, the exchangeable solutions had δ7Li values 7‰ lower than the initial solution. The average difference between the residual solid and initial solution δ7Li values (Δ7Liresidue-solution) for the synthesised layer silicates was −16.6 ± 1.7‰ at 20 °C, in agreement with modelling studies, extrapolations from high temperature experimental data and field observations. Three bonding environments were identified from 7Li-NMR spectra which were present in both bulk and residual solid 7Li-NMR spectra, implying that some exchangeable Li remains after treatment with ammonium chloride. The 7Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the 7Li-NMR data with mass balance constraints we calculated a fractionation factor, based on a Monte Carlo minimum misfit method, for each bonding environment. The calculated values are −21.5 ± 1.1‰, −0.2 ± 1.9‰ and 15.0 ± 12.3‰ for octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1σ). The bulk fractionation factor (Δ7Libulk-solution) is dependent on the chemistry of the initial solution. The higher the Na concentration in the initial solution the lower the bulk δ7Li value. We suggest this is due to Na outcompeting Li for interlayer sites and as interlayer Li has a high δ7Li value relative to octahedral Li, increased Na serves to lower the bulk δ7Li value. Three experiments conducted at higher pH exhibited lower δ7Li values in the residual solid. This could either be a kinetic effect, resulting from the higher reaction rate at high pH, or an equilibrium effect resulting from reduced Li incorporation in the residual solid and/or a change in Li speciation in solution. This study highlights the power of 7Li-NMR in experimental studies of clay synthesis to target site specific Li isotope fractionation factors which can then be used to provide much needed constraints on field processes.