Experimental constraints on Li isotope fractionation during clay formation

Abstract

Abstract 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 6 Li compared to the initial solution. On average, the exchangeable solutions had δ 7 Li\xa0values 7‰\xa0lower than the initial solution. The average difference between the residual solid and initial solution δ 7 Li\xa0values ( Δ 7 Li residue - solution ) for the synthesised layer silicates was −16.6\u202f±\u202f1.7‰\xa0at 20\u202f\u202f ° C, in agreement with modelling studies, extrapolations from high temperature experimental data and field observations. Three bonding environments were identified from 7 Li-NMR spectra which were present in both bulk and residual solid 7 Li-NMR spectra, implying that some exchangeable Li remains after treatment with ammonium chloride. The 7 Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the 7 Li-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\u202f±\u202f1.1‰, −0.2\u202f±\u202f1.9‰\xa0and 15.0\u202f±\u202f12.3‰\xa0for octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1 σ ). The bulk fractionation factor ( Δ 7 Li bulk - solution ) is dependent on the chemistry of the initial solution. The higher the Na concentration in the initial solution the lower the bulk δ 7 Li\xa0value. We suggest this is due to Na outcompeting Li for interlayer sites and as interlayer Li has a high δ 7 Li\xa0value relative to octahedral Li, increased Na serves to lower the bulk δ 7 Li\xa0value. Three experiments conducted at higher pH exhibited lower δ 7 Li\xa0values 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 7 Li-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.