High-resolution solid-state 7Li NMR study of lithium incorporation in calcite and aragonite

The concentration of Li and its isotope compositions in CaCO₃ minerals have been proposed as emerging paleo-environmental tools. Yet the bonding environment of Li in calcite and aragonite, a prerequisite for interpreting mechanisms controlling Li incorporation and isotope fractionation, is not known. In this study, the atomic-scale environment of lithium in two synthetic samples of calcite and aragonite, is determined for the first time using ⁷Li Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). The spectra display several overlapping signals with ⁷Li chemical shift ranging between −2 and + 1 ppm in the calcite sample, and spanning a narrower range, from −1 to 0 ppm, in the aragonite sample. This attests to a diversity of Li atomic-scale environments, which are interpreted in the light of a theoretical study of Li-bearing calcite and aragonite models using the Density Functional Theory (DFT). In the aragonite sample, Li is dominantly located at neighbouring substitutional and interstitial sites, the interstitial Li atom ensuring the charge balance of the Li for Ca substitution. In the calcite sample, Li ions occur at substitutional and interstitial sites, but additional mechanisms could ensure the charge balance, notably involving the incorporation of H⁺ ions at nearby carbonate sites. This variability of Li solid-state environment could explain why it is challenging to identify the dominant factors controlling the Li incorporation in inorganic calcium carbonates from aqueous solutions.