Fluorine incorporation into calcite, aragonite and vaterite CaCO3: Computational chemistry insights and geochemistry implications

The abundant occurrence of calcium carbonate minerals in marine sediments and their high fluorine content suggests that fluorine is a good candidate for reconstructing pale-oceanographic parameters. However, the potential of fluorine as a paleoproxy had hardly been explored, and fundamental insights into the behaviour of fluorine in biogenic carbonates and marine sediments is required. A first-principles modelling approach is used here to analyse the incorporation mechanisms of fluorine into crystalline calcium carbonates. We compute F incorporation into the CaCO3 lattice via a number of mechanisms, but concentrate on comparison of the energetics of the two easiest substitution mechanisms: replacing one oxygen atom within the carbonate group to form a (CO2F)(-) group as against a substitution involving replacement of the CO3 group by two fluorine ions to form a CaF2 defect. These incorporation mechanisms are fundamentally different from that of iodine into calcium carbonates, where a carbon atom is replaced. Our simulations suggest that the substitution of CO32- by F-2(2-) is the most favoured and that fluorine is preferentially incorporated into the three naturally-occurring polymorphs of calcium carbonate in the order greater than or similar to vaterite aragonite >> calcite. These results explain the previously-reported preponderance of fluorine in aragonite corals, and lend support to the use of F/Ca as a proxy for ocean pCO(2). (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The incorporation mechanisms of fluorine into crystalline calcium carbonates differ from that of iodine, with fluorine preferentially incorporating into vaterite and aragonite over calcite. These results support the use of F/Ca as a proxy for ocean pCO(2).