Calcium isotope fractionation upon experimental apatite formation

Abstract

Carbonated hydroxy-apatite (Ca10-b(NH4,K,Na)2b(PO4)6-y[(CO3)2x+(3/2)y(OH)2–2x]; CHAP) was synthesized in batch-type experiments by mixing of calcium (Ca)- and phosphate-bearing aqueous solutions and the transformation of calcite powder in aqueous solution between 11° and 65 °C. Compositional changes of the mother solution and solid phase products were followed by powder XRD, scanning-electron microscopy (SEM-EDX), Raman spectroscopy, and elemental analysis. It was found that the transformation at pH values between 7.5 and 7.9 occurs via the dissolution of calcite followed by the precipitation of CHAP. In the mixing approach, crystallization of CHAP took place via precipitation of metastable brushite (CaH(PO4).2H2O) as precursor phase that was further transformed into apatite.Calcium isotope fractionation led to an enrichment of the light isotope by about −0.5 to −1.1‰ in the solid CHAP compared to the aqueous solution, consistently in all experimental approaches, essentially independent of temperature. In some experiments metastable brushite formed prior to transformation to CHAP and showed a reduced fractionation compared to the CHAP. The observed Ca isotope fractionation into the CHAP lattice resembles that of natural phosphorites and lies within the range of recently published experiments.