Incorporation of fluoride in calcite: Insights from solid-state NMR, NanoSIMS, APT, HRSTEM and DFT calculations

Abstract

Despite the abundant occurrence and large variation in fluorine (F) in calcium carbonate minerals in geological settings, it remains poorly understood to what degree F can be incorporated into calcium carbonate minerals. Particularly, molecular-scale evidence for F incorporation modes in the calcite structure is lacking. In this study, we conducted coprecipitation experiments for F with calcium carbonate to reveal the nature of F partitioning in calcite minerals. The ¹⁹F solid-state NMR spectra yield four peaks at −82, −97, −107 and −108 ppm, suggesting that F can be taken up by calcite via at least four mechanisms. The −108 ppm peak occurs at F⁻ concentrations above 2 mM and is consistent with fluorite (CaF₂), suggesting that fluorite precipitation predominates at high [F]. Combined with ¹⁹F chemical shifts, ¹⁹F{¹³C} rotational echo double resonance (REDOR) techniques, and DFT-based quantum computations, the ¹⁹F NMR signals at −82, −97 and −107 ppm are assigned to surface adsorbed species, surface incorporated “CaF₂” defects and incorporated “CaF₂” defects within the bulk of calcite, respectively. Visualization via three-dimensional atomic probe topography (3D APT), high-resolution scanning transmission microscopy (HRSTEM) and nanoscale secondary ion mass spectrometry (NanoSIMS) analysis further supported that F can be incorporated into the bulk structure of calcite. These findings provide atomic and molecular insight into F incorporation in carbonate minerals and are helpful for the application of carbonate materials as potential geochemical proxies.