Boron Coprecipitation with Calcite: Distinguishing Calcite-Hosted B by NMR Spectroscopy

The boron isotope composition for boron incorporated in marine calcium carbonate minerals has become an increasingly important proxy for seawater pH and thus a key constraint for the concentration of atmospheric carbon dioxide in the geologic past. We report here the results of a solid-state NMR study undertaken to investigate the nature of the boron substitution in calcite and its relationship to the calcite structure. Calcite/boron coprecipitates with B concentrations ranging from 149 to 1260 μg/g were prepared using the constant rate of addition method with 99% ¹³C-carbonate to facilitate the application of double-resonance NMR techniques. The ¹¹B MAS/NMR spectra indicate the presence of both three-coordinated (B(3)) and four-coordinated boron (B(4)) in all of the synthetic samples, with subequal abundances. Strong dephasing effects were observed in ¹¹B-observed/¹³C rotational-echo double-resonance (REDOR) experiments for both B(3) and B(4), indicating that both types of coordination environment occur in the calcite structure and in atomically close proximity to carbonate groups. The variation in the REDOR spectra with dephasing time for the sample with the highest B content reveals the presence of two distinct trigonal B environments that differ principally in the quadrupolar asymmetry parameter (η). Only the asymmetric B(3) (η ≅ 0.65) occurs in the REDOR difference spectra and can be assigned to calcite-hosted boron. The signal from an axially symmetric trigonal environment (η ≅ 0) dominates the dephased REDOR spectra at intermediate dephasing times and exhibits rapid spin-spin relaxation, suggesting assignment to B[OH]₃ groups external to calcite. The symmetric B(3) accounts for ∼15% of the B for samples with the highest B content but appears to be much less abundant or absent in samples with a lower B content, including modern and fossil brachiopods. Model-free decomposition of the ¹¹B spin-echo spectra corroborates this interpretation of the REDOR results. Comparison of the NMR parameters calculated in a previous study with those measured for B(4) and the asymmetric B(3) indicates assignment, respectively, to B[OH]₄^- and BO₂[OH]^2- groups in calcite. These results are consistent with the incorporation of trigonal B in calcite during coprecipitation but inconclusive as to whether it results from deprotonation of boric acid or coordination change of the borate ion through deprotonation and hydroxyl transfer reactions. In either case, incorporation of trigonal B must involve multiple hydrolytic processes at the calcite/fluid interface during crystal growth, which converge to yield a surprisingly structurally homogeneous B(3) substitution.