The demonstration by Lippmaa et al. in 1980 of the potential for high-resolution MAS/NMR spectroscopy of inorganic solids led ultimately to fundamental advances in our understanding of geological materials. That line of inquiry continues through the present, illustrated by recent results for boron substitution in calcite that show the presence of multiple sites with geochemical implications. Based on the large 10B/11B isotopic fractionation between aqueous boric acid and its first hydrolysis product, geoscientists have developed paleo-pH proxies from boron that occurs in calcium carbonate minerals. However, proxy offsets exist depending on the origin of the mineral, suggesting the effects of secondary factors. Although isotopic analyses suggest that only borate ions are removed from solution during calcite precipitation, high-resolution 11B MAS/NMR spectroscopy reveals the presence of both 3- and 4-coordinated B in the solid. We investigated the nature of such calcite-hosted boron defects using 11B-13C double resonance methods. For synthetic 13C-enriched calcite, 11B-13C rotational echo double resonance (REDOR) yields complete dephasing for the trigonal B and nearly so for the tetrahedral sites. These results indicate that substantially all the boron in both coordination environments occurs in the mineral structure, in close proximity to carbonate. Variation in the trigonal B spectral profile with spin-echo delay suggests the presence of multiple 3-coordinated environments. This interpretation is supported by a model-free multivariate curve resolution analysis, which decomposes the resonance of trigonal B into two components with recognizable quadrupolar peak shapes. These components differ principally in the quadrupolar asymmetry parameter which can be attributed to distinct protonation states. These results offer promise for using NMR to relate geochemical proxy offset effects to variations in substitution defect structure.

Insights from NMR spectroscopy on the diversity of boron defects in calcite

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.