Total X-ray scattering and pair distribution function analysis are combined with nuclear magnetic resonance spectroscopy to identify key differences in structural properties between biogenic and synthetic samples of amorphous calcium carbonate (ACC). Biogenic samples studied are gastroliths taken from the American lobster and are composed of hydrated ACC containing minor impurities. X-ray pair distribution functions reveal that the short- and medium-range structure found in synthetic ACC also occurs in gastrolith ACC, notably with atomic pair correlations extending up to ∼10 Å. The 13C NMR spectra of gastrolith ACC show a distribution of carbonate environments as seen in synthetic hydrated ACC. However, 1H NMR spectroscopy reveals that a mobile H2O component and hydroxyl groups found in synthetic hydrated ACC are absent in the gastrolith ACC. This difference may arise from differences in local conditions of ACC formation. The 31P NMR results indicate that inorganic phosphate is the principal form of the minor phosphorus. Gastrolith that was allowed to age shows the presence of calcite and vaterite, as well as residual ACC. 31P NMR also reveals trace amounts of monetite (CaHPO4) in aged samples, raising the possibility that fresh gastrolith ACC may contain a minor component of amorphous calcium phosphate. The findings suggest that important differences in the hydrous components between synthetic and biogenic hydrated ACC influence stability of the amorphous phase and its transformation to crystalline forms, thereby extending the foundation for advanced materials applications in engineered systems.

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Organic phosphorus incorporated in calcite during laboratory precipitation experiments and in natural cave deposits was investigated by solid-state NMR spectroscopy. For calcite precipitated in the presence of organic phosphoesters of varying size and functionality, solid-state 31 P{ 1 H} CP/MAS NMR shows that the phosphoesters were incorporated intact into the solid. Systematic changes in the 31 P NMR chemical shift of the phosphate group were observed between the solid phosphoester and that incorporated in the solid precipitate, yielding 31 P NMR chemical shifts of the coprecipitates in the range of +1.8 to -2.2 ppm.These chemical shifts are distinct from that of similarly prepared calcite coprecipitated with inorganic phosphate, 3.5 ppm. Only minor changes were noted in the phosphoester 31 P chemical shift anisotropy (CSA) which suggests no significant change in the local structure of the phosphate group, which is dominated by C-O-P bonding. Close spatial proximity of the organic phosphate group to calcite structural components was revealed by 31 P/ 13 C rotational echo double resonance (REDOR) experiments for coprecipitates prepared with 13 C-labeled carbonate. All coprecipitates showed significant 31 P dephasing effects upon 13 C-irradiation, signaling atomic-scale proximity to carbonate carbon. The dephasing rate for smaller organophosphate molecules is similar to that observed for inorganic phosphate, whereas much slower dephasing was observed for larger molecules having long and/or bulky side-chains. This result suggests that small organic molecules can be tightly enclosed within the calcite structure, whereas significant structural disruption required to accommodate the larger organic molecules leads to longer phosphate-carbonate distances. Comparison of 31 P NMR spectroscopic data from the synthetic coprecipitates with those from calcite moonmilk speleothems indicates that phosphorus occurs mainly as inorganic orthophosphate in the natural deposits, although small signals occur with characteristics consistent with phosphate monoesters. The results of this study indicate that trace-to minor concentrations of dissolved organic molecules can be effectively taken up during calcite precipitation and incorporated in the structure, leaving a resilient record of materials present during crystallization.