Structural Basis for Stabilization and Transformation Behavior of Amorphous Calcium Carbonate

Calcium carbonate biomineralization is one of the most important processes in the carbon cycle, and the inorganic formation of calcium carbonate minerals is a key component of many carbon storage strategies.An intensive research effort has demonstrated that biologic and inorganic formation of calcium carbonate minerals from supersaturated solutions commonly occurs via a metastable amorphous precursor: amorphous calcium carbonate (ACC).In biomineralization, ACC may serve several functions, allowing construction of complex functional structures and temporary storage of calcium. The presence of inorganic and organic additives serves to temporarily stabilize the amorphous state and structured templates are thought to direct assembly and further control transformation to more stable, crystalline forms, such as calcite and aragonite.ACC, which is typically hydrated upon initial formation, exhibits variable composition, properties and transformation behavior, depending on conditions of formation or according to biomineralizing organism. A fundamental limitation in our ability to integrate ACC into quantitative geochemical modeling is the challenge in evaluating its structural properties.Previous studies combining synchrotron X-raytotal scattering methods and1H and13C NMR spectroscopy have resulted in major advances in understanding structure of this amorphous phase.The application of reverse Monte Carlo modeling of total scattering data allowed has allowed development of the first structure model for hydrated ACC, which has served as a benchmark for computational studies.Despite these advances, there is still a poor understanding of the functionally distinct types of molecular water in ACC and their role in its stabilization and transformation to crystalline forms. The research program proposed here builds on our previous successful approach, adding the unique capability of neutron total scattering to overcome the inherently poor sensitivity of H for X-rays.Combined with deuterium (2H) substitution methods, neutron total scattering and pair distribution function analysis permit direct evaluation of molecular water components, allowing much-needed improvement of ACC structure models.Parallel NMR experiments that also take advantage of deuterium substitution and employ double resonance techniques will permit assignment of distinct types of molecular water and their functional roles in the stabilization and transformation of ACC.Heating experiments can be employed to achieve different hydration states leading to the onset of crystallization. In situ transformation studies will allow direct characterization of ACC crystallization kinetics. The structural information obtained in this study will provide a basis for understanding energetic and kinetic differences among varieties of ACC and associated transformation pathways.Structure models that result from this research will be used as a basis for computational modeling that examines structural interactions of ACC with inorganic and organic additives, with templates, and transformation to different crystalline forms of calcium carbonate.A long-term goal is to develop structural data for different forms of ACC that enable their inclusion in quantitative modeling of geochemical processes.Experimental structure models that account for differences in H2O content and the presence of various additives can also serve as a foundation for bio-inspired synthesis to develop functional materials.