THE SOLUBILITY AND STRUCTURE OF AMORPHOUS CALCIUM CARBONATE(S) (ACC): TOWARD AN UNDERSTANDING OF CHEMICAL AND STRUCTURAL EVOLUTION DURING MINERALIZATION

Recent advances show the widespread occurrence of multistep pathways to mineralization in biological and geological settings (De Yoreo et al., 2015, Science). These findings highlight the importance of understanding the role of amorphous intermediates in determining isotopic compositions of final products. For example, amorphous calcium carbonate (ACC) often forms as a reactive intermediate in biomineralization and little is known about its structural and chemical properties. The complexity of these systems is seen in a recent tracer study of the ACC to calcite transformation that qualitatively showed the importance of Mg content in determining the isotopic composition of final products (Giuffre et al., 2015, GCA). Given that the relationships between the isotopic compositions of initial and final mineral products are not understood, an understanding of the structure and properties of ACC is critically needed to establish the mechanistic basis for the signatures recorded in carbonate minerals.

This experimental study quantified the chemical and physical properties of ACC and its evolution to final products. We first determined ACC solubility under controlled chemical conditions using a flow-through reactor developed by our research group (Blue et al., 2017, GCA). The experimental design varied Mg concentration and total alkalinity while maintaining a mild pH. ACC solubility was measured at specific time points during the precipitation (from super- and undersaturated conditions) and during its subsequent evolution. Parallel experiments characterized the structure of the corresponding amorphous products using in situ pair distribution function (PDF) and small-angle x-ray scattering (SAXS) analyses.

The measurements demonstrate at least two types of ACC with structure-specific solubility are produced by tuning Mg concentration and alkalinity. We also find Mg-dependent and temporal changes in the short-range ordering of ACC after precipitation. This suggests elemental and isotopic signatures contained in the final crystalline product record the pathway-dependent evolution of the amorphous intermediate. Insights from this study hold promise for establishing a process-based understanding of mineralization pathways and the factors that determine the signatures recorded in carbonate biominerals.