The solubility and structure(s) of amorphous calcium carbonate(s) (ACC) under controlled conditions*

Recent advances show multistep pathways to mineralization are widespread in biological and geological settings (De Yoreo et al., 2015, Science). The findings highlight the importance of amorphous intermediates in determining the composition of final crystalline products and indicate chemical signatures could record the properties of the initial amorphous phase. Thus, it is critical to understand the structure and physical properties of ACC and establish the mechanistic basis for the formation of 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 group (Blue et al., 2017, GCA). The experimental design varied Mg concentration and total alkalinity at an environmentally relevant pH. ACC solubility was measured at specific time points from super- and under-saturated conditions and the subsequent evolution of the ACC was monitored by small-angle x-ray scattering (SAXS). Parallel experiments characterized the composition and structure of the corresponding amorphous products using Thermogravimetric Analysis (TGA) and in situ pair distribution function (PDF) analyses.

The measurements demonstrate at least two types of ACC are produced by tuning Mg concentration and alkalinity. Each “phase” exhibits a different short-range order with distinct structural properties. TGA independently confirms two structure-dependent compositions. We also find temporal evolution of ACC structure and morphology after precipitation. This suggests composition of the final crystalline product is dependent on pathway/evolution of the amorphous intermediate. The findings hold promise for quantifying chemical and structural properties of ACC and reconciling discrepancies in the literature.