Water Activity Controls the Stability of Amorphous Ca–Mg- and Mg-Carbonates

Amorphous carbonates are increasingly recognized as important precursors in biomineralization and abiotic carbonate precipitation at ambient temperatures. Understanding the conditions and factors that influence the stability of amorphous Ca-, Ca–Mg-, and Mg-carbonates (ACC, ACMC, and AMC) is essential for understanding how organisms control biomineralization, the creation of functional carbonate nanomaterials, and enhancing engineered and natural CO2 storage processes. Here, we constrain the intersecting effects of precursor compositions (Mg content) and the action of physisorbed water on the reaction rates and crystallization pathways of ACMC by reacting amorphous carbonates, Ca1–xMgxCO3·nH2O, with compositions of 0.16 ≤ x ≤ 1 under relative humidities (RHs) of 5 to 98 ± 2%. We report the persistence of synthetic amorphous carbonate phases at ≤53% RH on similar time scales to biogenic ACC and ACMC (≥7 months). By contrast, transformation to crystalline phases occurs before 30 days for all compositions of ACMC at 98% RH (aw ∼ 1) and by 210 days for high and low Mg precursors at 75% RH. These results suggest that a threshold amount of physisorbed H2O generated between 75 and 98% RH facilitates crystallization, indicating that structural H2O in the amorphous material is not the primary agent of recrystallization in the absence of bulk water.