posted on 2024-02-23, 04:29authored byAvni S. Patel, Maija J. Raudsepp, Sasha Wilson, Anna L. Harrison
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.