Carbonate mineral precipitation has been identified as a significant and isotopically distinct sink in the global carbon cycle. Amorphous calcium carbonate (ACC) occurs as a precursor to geological and biogenic calcium carbonate (CaCO₃). The transformation pathway and transformation mechanisms of impurity-doped ACC to crystalline CaCO₃remain controversial and inconsistent, with respect to the ACC local conditions, subsequent crystalline polymorph selection, resulting morphologies, etc. We investigated the transformation of impurity-free and Mg/Sr/Ba-doped ACC to calcite in both solution and dry conditions utilizing operando time-resolved synchrotron X-ray diffraction (TRXRD) and reactive transport modeling.

Results demonstrate the mechanism of the amorphous to crystalline phase transformation can be differentiated by observing the lattice changes using TRXRD techniques. Impurities drive the polymorph selection and inhibits crystal growth. Specifically, we observe that in water-abundant ambient environments, pure ACC transforms to calcite through a dissolution-reprecipitation mechanism and a unit cell volume contraction after initiation of crystal growth (Chen et al., 2024). Mg, Sr, or Ba-doped ACC (Impurity/Ca ratio = 0, 1/3, 2/1, 3/1) still undergoes dissolution and reprecipitation mechanism in water abundant conditions, but impurity ions showed different effect in ACC dissolution, CaCO₃ nucleation and growth. Under water-limited conditions, in contrast, pure ACC transforms to calcite via a solid-state transformation mechanism and a unit cell expansion during crystal growth. Mg, Sr, or Ba-doped ACC still undergoes dissolution and reprecipitation mechanism in water abundant conditions, but impurity ions showed different effects for ACC dissolution, CaCO₃ nucleation and growth. Impurity ions elevate the activation energy to crystalline phase transformation. Reactive transport modeling of the dissolution-precipitation kinetics suggests that ACC must be present as compact molecular aggregates, which create calcite-supersaturated solutions in their pore-spaces during dissolution. These various transformation mechanisms drive diverse geological and biological carbonates formations, impacting their use as paleo-environmental markers and functional materials synthesis.

Craig Bridges

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