Deciphering the Chemical and Physical Basis for the CaCO3 Polymorphs that Form by an Amorphous Pathway

Abstract:

The formation of calcite and aragonite biominerals by pathways involving metastable intermediates is increasingly recognized. Initial precipitates evolve along a multistep pathway that is not well understood. Previous studies of amorphous calcium carbonate (ACC) provide important insights, but relationships between ACC composition and the subsequent crystalline polymorphs are not yet established.

This experimental study quantified control of initial solution composition on the polymorphs that transform from ACC. Using a flow-through reactor to regulate solution chemistry without organic additives, ACC was synthesized with variable Mg contents by tuning input solution Mg/Ca ratio, total carbonate concentration, and pH (Blue and Dove, 2015, GCA). This ACC was transformed to crystalline products under stirred or quiescent conditions while monitoring solution and solid compositions.

By quantifying solution chemistry throughout the transformation, we find pH and Mg/Ca evolve along polymorph-specific trajectories. The relations predict the initial crystalline phase to form and suggest incipient differences in the initial ACC. The first crystalline polymorph appears controlled by fast kinetics and determined by a systematic relationship to solution Mg2+/Ca2+ and CO3=/Ca2+ ratios. Stirring versus quiescent conditions further direct the initial crystalline polymorph to form. Over longer time periods, thermodynamic factors can drive the initial phase to transform again.

The results suggest a quantitative framework for predicting the products to transform from ACC that is based on the interplay of chemistry and physical conditions. Organic additives and extreme pH are not required. The findings reconcile apparent discrepancies between previous studies of ACC and suggest monohydrocalcite may be an overlooked transient phase in forming some deposits of aragonite and calcite. They also 1) support the idea that calcite composition is determined by local Mg²⁺/Ca²⁺ and isotope conditions (Giuffre et al., in press, GCA) at the time of initial precipitation; and 2) demonstrate the importance of kinetic versus thermodynamic factors in regulating multistep pathways (De Yoreo et al., 2015, Science) to determine the polymorphs/compositions that form in biological and geological settings.