Formation of Monolithic Magnesium-Calcium Carbonates Through Aggregation of Carbonate-Cluster Nanodroplets

Mineral nucleation is a fundamental process of paramount importance to various fields, including geology, biomineralization, and industrial manufacturing. Recently, studies on the biominerals of calcium carbonate and phosphate have revealed the presence of a dense liquid phase prior to the formation of an amorphous solid phase. Here, we show that liquid-liquid phase separation occurs in a non-aqueous solution where magnesium and calcium carbonates are dissolved in ethanol with excess triethylamine. The nanodroplets can be kinetically trapped by triethylamine. We found that molecular complexes of triethylamine, CO₂, and ethanol molecules are in constant exchange with sizable carbonate clusters. We argue that nanodroplets comprising highly solvated carbonate clusters are the key entities in the nucleation process. Remarkably, the coalescence of the nanodroplets leads to the formation of a monolithic magnesium-calcium carbonate containing two distinct domains with different Mg content.