Transformation of amorphous calcium carbonate in the presence of magnesium, phosphate, and mineral surfaces

Amorphous calcium carbonate (ACC) is known as a precursor to crystalline calcium carbonate polymorphs (CaCO3) in various natural and synthetic systems. While ACC has been shown to form by heterogeneous nucleation in biological systems, it is less obvious whether it plays any role in inorganic settings where carbonate minerals grow on pre-existing surfaces. Tight assemblies of calcium carbonate (CaCO3) phases and clay minerals occur in rock-forming quantities in marlstones, are abundant in Earth’s critical zone, in the sediments of aqueous environments, and in the atmosphere as aerosol particles. Previous studies suggested that the co-occurrence of clay and carbonate minerals (especially smectite and calcite) results from the heterogeneous nucleation of CaCO3 on the surface of smectite. In order to understand the role of ACC in the heterogeneous nucleation of carbonate minerals and the effects of common silicate mineral surfaces on the stability of ACC, we synthesized ACC both in the presence and absence of smectite, kaolinite, and quartz, monitored the pH of the solution during the experiments, and studied the precipitated phases with various transmission electron microscopy techniques. Since Mg2+ and (PO43−) ions are known to prolong the lifetime of ACC and are important components in natural aqueous environments, we tested their effects on ACC transformation in both homogeneous and heterogeneous systems. In all studied systems ACC was the first condensed carbonate phase. The presence of smectite in the solution reduced the lifetime of ACC and led to the rapid formation of calcite. We suggest that the main factor driving the transition was the enhanced growth of ACC particles on the surfaces of smectite flakes. On the other hand, the presence of kaolinite and quartz slightly increased ACC stability; ACC typically rimmed the silicate particles and transformed slower than separated, individual ACC particles. When Mg2+ was present in the solution, ACC stability significantly increased and led to the formation of aragonite instead of calcite in both homogeneous and heterogeneous systems. The presence of (PO4)3− ions caused only a minor delay in ACC transformation and the crystalline product was calcite, irrespective of the presence of silicate minerals. These observations suggest that smectite minerals play important roles in the formation of carbonate minerals by providing a suitable surface for the rapid growth and coalescence of ACC particles, leading to larger particle sizes which, in turn, result in the conversion of ACC into calcite.