Beauty and Complexity of Calcium Carbonate Precipitation: Optical Microscopy and in situ Raman Microspectroscopy Characterization

Calcium carbonate is one of the most studied systems in several disciplines (chemistry, biology, geology, etc.). Its crystallization process has been highly debated, and contradicting claims can be found in the literature. The controversy may be attributed to various initial conditions (concentration, pH, with or without additives), different precipitation protocols among literature, and the methods adopted with a large gap in the length scale: electron microscopy at the nano- and spectroscopic means at the bulk scale. Here, we investigated calcium carbonate precipitation by combining optical microscopy with in situ Raman microspectroscopy as a mesoscopic length scale technique. The precipitation of calcium carbonate was performed by mixing CaCl2 and Na2CO3 from a wide range of supersaturations without stirring to screen all possible crystallization pathways. We categorized and characterized the initial precipitates observed upon mixing as gel, gel + amorphous calcium carbonate (ACC), ACC, crystal, and no precipitate. Calcite and vaterite were observed in all mixed solutions, while aragonite was observed in most of them. This study shows how complex and heterogeneous yet beautiful CaCO3 precipitation is, and it emphasizes the importance of studying the system at the length scale between nano and bulk as well as that of initial conditions in order for the debates to converge in the near future.