Possible embryos and precursors of crystalline nuclei of calcium carbonate observed by liquid-cell transmission electron microscopy.

Several different building blocks or precursors play an important role in the early stages of the crystallization of calcium carbonate (CaCO 3 ). Many studies have been conducted over a long period to elucidate the fundamental processes involved in this crystallization. Here, we report the role of an amorphous phase and embryo at the beginning of the nucleation of CaCO 3 from solutions of relatively low supersaturation. Prenucleation crystals formed in amorphous calcium carbonate (ACC) at a significantly large formation rate of 2 × 10 22 m -3 s -1 , suggesting that a low interfacial energy exists between the ACC and crystals. Only one calcite crystal exceeded the size for a critical nucleus (∼10 4 molecules) in 150 pre-nucleation crystals. Each pre-nucleation crystal might consist of a different polymorph, and ACCs have a similar composition and structure. A particle-detection algorithm, used in conjunction with machine learning, suggested that an embryo with a characteristic structure exists in solution and might play a crucial role in nucleation. No similar embryonic structure could be observed immediately after the dissolution of pre-nucleation particles, implying that their dissolution process is not simply the reverse process of their growth. This method should provide a new approach to understanding nucleation processes.