There are three recognized crystalline polymorphs of calcium carbonate (CaCO3): calcite, aragonite, and vaterite, along with an amorphous Calcium Carbonate (ACC) phase. Interestingly, nature uses the distinct characteristics of these phases effectively by somehow choosing the appropriate one for a given purpose. A notable example is found in aquatic organisms like oysters, mussels, and abalones. These creatures produce CaCO3 shells with a unique structure—layers of calcite form the outer part, providing protection to the soft body from enemies, while the inner surface is crafted from a shiny and smooth nanoscale aragonite, known as nacre or mother-of-pearl.
To gain insight into how nature selectively chooses a polymorph, it will help us mimic natural processes and gain control over crystallisation. This requires a deep understanding of the mechanisms involved. Studying the early stages of crystallisation in the laboratory in bulk solutions is challenging because it occurs rapidly at the atomic scale. To address this issue, the crystallization of CaCO3 was studied at very low levels of supersaturation, as existing literature primarily focuses on higher supersaturations.
In this thesis the early stages of crystallisation of CaCO3 in bulk solutions was investigated at room temperature. For this purpose, CaCO3 crystallisation was monitored at different times and different supersaturation levels for two different systems: equimolar reactant concentrations (0.5, 1.0, 2.5 and 4.5 mM) and carbonate rich conditions (Na2CO3:CaCl2 in a 200:1 molar ratio). The shape (polymorph) and size of the crystals were studied as a function of reaction time and supersaturation by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Cryogenic (cryo)-TEM. Energy dispersive x-ray spectroscopy and electron diffraction patterns were used to confirm the presence of CaCO3 and determine the polymorphs.
At higher supersaturated concentrations, nano-sized crystals of spherical vaterite and rhombohedral calcite were observed with well-defined morphologies which grew over time (minutes), together with the presence of some ACC at the shortest reaction time (5 seconds) of the crystallisation process, but no evidence was found for the presence of aragonite. As super-saturation decreases to 1.0 and 0.5 mM, there is an increased proportion of ACC and crystalline particles were smaller due to slower reaction kinetics. At the concentration of 0.5 mM after 5 minutes, a diffuse region of higher contrast was observed from TEM, which was not seen at any other concentrations. We attribute this to potentially the remains of a dense liquid-like precursor structure. Interestingly, a carbonate-rich environment led to the formation of aragonite at room temperature, with no vaterite or calcite observed.
Additionally, a range of different sample preparation methods including air-drying, vacuum filtering, ethanol quenching and plunge freezing-vacuum drying as well as plunge freezing and direct examination in cryo-TEM, were investigated. Significant differences have been observed especially in terms of the amount and size of ACC particles in ethanol quenching techniques since ethanol is known to affect both the polymorphs and morphologies in crystallization solutions. Notably, in ethanol quenching techniques, the formation of a network of ACC particles was observed, leading to the conclusion that ethanol contributed to the formation of this network.
Overall, the process of CaCO3 polymorph crystallisation is highly complex, and modifying a specific parameter can result in the preferential formation of a different polymorph. The research findings of this thesis demonstrate that electron microscopy techniques such as TEM and particularly Cryo-TEM are robust, offering insights into the complex nature of CaCO3 polymorph crystallisation, allowing the study of crystallization in its native state.
