Nonclassical and Classical Crystallization: The Formation of Spindle-Shaped CaCO3 Covered with Abundant Nanoscale Rhombic Calcite Subunits

As an essential inorganic material, calcium carbonate (CaCO3) is one of the most abundant minerals in nature, whose preparation by the carbonation of calcium hydroxide [Ca(OH)2] suspension involves complex physical and chemical processes due to the presence of gas–solid–liquid phases. It is challenging to accurately predict the final morphology of CaCO3. In this paper, a novel spindle-shaped CaCO3 covered with a mass of nanoscale rhombic calcite subunits was prepared via the carbonation method. This structure was mainly formed by the aggregation of nanoparticles in the pre-reaction stage and turned into a classical ion–ion growth process at the stage of the late reaction. During the pre-reaction period, solid Ca(OH)2 kept the solution supersaturated and made the aggregation of nanoparticles dominate and assemble to form the nanochains of CaCO3. Subsequently, the aggregation of nanochains formed a dumbbell shape, which was accompanied by the recrystallization of nanoparticles inside aggregates to create an ordered single-crystal spindle structure. Finally, the concentration of Ca(OH)2 decreased, and a layer of nanoscale rhombic calcite subunits grew on the surface of spindle CaCO3. This work will provide new insights into the industrial preparation of CaCO3 with controlled morphology and contribute to understanding the biomineralization of aggregation-based growth in aqueous solutions.