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

As an essential inorganic material, calcium carbonate (CaCO 3 ) 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 CaCO 3 . In this paper, a novel spindle-shaped CaCO 3 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 CaCO 3 . 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 CaCO 3 . This work will provide new insights into the industrial preparation of CaCO 3 with controlled morphology and contribute to understanding the biomineralization of aggregation-based growth in aqueous solutions.