Abstract
Calcium carbonate (CaCO3) is a widely occurring mineral that is crucial to many natural and synthetic processes. CaCO3 crystallizes in three anhydrous polymorphs and often serves as a model system to study nucleation and growth phenomena. In this article, we employ coherent diffraction imaging coupled with wide-angle x-ray diffraction (CXDI-WAXD) to investigate the three-dimensional (3D) morphology and crystal structure of CaCO3 microparticles crystallized under different reaction temperatures. We report particles containing both vaterite and aragonite, which we interpret as an intermediate growth stage between early vaterite and later aragonite formation. The results are based on the density variations observed in the 3D tomographic CXDI images, x-ray diffraction, and focussed ion beam-transmission electron microscopy study of the mixed vaterite-aragonite microparticles. A computational study based on Monte Carlo and molecular dynamics energy minimization was used to identify the small energetic variations between the different crystalline forms of CaCO3. The results presented in this article provide a better understanding of the morphology-structure correlation for CaCO3 polymorphs, depicting a transitional stage where both vaterite and aragonite coexist.