Shape change of calcite single crystals to accommodate interfacial curvature: Crystallization in presence of Mg²⁺ ions and agarose gel-networks

Synthetic calcite single crystals, due to their strong crystal habit, tend to grow into characteristic rhombohedra. In the nature, biogenic calcite crystals form composites together with biomacromolecular materials, spurring investigations of how the growing calcite single crystals change their habit to satisfy the curvature of the organic phase. In this work, we examine calcite crystallization on a flat surface of glass slide and a curved surface of polystyrene (PS) sphere. The crystals exhibit tiny contact area onto the glass substrate that is averagely only 15% of their projected area on the substrate. In sharp contrast, the contact area greatly increase to above 75% of the projected area, once magnesium ions or agarose gel networks are introduced into the crystallization media. Furthermore, the calcite crystals form rough and step-like interfaces with a curved surface. However, the interfaces become smooth and curved as the crystals grow in presence of magnesium ions or agarose gel networks. The discrepancy between the interfacial structures implies kinetic effects of the additives on the crystallization around the surfaces. This work may provide implications for understanding the formation mechanisms of single-crystal composite materials.