Ambient Formation of Spherulites of Mg-Calcite in an Aqueous Lipid Solution through the Interplay between Multiple Pathways

Nanograins of Mg-containing amorphous calcium carbonate were prepared in an L-alpha-lecithin solution from which spherulitic crystallites of high-Mg calcite were subsequently formed. The structures of the spherulites of high-Mg calcite were carefully characterized using powder X-ray diffraction, electron microscopy, and solid-state NMR spectroscopy. While the initial growth of Mg-calcite spherulites occurs through the particle attachment pathway, the process of ion-by-ion addition appears to be the predominant mechanism of formation near the end stage of the crystal growth. Consequently, the outer region of the Mg-calcite spherulites displays a higher crystallinity than the inner region, which is a direct consequence of the interplay between multiple pathways.