Growth process and crystallographic properties of ammonia-induced vaterite

Abstract

Metastable vaterite crystals were synthesized by increasing the pH and\nconsequently the saturation states of Ca and CO3 containing solutions using an\nammonia diffusion method. SEM and TEM analyses indicate that vaterite grains\nproduced by this method are polycrystalline aggregates with a final morphology\nthat has a sixfold-symmetry. The primary structure develops within an hour and\nis almost a spherical assemblage of nanoparticles (5 to 10 nm) with random\norientation, followed by the formation of hexagonal platelets (1 to 2um), which\nare first composed of nanoparticles and that develop further into single\ncrystals. As determined using transmission electron microscopy, these hexagonal\ncrystallites are terminated by (001) surfaces and are bounded by 110 edges. The\nhexagonal crystals subsequently stack to form the petals (20um wide, 1um thick)\nof the final flower-like vaterite morphology. The large flakes gradually tilt\ntoward the center as growth progresses so that their positions become more and\nmore vertical, which eventually leads to a depression in the center. Since this\nsequence encompasses several morphologies observed in previous studies\n(spheres, hexagons, flowers etc), they may actually represent different stages\nof growth rather than equilibrium morphologies for specific growth conditions\n