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

Materials Science