Synthesis and structure of hollow calcite particles

Hollow spherical particles of calcite with a diameter of 2-10 mu m were synthesized on a polyvinyliden difluoride ( PVDF) membrane blotted with crude organic matrices prepared from the pearl oyster shell in a supersaturated solution of calcium carbonate. Examination of the produced particles as a function of the reaction time showed that the surface topography of the particles changed from a smooth surface to a ragged one with fine {104} facets of calcite crystals. Cross-sections and thin specimens of the particles were prepared by using the focused ion beam technique to observe the inside and determine the crystalline phases, respectively. Transmission electron microscopy (TEM) analysis showed that the particles with a short reaction time consist of vaterite and amorphous calcium carbonate (ACC). The ragged particles are polycrystalline calcite with a hollow structure ( especially in relatively smaller particles) or a void space inside the particles. Large calcite crystals form a crust of the sphere, and minute calcite crystallites exist inside with the void space. Such a hollow structure can be explained by the solid-to-solid transformation from vaterite or ACC to calcite initiating from the surface and proceeding toward the inside of the sphere, with volume reduction by the phase transition.