Controlled crystallization of unstable vaterite with distinct morphologies and their polymorphic transition to stable calcite

Vaterite with different morphologies, a thermodynamically unstable polymorph of calcium carbonate, was successfully synthesized by a simple injection-precipitation method. The phase composition and morphology of the products were characterized by the XRD, FT-IR, SEM, and TEM techniques. Experiments were performed at 37 and 25 degrees C using pH values of 1.5, 3.0, and 6.9. At 37 degrees C, precipitated vaterite has spindle-like morphology at the low pH 1.5 of the initial CaCl (sub 2) solution, and shows coexistence of the spindle-, spheroid-, and cauliflower-like morphologies at the intermediate pH 3.0, whereas it is spheroidal at pH 6.9. SAED analyses revealed that the spindle-like vaterite superstructures were self-assembled by the oriented aggregation of vaterite micro-crystals along the crystallographic c direction. At 25 degrees C, however, the low pH (1.5) led to coexistence of cauliflower- and spheroid-shaped vaterite, whereas spherulite-like vaterite was always obtained at either pH 3.0 or 6.9. We show that simple inorganic precipitation processes lead to complex and unusual morphologies with hierarchical structure. Therefore, care must be taken when morphological criteria are claimed as proof for biogenic origin of minerals. Moreover, time studies of the polymorphic transition revealed that, although solution-mediated dissolution of precursor vaterite and reprecipitation of secondary calcite always occur, Ostwald ripening finally contributes to the formation of rhombohedral calcite. Therefore, the polymorphic transition sequentially proceeds from vaterite through irregular calcite aggregates to stable calcite rhombohedra.