The effect of lanthanum ion on the crystallization of calcium carbonate was investigated using CaCl 2-NaHCO 3 solutions. A sampling vessel enabling us to sample a small aliquot of a solution at any time almost under a closed condition was invented for the experiments. The abundance of the crystallographic polymorphs in the precipitate was determined by powder X-ray diffraction. The formation of calcium carbonate from lanthanum-doped solutions can be featured by three stages: a spontaneous and rapid precipitation of vaterite, a transitive static stage and a delayed formation of calcite. Almost all lanthanum (> 95%) in the solution system (dissolved + suspended solid phases) was proved to be incorporated in the initially formed vaterite, and lanthanum in the vaterite seemed to be immobilized throughout the experiment. The presence of lanthanum in the starting solution (10 -4 of calcium in mole) stabilized the initial vaterite crystal, preserved it much longer than one month and increased the solubility of CaCO 3 to the level of vaterite. It is suggested that the lanthanum ion prohibited either the transformation initially formed vaterite to calcite or the overgrowth of calcite.
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