Effects of trace lanthanum ion on the stability of vaterite and transformation from vaterite to calcite in an aquatic system

The effect of lanthanum ion on the crystallization of calcium carbonate was investigated using CaCl2-NaHCO3 solutions. A sampling vessel enabling us t o sample a small aliquot of a solution at any time almost under a closed co ndition was invented for the experiments. The abundance of the crystallogra phic polymorphs in the precipitate was determined by powder X-ray diffracti on. The formation of calcium carbonate from lanthanum-doped solutions can b e featured by three stages: a spontaneous and rapid precipitation of vateri te, a transitive static stage and a delayed formation of calcite. Almost al l lanthanum (> 95%) in the solution system (dissolved + suspended solid pha ses) was proved to be incorporated in the initially formed vaterite, and la nthanum in the vaterite seemed to be immobilized throughout the experiment. The presence of lanthanum in the starting solution (10(-4) of calcium in m ore) stabilized the initial vaterite crystal, preserved it much longer than one month and increased the solubility of CaCO3 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.