Abstract

The precipitation of CaCO3 is a widely occurring process in nature as well as an important process in industry. CaCO3 nucleates in three crystalline polymorphs calcite, aragonite and vaterite. Biological systems control polymorphism by the production of particular macromolecules or by regulating diffusion rate of inorganic components. Although the effect of experimental variables on the polymorphs has been studied by many authors, the favorable conditions for getting each polymorph are generally uncertain. Aragonite is less abundant than calcite and is formed under a very narrow range of physicochemical conditions and is easily transformed into calcite by changes in the environment. The crystallization experiments were done using a chamber consisting of a plastic Petri dish having a central hole in its bottom glued to a plastic cylindrical vessel. Inside the chamber, micro-bridges were filled with CaCl2 solution in TRIS buffer pH 9. The central hole allows the diffusion of CO2 vapor from the (NH4)HCO3 into the buffered CaCl2 solution. By varying the hole diameter of the CaCl2 -containing chamber and the temperature of the experiments this rate was changed. The hole diameter was varied from 3 mm to 30 mm and the temperatures from 20 to 70°C. It was found that by increasing the hole diameter and/or the temperature, a notorious increase in the aragonite form is observed. By combining these factors appropriately even vaterite crystals were obtained. Since the kinetic of crystallization is a determinant factor of the morphology, a convenient design of the crystallization chamber can conduct the crystallization of CaCO3 towards a specific crystalline form. © 2004 Materials Research Society.