Vaterite is generally known as an unstable form of calcium carbonate, but pure vaterite was obtained from the reaction of concentrated aqueous solutions of the CaCl2-Na2CO3 system. Ordinarily in the mechanism of precipitation reaction an important part is played by the degree of concentration of the reactants, whose influence is especially noticeable on the number of the stages. From diluted aqueous solutions of reactants in the CaCl2-Na2CO3 system, it is known that there is only one step of reaction as follows;
In concentrated reactant a transparent gelatinic membrane appears at the phase boundary of CaCl2 solution (>2N) and Na2CO3 solution (>2N) in the initial stage, which becomes translucent in 15 minutes or more, and the following process of reaction has been observed; CaCl2+Na2CO3→Oriented substance in the initial period at the interface. (a transparent gelatinic membrance)
→CaCO3 (Vaterite) or CaCO3 (Calcite) at room temperature.
Studies were made of the X-ray diffraction, the infrared absorption spectra, the high-resoltion nuclear magnetic resonance and the electron spin resonance, during the transparent stage and the following stages at the room temperature or 77°K. The result shows that the gelatinic substance is in an amorphous state showing a broad peak at about 15° (θ) by the x-ray diffraction and that CO32-ions are not distributed at random but orient themselves in a definite way, making an angle of 45°-90° with the interface as revealed by the polarized infrared absorption.
The experiment also indicates that the gelatinic substance is viscous in spite of its high water content, and that its watering redient has been restricted by stronger hydrogen-bonds than the original solution. It is interesting to notice the presence of from three to five broad lines of proton magnetic resonance absorption (60Mc) extending from the water level to high magnetic fields.
The E.S.R. signal of gelatinic substance appears at the point of 2.004 (G-Value) at 77°K. It is possible that this points to the presence of H2O-.
This fact suggests that H2O gets strongly polarized in the ionic fields and makes a spur of radical in the gelatinic substance.
The gelatinic substance gradually precipitates by separation of H2O and produces the double salts at room temperature as follows: CaCO3·Na2CO2·nH2O (n≅13), CaCO3·Na2CO3·5H2O (Gaylussite), CaCO3·Na2CO3·2H2O and CaCO3·Na2CO3·H2O (Pirssonite). The major product of the double salts is gaylussite and it is subject to decomposition into vaterite under cectain conditions.
The production of pure vaterite is possible under the following condition.
Original solution; 6N CaCl2-2N Na2CO3,
Reaction temperature; 20-30°C,
Mixing ratio (mol ratio); CaCl2:Na2CO3=1:3,
Reaction time; 30-50min,
The stirrer must be worked slowly (200r/min). When the precipitation is complete the precipitate must be filtered rapidly and washed in the mixture of H2O (70%) -Methanol (30%) and then with ether.