*4.3. Di*ff*usion Method*

Calcium carbonate in vaterite form can be produced by a diffusion method, in which ammonium carbonate [52] or ammonium bicarbonate [61] is used. These compounds slowly decompose the formed ammonia gas and carbon dioxide. The precipitation is carried out in a closed vessel containing a container with a calcium salt solution and a container with solid carbonates. The formed gases diffuse into the calcium salt solution. After the absorption of ammonia and carbon dioxide, the calcium carbonate is formed in the liquid phase according to the reaction described by Equation (3).

## *4.4. Synthesis in Emulsions and Microemulsions*

The reaction between calcium ions and carbonate ions may take place in the emulsion system. A water-in-oil emulsion containing a single reagent solution as the dispersed phase can be used for the reaction. A second reactant solution is added to the emulsion and precipitation occurs in the dispersed aqueous phase [62]. A stable double water-in-oil-in-water emulsion can be used for vaterite synthesis as well. In this case, calcium ions dissolved in the external phase are extracted into the organic phase containing oil-soluble extractant and stripped into the internal aqueous phase, where the reaction with carbonate ions occurs [63]. Also, the use of microemulsions is proposed for the production of vaterite. For this purpose, two microemulsions containing Ca2<sup>+</sup> and CO3 <sup>2</sup><sup>−</sup> ions are prepared. Then, these microemulsions are mixed together to produce calcium carbonate [64].

Another approach to calcium carbonate precipitation in a microemulsion system is the decomposition of calcium bicarbonate:

$$\text{Ca(HCO}\_3\text{)}\_2 \leftrightarrow \text{CaCO}\_3 + \text{CO}\_2 + \text{H}\_2\text{O} \tag{4}$$

When a saturated solution of calcium bicarbonate is a water phase in a water-in-oil emulsion, vaterite is formed during the slow desorption of carbon dioxide from an aqueous phase. Sponge-like vaterite spheroids were produced using this method [65].
