*4.5. Synthesis Using a Precursor of Carbonate Ions*

The synthesis of calcium carbonate can be carried out using a substance that forms carbonate ions in the reaction environment. Such carbonate ion promoters may be ammonium carbamate [58] or urea [66,67]. Ammonium carbamate hydrolyzes in aqueous solutions to form ammonium carbonate:

$$\mathrm{NH\_2CO\_2NH\_4} + \mathrm{H\_2O} \leftrightarrow \mathrm{(NH\_4)\_2CO\_3} \tag{5}$$

Ammonium carbonate is also the product in the reaction of urea with water:

$$2\text{ CO(NH}\_2\text{)}\_2 + 2\text{H}\_2\text{O} \leftrightarrow (\text{NH}\_4)\_2\text{CO}\_3\tag{6}$$

A high concentration of both soluble calcium salts and carbonate ions precursors is necessary to precipitate vaterite polymorph [58,66]. Successful vaterite precipitation has been carried out at temperatures of 15, 25, and 50 ◦C, when ammonium carbamate has been used [58]. While, the reaction in the solution containing soluble calcium salt and urea required a temperature of 90 ◦C [66].
