**2. Vaterite Properties**

Synthetic vaterite particles are usually produced as polycrystalline spheres. The main advantages of such particles' morphology are the porous structure, large surface area, and greater hydrophilicity in comparison to more stable calcite or aragonite [10,19,20]. Other morphological forms of vaterite can be also obtained, i.e., plates [21], hexagonal crystals [22], lenses [23], lamellar aggregates [24], florets, or rosettes [21,25] as well as microtablets [26]. Examples of vaterite particles are shown in Figure 2.

**Figure 2.** Vaterite particles: (**a**) and (**b**) typical spherical particles; (**c**) spherical and lens-like particles; (**d**) deformed lens-like and crossed lens-like particles.

Vaterite has a hexagonal crystal system, but the exact crystal structure of vaterite is still under discussion. The analysis of experimental data is consistent in that all vaterite structures belong to the order-disorder (OD) family [27]. It means that the occurrence of multiple polytypes on the microto macroscopic scale, as well as considerable stacking disorder, are both to be expected. Recently Burgess and Bryce [28] used the combined 43Ca solid-state nuclear magnetic resonance spectroscopic and computational method to indicate two crystal structures, i.e., the hexagonal lattice, P3221, and

monoclinic lattice, C2, which have the best agreement between the simulated spectra and diffractograms with the experimental data.

Selected properties of the vaterite are summarized in Table 1.


**Table 1.** Selected properties of vaterite.

<sup>1</sup> Solubility product; <sup>2</sup> Volumetric thermal expansion coefficient.

The surface of the vaterite particles is usually hydrophilic. The hydrophobic vaterite can be obtained by the adsorption of amphiphilic molecules, e.g., oleic acid, at the interface of the produced vaterite [32]. The charge of vaterite particles depends on the composition of the solution and its pH. The values of the ζ-potential were negative when the vaterite particles were dispersed in saturated CaCO3 solution at pH 9.0 (−4 mV) and at pH 10.6 (−26 mV) [33]. Another experiment showed that when the solution was composed from 0.01 mol/dm3 CaCl2, 0.002 mol/dm<sup>3</sup> Na2CO3, and 0.5 mol/dm3 NaCl, the charge of the vaterite particles was positive in the range of pH 7.5 to 9.9 [34]. The addition of organic compounds, like polypeptides or fulvic acid, can change the charge of the vaterite particles due to its adsorption at the precipitated crystal interface [33,34].

The mechanical properties of synthetic vaterite particles were determined using nanoidentation analysis [30]. The elastic modulus was found to be in the range of 16 to 61 GPa and the calculated mean value of this parameter was 31 GPa. The determined hardness of vaterite was in the range of 4.2 to 0.3 GPa with a mean value of 0.9 GPa.

During the heating of vaterite particles, thermal transformation and decomposition occurs. The exothermic transformation of vaterite into calcite takes place at temperatures between 395 and 540 ◦C [6,35,36]. The exact transformation temperature of vaterite into calcite depends on the particle characteristics, the presence of additives, and the heating rate. The shift of the vaterite transformation to a lower temperature may be observed when calcite is present in the sample [35,36] or in the case of the incorporation of organic molecules or foreign ions into the vaterite particles [6,37]. Also, the coexistence of pure vaterite and vaterite in contact with the calcite phase (e.g., vaterite particles covered by a calcite layer) [36] can cause the appearance of a broad range of transformation temperatures.

Recently, a report on the pressure-induced phase transition of vaterite was presented [38]. With increasing pressure, vaterite transformed to high-pressure vaterite forms (vaterite II, vaterite III, and vaterite IV) or partially to calcite. All phase transitions related to vaterite were reversible, except for vaterite II to calcite III.

The presence of vaterite in calcium carbonate samples can be determined using Fourier transformed infrared spectroscopy (FTIR) [39], powder X-ray diffraction (PXRD), Raman spectroscopy [40], or 43Ca solid state nuclear magnetic resonance (43Ca ssNMR) [41]. Characteristic peaks of calcite, aragonite, and vaterite, obtained in spectra or diffractograms that allow these polymorphic forms to be distinguished by FTIR, XRD, Raman spectroscopy, and 43Ca ssNMR, are listed in Table 2.


**Table 2.** Characteristic peaks of CaCO3 polymorphs in FTIR, XRD, Raman spectroscopy, and 43Ca ssNMR analysis.

<sup>1</sup> The isotropic chemical shift.
