## 3 Results and Discussion
### XRD, FTIR and Raman  
XRD study of disks immersed in carbonated water revealed the formation of crystalline phases at the surface. Figure 3 shows the evolution of the XRD pattern for the bare Mg disk (Figure 3a) after one hour (Figure 3b). The XRD pattern after 1 h presents prominent Mg peaks, marked with'm' in Figure 3a, and a strong additional feature at ~30 deg, corresponding to the main peak of calcite calcium carbonate -(014)-, accompanied by a doublet in the 47-49 deg region, characteristic of the second most intense feature of calcium carbonate -(018) and (116)-. Increased immersion times to 1 day (Figure 3c) results in the increase of the intensity of the main carbonate peaks relative to the metal peaks, which are still clearly observed. After one week, the metal peaks cannot be noticed in the XRD pattern,  
Figure 1: Mg disks immersed in solution during carbonate coating: top views of (**a**) beaker with several Mg disks. (**b**) higher detail image, showing the gas column emanating from each disk, (**c**) one disk after 1 h immersion. and instead, additional minor peaks, marked with \"*\", appear in the pattern (Figure 3d) that can be indexed to aragonite, another allotrope of calcium carbonate (CaCO\({}_{3}\)) with orthorhombic symmetry. The relative intensity of the aragonite peaks is small, so the quantity of aragonite can be estimated at about one order of magnitude below that of calcite, as the main crystalline phase at the disk's surface. The mechanistic crystallization of the coating along the surface by attaching oriented particles is discussed below in the manuscript (Section 3.5), explaining the two-dimensional growth of calcite coating, not in height, until completely coating the Mg surface. The magnesium content in the lattice of the calcite phase was determined using the method in ref [31], giving values of Mg presence between 1-2 mol%. Figure 3: XRD patterns showing the evolution with carbonatation time of Mg disks: (**a**) before immersion, and after (**b**) 1 h, (**c**) 1 day, (**d**) 1 week immersion times (c: calcite; *: aragonite; m: Mg). Figure 2: Optical images showing the evolution with carbonatation time of Mg disks: (**a**) before immersion, and after (**b**) 15 min, (**c**) 1 h, (**d**) 1 week immersion times. The samples immersed less than 1h were additionally investigated by vibrational spectroscopy in order to study the presence of amorphous or disordered phases and ascertain the deposition of chemical species on the surface of Mg during the first hour of immersion in carbonated solution, previous to the crystallization of calcite observed in the XRD analysis. The FTIR spectra of samples immersed for 1 h (Figure 4) displays the characteristic vibrations of the carbonate group in well -ordered, crystalline calcite, with a sharp peak at 713 cm\({}^{-1}\) (\(\upsilon_{4}\)-symmetric CO\({}_{3}\) bending) as well as the carbonate group vibrations at 873 cm\({}^{-1}\) (\(\upsilon_{2}\)-asymmetric CO\({}_{3}\) bending) and 1083 cm\({}^{-1}\) (\(\upsilon_{1}\)-symmetric CO\({}_{3}\) stretching) [32; 33; 34]. The sharp peak at 1796 is assigned to the \(\upsilon_{1}+\upsilon_{4}\) overtone. The FTIR spectra for 30 min immersion time exhibit weak \(\upsilon_{4}\) and (\(\upsilon_{1}+\upsilon_{4}\)) bands, with a clear decrease in the relative intensity between the \(\upsilon_{4}\) and the \(\upsilon_{2}\) bands, which indicates the predominance of disordered carbonate. The strong \(\upsilon_{2}\) absorption band at \(\sim\)866 cm\({}^{-1}\) and the split peak at 1420 and 1475 cm\({}^{-1}\) in the FT-IR spectra (Figure 4) have been described as characteristic of amorphous calcium carbonate (ACC) [35]. There are two main ACC absorptions around 1450 cm\({}^{-1}\), as compared to a single absorption for the crystalline forms calcite and aragonite. The splitting is related to the \(\upsilon_{3}\)-asymmetric CO\({}_{3}\) stretching around the carbonates.