## 3 Results
Additionally, the drying process  
Figure 2: Scanning electron micrographs of carbonate products (acquired in variable pressure mode), showing representative morphologies from aqueous syntheses (**a–c**), from scCO\({}_{2}\)-od (**d**) scCO\({}_{2}\)-hpd (**e**). applied to the samples influenced the surface appearance, given that the sample dried over a hot plate has a rougher surface (Figure 2e). Considering the morphology observed by SEM, the DSA product and the supercritical products were characterized further. The diffractogram of the scCO\({}_{2}\)-od carbonate product (Figure 3a) matches the diffraction pattern of nesquehonite, (MgCO\({}_{3}\cdot\)3H\({}_{2}\)O). These crystals look similar to those found by [PERSON] et al. [24] using CO\({}_{2}\) at a pressure of 1 bar, using sodium carbonate as the base, and CO\({}_{2}\) at temperatures and pressures above the critical point (120 \({}^{\circ}\)C, 100 bar) but they found precipitation of hydromagnesite and magnesite exclusively. The latter was favored by longer reaction times. By contrast, [PERSON] et al. found mostly nesquehonite when using supercritical CO\({}_{2}\) at moderate temperatures (\(\leq\)60 \({}^{\circ}\)C), and when using short chain alkanolamines as the base, with hydromagnesite favored at higher temperatures (70 \({}^{\circ}\)C) [19]. XRD of commercial carbonate showed that it was hydromagnesite, which is typically the more stable hydrated phase of magnesium carbonate [25]. The diffractogram of the DSA material (Figure 3b) corresponds to a mixture of crystalline structures. Rietveld refinement was used to correlate those peaks to four structures: magnesite (MgCO\({}_{3}\)), nesquehonite (MgCO\({}_{3}\cdot\)3H\({}_{2}\)O), hydromagnesite (MgS(CO\({}_{3}\))\({}_{4}\)(OH)\({}_{2}\cdot\)4H\({}_{2}\)O), and lansfordite (MgCO\({}_{3}\cdot\)5H\({}_{2}\)O), with a good match found only for the last three. For the carbonate obtained with the MgSO\({}_{4}\cdot\)7H\({}_{2}\)O precursor, the DSA morphology with elongated crystals was favored over regular spherulitic growth as previously reported [3]. The diffractogram of the scCO\({}_{2}\) carbonate product with hot plate drying (Figure 3c) shows a mixture of crystalline structures of hydrated magnesium carbonate. According to the Rietveld refinement the identified structures correspond to magnesite, hydromagnesite, usequehonite, and lansfordite. The proportion of each of the phases is shown in Table 2. The difference between the composition of both scCO\({}_{2}\) products was expected since magnesium carbonate is sensitive to temperature and dehydrates when heated at moderately high temperatures [3]. The scCO\({}_{2}\)-hpd product contains a higher percentage of a more hydrated phase, lansfordite and hydromagnesite, compared with the scCO\({}_{2}\)-od for which the main component is nesquehonite. Figure 3: X-ray diffraction patterns of (**a**) DSA (dumbbell-shaped aggregates) product, (**b**) magnesium carbonate, from supercritical synthesis, hot plate dried, scCO\({}_{2}\)-hpd, and (**c**) magnesium carbonate from supercritical synthesis, oven dried, scCO\({}_{2}\)-od. Reference peaks of crystalline structures with the highest correlation are shown as determined by Rietveld refinement. The structures of the carbonates produced by the aqueous route changed depending on the precursor. For nitrate, the dried product showed a mixture of dypingite and hydromagnesite (Figure 4a). With sulfate as precursor, in samples that did not show the DSA morphology, the peaks correspond to a mixture of nesuehouite and hydromagnesite (Figure 4b), without peaks for magnesium sulfates. We note that since we dried these products at relatively high temperatures there may have been partial dehydration of the precipitate. Raman spectra of the DSA product and scCO\({}_{2}\)-od resebuehonite (Figure 5) show peaks characteristic of the carbonate ion (CO\({}_{3}^{2}\)-) at 1107 and 1400-1500 cm\({}^{-1}\)[26].