## 3 Results  
The net chemical reaction for the synthesis of magnesium carbonate is shown in Equations (1) and (2): a Mg\({}^{2+}\) precursor (MgX, X represents the anion) reacts in an alkaline \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Experimental** & **Precursors Salts** & **Reaction Time** & \begin{tabular}{c} **Reaction** \\ **Temperature** \\ \end{tabular} & \
\begin{tabular}{c} **Drying** \\ **Temperature** \\ \end{tabular} \\ \hline \multirow{3}{*}{
\begin{tabular}{c} Aqueous \\ Synthesis \\ \end{tabular} } & MgCl\({}_{2}\)-6H\({}_{2}\)O & \multirow{3}{*}{25 and 60 min} & \multirow{3}{*}{5, 21, and 70 \({}^{\circ}\)C} & \multirow{3}{*}{40 \({}^{\circ}\)C} \\ \cline{2-2} \cline{4-4}  & MgSO\({}_{4}\)-7H\({}_{2}\)O & & & \\ \hline \multirow{2}{*}{scCO\({}_{2}\) Synthesis} & Mg(NO\({}_{3}\))\({}_{2}\)-6H\({}_{2}\)O & \multirow{2}{*}{12 h} & \multirow{2}{*}{35–40 \({}^{\circ}\)C} & \multirow{2}{*}{40 \({}^{\circ}\)C} \\ \cline{2-2} \cline{4-4}  & Mg(NO\({}_{3}\))\({}_{2}\)-6H\({}_{2}\)O & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Design of experiments. medium to form Mg(OH)\({}_{2}\), followed by a carbonation reaction producing MgCO\({}_{3}\). At the lowest temperature (5 \({}^{\circ}\)C), due to slow kinetics, no precipitate was formed. \[\text{MgX}+\text{ 2 OH}^{-}\left(\text{aq}.\right)\rightarrow\text{ Mg(OH)}_{2} \tag{1}\]  
\[\text{Mg(OH)}_{2}+\text{ HCO}_{3}^{-}\left(\text{aq}.\right)\rightarrow\text{ MgCO}_{3}+\text{H}_{2}\text{O}+\text{OH}^{-}\left(\text{aq}.\right) \tag{2}\]  
Different MgCO\({}_{3}\) structures were obtained, including hydromagnesite (Mg5(CO3)\({}_{4}\)(OH)\({}_{2}\)-4H\({}_{2}\)O), dypingite (Mg5(CO3)\({}_{4}\)(OH)\({}_{2}\)-5H\({}_{2}\)O), lansforide (MgCO3\({}_{5}\)-5H\({}_{2}\)O), magnesite (MgCO3), and usebonite (MgCO3\({}_{3}\)-3H\({}_{2}\)O) according to XRD analysis. The density of these hydrated magnesium carbonates depends on their structure, hydromagnesite usually is 2-2.5 times denser than dypingite [3], therefore, it would present less specific surface area. For the reaction with Mg(NO3)\({}_{2}\)-6H\({}_{2}\)O as precursor at 70 \({}^{\circ}\)C, spherical nanoparticles were observed (Figure 2a), these are hydromagnesite according to XRD results (shown below). [PERSON] et al. [3] have reported similar hydromagnesite spherical structures. The carbonate yields at 70 \({}^{\circ}\)C were higher than at 21 \({}^{\circ}\)C, which is expected, since the solubility of CO\({}_{2}\) decreases at higher temperatures, and the formation of hydromagnesite is not thermodynamically favorable below 55 \({}^{\circ}\)C [21]. With MgSO\({}_{4}\) as precursor (25 min reaction, 21 \({}^{\circ}\)C), we obtained a product with a morphology of rods of ca. 20 \(\upmu\)m, conjoined and extending from a compact center (Figure 2b). This morphology is usually known as dumbbell-shaped aggregates (DSA) [22]. This type of morphology can appear as an intermediate state of spherulitic growth of many minerals. However, we should remark that in our material we did not observe spherulitic crystals, which may indicate that this nonequilibrium morphology could have been favored under our synthesis conditions. One way this might have happened is by the appearance of a large number of nuclei at the beginning, which then grow and deplete precursors before crystals can achieve the spherulitic morphology. Using magnesium chloride and nitrate as precursors, at the same reaction temperature, we obtained products with two different morphologies (Figure 2c), containing magnesium hydroxide [23] in addition to the carbonate. The synthesis with scCO\({}_{2}\) generated acicular crystals exclusively, ca 10 \(\upmu\)m long (Figure 2d). Our results show that carbonate crystal sizes changed with synthesis temperature, in a matter consistent with the reported trend [23] that smaller crystals are favored at higher temperature.