## 2 Materials and Methods  
Aqueous synthesis by \(\text{CO}_{2}\) sequestration was based on the method of [PERSON] et al. [16]. We bubbled \(\text{CO}_{2}\) through a Pasteur pipette into magnesium solutions in deionized water in a stoppered flask, with a flow rate equivalent to 1.1 g/min (Figure 1a).  
Figure 1: Schematic diagrams showing the experimental method for aqueous synthesis, (**a**) bubbling \(\text{CO}_{2}\) gas, and (**b**) supercritical fluid (SCF) closed reactor (kept at constant temperature in a hot water bath, not shown for clarity).  
Three precursors salts (all reactive grade, CTR Scientific, Monterrey, Mexico) were used for both routes: MgCl\({}_{2}\)-6H\({}_{2}\)O (10 mL, 6.07 M), Mg(NO\({}_{3}\))\({}_{2}\)-6H\({}_{2}\)O (10 mL, 6.07 M), MgSO\({}_{4}\)-7H\({}_{2}\)O (20 mL, 3.036 M). The pH was adjusted to 8-9 with NH\({}_{4}\)OH and reactions were carried out for 25 and 60 min at 5, 21, and 70 \({}^{\circ}\)C, as shown in Table 1.  
In the supercritical synthesis, we added 60 g of CO\({}_{2}\) (enough to reach a pressure \(>\)74 bar) in a custom-made reactor, with a volume of 240 mL (Figure 1b), containing 10 mL of 6.07 M Mg(NO\({}_{3}\))\({}_{2}\)-6H\({}_{2}\)O (pH adjusted to 8-9 with NH\({}_{4}\)OH). The reactor was hermetically closed and placed in a water bath at 35-40 \({}^{\circ}\)C to reach and maintain supercritical conditions, monitored by observation of the phase change through a window. After 12 h of reaction, the reactor was depressurized. Solid products were collected and washed five times with 5 mL of distilled water, then oven-dried for 12 h at 100 \({}^{\circ}\)C (scCO\({}_{2}\)-od). To analyze the effect of the drying temperature on the material structure, the same synthesis procedure was carried out, and the sample was dried over a hot plate at 40 \({}^{\circ}\)C for 12 h (scCO\({}_{2}\)-hyd).  
Morphology was observed with a Zeiss EVO MA scanning electron microscope (SEM), from Carl Zeiss AG (Oberkoche, Germany). Some samples were sputter coated with 5 nm of Au, to prevent charging.  
Diffraction patterns of selected products were obtained with a Siemens/Bruker D5000 diffractometer from Bruker AXS (Karlsruhe, Germany), using Cu K\({}_{\alpha}\) radiation (\(\lambda\) = 1.54184 A), 20 angle from 5\({}^{\circ}\) to 70\({}^{\circ}\) with a scan rate of 5 \({}^{\circ}\)/min. Profex, an open source program by [PERSON] (www.profex-xrd.org), was used for Rietveld refinement. Raman spectroscopy for the samples scCO\({}_{2}\)-od and aqueous route was performed in an OceanOptics (Orlando, FL, USA) QE65000 equipment (785.0 nm laser, 5 s acquisition time). Raman spectroscopy for the scCO\({}_{2}\)-hyd samples was performed in a Jasco NRS-5100 equipment (785.0 nm laser, 60 s acquisition time), from JASCO Corporation (Hachioji, Japan). BET analysis was performed in a Quantachrome Autosorb iQ\({}^{\circledR}\), from Anton Paar Instrumentsn (Boynton Beach, FL, USA), in a 6 mm cell, samples were previously degassed for 2 h at 80 \({}^{\circ}\)C followed by 10 h at 100 \({}^{\circ}\)C.  
For comparison, another magnesium carbonate sample was prepared by aqueous route by reaction of magnesium chloride, MgCl\({}_{2}\)-6H\({}_{2}\)O [Products Quimicos Monterrey S.A. de C.V., Monterrey, Mexico, ACS grade], with potassium hydroxide [KOH, Products Quimicos Monterrey S.A. de C.V., ACS grade] and sodium bicarbonate [NaHCO\({}_{3}\), CTR Scientific, Monterrey, Mexico, ACS grade], by dissolving 0.1 mole of each in 100 mL of water.  
A commercial magnesium carbonate (Gym Chalk, Gibson Athletic, Denver, CO, USA, purchased at Amazon.com) was also characterized by XRD and BET for comparison.