## 2 Materials and Methods  
### Obtaining MgO from serpentinite  
The flow chart for obtaining MgO from serpentinite is presented in Figure 1. In this work, an average sample of serpentinite from the \"Bedenskoye\" deposit, having a composition presented in Table 1, was used as a source of magnesium oxide. Several repeated experiments were carried out to decompose serpentinite and remove magnesium hydroxide. In each of them, 100 g of finely ground serpentinite powder (with a grain boundary size d < 45 \(\upmu\)m), obtained in a ball mill with ceramic balls, were placed in a laboratory autoclave with a mechanical stirrer, and 650 g (520 mL) of circulating solution containing 35% NaHSO4, up to 7% MgSO4 and up to 10% (NH4)2 SO4 were added. The mixture was stirred for 4 h at 120 \({}^{\circ}\)C. The obtained hot suspension was filtered under reduced pressure created by a water-jet pump on a Buchner filter with a thermostatic shirt at 95 \({}^{\circ}\)C to obtain filtrate No. 1, and then at the same temperature, the filter precipitate was washed by 250 mL of hot deionized water. Filtrate No. 1 and wash water were combined, and 912-925 g (730-740 mL) of mixed solution, containing no less than 3.4% of Mg(II) and no more than 0.08% of Fe(II, III), were obtained in different experiments that corresponded to the degree of magnesium removal--no less than 80%, and iron--up to 11.5%. The solution was placed in a cold-storage, where it was cooled to +4 \({}^{\circ}\)C and kept there for 2 h. As a result, a suspension was obtained, which was filtered on a thermostatted Buechner filter at 5 \({}^{\circ}\)C. Filtrate No. 2 (no more than 490 g with up to 0.5% magnesium and 0.1% iron) and a glassy precipitate with a pale yellowish-pink color were obtained. Chemical analysis of such precipitate (no less than 370 g), dried at 105 \({}^{\circ}\)C for 1 h until the loss of more than 10% moisture, showed a magnesium content of 6.5% and a molar ratio of Mg2+:NH4+SO4\({}^{-2}\) = 1:2:2 that corresponded to the double salt--magnesium ammonium sulfate (Boussinesqultite). The iron content in the dry sediment did not exceed 0.06%. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Content** & **MgO** & **SiO2** & **Fe3O4** & **CaO** & **NiO** & **Al2O3** & **Cr2O3** & **MnO** & **SO3** & **K2O** & **Na2O** & **LOI \({}^{*}\)** \\ \hline (wt\%) & 38.05 & 39.96 & 8.55 & 0.63 & 0.36 & 0.33 & 0.32 & 0.11 & 0.06 & 0.02 & 0.01 & 11.30 \\ \hline \hline \end{tabular} \({}^{*}\) Loss on Ignition.  
\end{table} 
Table 1: Chemical composition of serpentinite. Figure 1: Flow chart for obtaining MgO from serpentinite. The precipitate was dissolved in 650 mL of hot deionized water (95 \({}^{\circ}\)C), and a 25% ammonia solution (purity qualification \"analytical grade\") was added to the obtained solution up to pH of 8, and 2 g of ammonium carbonate, (NH4)2 CO3 (\"reagent grade\"), was also added. A hot suspension of light green color was obtained, which was placed in an autoclave, in which the temperature was maintained at 95 \({}^{\circ}\)C. Air was passed through the suspension, adjusting the outlet of gases from the autoclave so that an excess pressure is created in it. The mixture was kept for 4 h with stirring. A light brown suspension was discharged from the autoclave, which was filtered through hot thermostatted Buchner filter with a water-jet pump using a filter with pore size 1-2.5 nm. The resulting filtrate No. 3 was again placed in an autoclave, previously washed with deionized water, and then filtrate No. 3 was cooled to room temperature. 80 mL of the solution of 25% ammonia were poured into the contents of the autoclave, and 45 g of ammonium carbonate were added. The autoclave was closed, the temperature was raised up to 65 \({}^{\circ}\)C, and the mixture was stirred under excess pressure for 4 h. A suspension of white color with a bluish sheet was obtained, which was filtered through a thermostatted Buchner filter using a paper filter with pore size 1-2.5 nm. Filtrate No.