In the work, the preparation of fine-grained silicon was performed in combustion mode by magnesiothermic reduction of silica, obtained from serpentine mineral, which is characterized by a high specific surface area (about 560 m²/g) and high reaction activity. It is manifested by the fact that the measured combustion temperature exceeded the adiabatic combustion temperature calculated with the ISMAN THERMO software, by about 200 °C. In order to mitigate the synthesis conditions, combustion products (Si+2MgO) were used as diluting agents allowing a significant decrease of combustion temperature and a finer silicon powder without contaminating the target product with incidental reagents and without decreasing the yield of the target product. Based on the measured values of combustion parameters (combustion temperature and combustion wave propagation velocity) vs diluent amount, it was estimated that the effective activation energy for SiO₂+2Mg reaction by the solid+liquid mechanism in the temperature interval 1400–2100 °C was about 65±3 kJ/mol.