This work examines factors, which can influence the formation of magnesium (Mg) carbonate phases produced during direct aqueous carbonation of heat‐activated lizardite, hydromagnesite, and the targeted magnesite phase. Carbon dioxide mass transfer and mixing during the course of the reaction was examined by varying impeller positioning and stirring speed of the dual impeller reactor. From a practical perspective, this provides some insight into the importance of reactor design, to achieve the highest possible magnesite yield. At the lowest stirring speed studied (100 rpm), two Mg‐carbonate phases were observed in all samples. By simply increasing the stirring speed, hydromagnesite was observed only during the initial stages of reaction and magnesite formation dominated thereafter. Higher yields of carbonate were obtained for the intermediate and maximum stirring speeds (450 and 600 rpm), respectively. Positioning of the uppermost impeller near to the liquid surface was also found to favor the formation of magnesite. Thermodynamic simulations (using OLIAnalyzer 9.2) were in a good agreement with the experimental results.