In this paper, we present an experimental study on the precipitation of magnesium carbonates (Mg-carbonates) from a solution containing magnesium chloride and sodium carbonate and in the presence of supercritical carbon dioxide. We performed homogeneous (unseeded) batch precipitation experiments at 100 bar of CO2 and at 90 degrees C, 120 degrees C, and 150 degrees C. The system was monitored with online temperature and pressure sensors and an online Raman spectroscopy probe. We investigated the effect of temperature and solution composition, i.e., supersaturation ratio, pH, speciation, ionic strength, and water activity on the mechanism and the kinetics of the precipitation of Mg-carbonates. Raman spectroscopy measurements allowed us to follow the temporal evolution of the solution and suspension composition and showed two Mg-carbonates can form under the investigated conditions, i.e., magnesite and hydromagnesite. The precipitation of these two phases occurred between pH 5.5 and 6.5 and was influenced by temperature, supersaturation ratio with respect to magnesite (S-M) and hydromagnesite (S-H), and the initial concentration of magnesium. At all investigated temperatures, we observed direct precipitation of magnesite while at 120 degrees C and 150 degrees C also simultaneous precipitation of magnesite and hydromagnesite followed by the transformation of the latter into the former. Under highly supersaturated conditions with respect to magnesite, S-M as large as 20, magnesite nucleated rapidly when the system was also supersaturated with respect to hydromagnesite, S-H as large as 1.5; whereas no nuclei formed within 20 h, otherwise. The analysis of the data collected at 120 degrees C and at 150 degrees C highlighted that the change in the type of mechanism was associated with the initial supersaturation ratios and the initial concentration of magnesium in solution. At 120 degrees C, the transformation process lasted for 2 h, slowing down the formation of magnesite, despite of the large S-M; whereas at 150 degrees C, the transformation process was only 5 min long, without affecting magnesite precipitation. (C) 2013 Elsevier B.V. All rights reserved.