[en] The reaction path in the MgO-CO₂-H₂O system at ambient temperatures and atmospheric CO₂ partial pressure(s), especially in high-ionic-strength brines, is of both geological interest and practical significance. Its practical importance lies mainly in the field of nuclear waste isolation. In the USA, industrial-grade MgO, consisting mainly of the mineral periclase, is the only engineered barrier certified by the Environmental Protection Agency (EPA) for emplacement in the Waste Isolation Pilot Plant (WIPP) for defense-related transuranic waste. The German Asse repository will employ a Mg(OH)₂-based engineered barrier consisting mainly of the mineral brucite. Therefore, the reaction of periclase or brucite with carbonated brines with high-ionic-strength is an important process likely to occur in nuclear waste repositories in salt formations where bulk MgO or Mg(OH)₂ will be employed as an engineered barrier. The reaction path in the system MgO-CO₂-H₂O in solutions with a wide range of ionic strengths was investigated experimentally in this study. The experimental results at ambient laboratory temperature and ambient laboratory atmospheric CO₂ partial pressure demonstrate that hydromagnesite (5424) (Mg₅(CO₃)₄(OH)₂ . 4H₂O) forms during the carbonation of brucite in a series of solutions with different ionic strengths. In Na-Mg-Cl-dominated brines such as Generic Weep Brine (GWB), a synthetic WIPP Salado Formation brine, Mg chloride hydroxide hydrate (Mg₃(OH)₅Cl . 4H₂O) also forms in addition to hydromagnesite (5424). The observation of nesquehonite (MgCO₃ . H₂O) and subsequent appearance of hydromagnesite (5424) in the experiments in a Na-Cl-dominated brine (ERDA-6) at room temperature and P_CO2=5x10^-2 atm allows estimation of the equilibrium constant (log K) for the following reaction: Mg₅(CO₃)₄(OH)₂.4H₂O+CO₂(g)+10H₂O=5MgCO₃.3H₂O as ∼2.5 at 25 deg. C. The log K for the above reaction at 5 deg. C is calculated to be ∼4.0 by using the Van't Hoff equation. By using these equilibrium constants, the co-existence of hydromagnesite (5424) with nesquehonite in various, natural occurrences such as in weathering products of the meteorites from the Antarctic and serpentine-rich mine tailings, can be well explained. Since the stoichiometric ratio of Mg to C is higher in hydromagnesite (5424) than in nesquehonite, this finding could have important implications for the sequestration of anthropogenic CO₂ in mafic and ultramafic rocks, suggesting that the sequestration of anthropogenic CO₂ is optimal in the stability field of nesquehonite