The System MgO-CO2-H2O at High Pressures and Temperatures

The system MgO-CO₂-H₂O has been studied up to 1,400? C and 4,000bars pressure using the sealed-capsule quenching technique.No melting was observed. At 1,000 bars pressure magnesite dissociatesat 780? C, and brucite at 635? C, to periclase and vapor. Theunivariant reaction MgCO₃?Mg(OH)₂ MgO + V proceeds at 630?C, at 1,000 bars and at 700? C, at 4,000 bars. Solubility measurementsshow that, at 1,000 bars and temperatures up to 1,000? C, lessthan 1.5 weight per cent MgO is dissolved in the vapor phase.Brucite is unstable in the presence of vapors containing morethan a small amount of CO₂. The maximum percentage of CO₂ ina vapor that can coexist with brucite increases with decreasingpressure and with increasing temperature: 6 weight per centCO₂ is the maximum at 630? C, 1,000 bars, and 4 weight per centat 700? C, 4,000 bars. The phase relations in the isobaric TXprism for 1,000 bars pressure are described. The results illustratetwo dissociation reactions, decarbonation and dehydration, occurringin the presence of a vapor phase containing two volatile components,H₂O and CO₂. Applications to metamorphism are briefly discussed.