[SMP33-P14] The Effects of Temperature and Pressure on Hydromagnesite
キーワード：ハイドロマグネサイト、マグネサイト、相変化、高圧、高温、CO2地中貯留
In recent years, the global warming is the most important environment problem. Therefore, attempts of CO2 geological storage have been made to reduce carbon dioxide in the atmosphere all over the world (Xue and Nakano 2008). Among some CO2 geological storage methods, the CO2 mineral trapping can store the carbon dioxide safety for a long time. In the case of geological storage, magnesium carbonate hydrates are important minerals since they precipitate easily from water saturated with carbon dioxide. The major magnesium carbonate hydrates formed from aqueous solution are nesquehonite MgCO3・3H2O and hydromagnesite Mg5(CO3)4(OH)2・4H2O. The stability of these minerals are critically important factors to assess environmental safety and phase stability over geological time scale, but they have not been fully investigated yet, especially of hydromagnesite. Here, we investigated the effects of pressure and temperature on hydromagnesite by using thermal analysis, high-temperature X-ray diffraction, X-ray total scattering, high-pressure X-ray diffraction, and high-pressure and high-temperature neutron diffraction techniques.
With temperature, hydromagnesite was decomposed into periclase MgO through the poor crystalline phase, accompanied with dehydration, dihydroxylation, and decarbonation of hydromagnesite. The a and c lattice parameters were monotonously increased as increasing temperature whereas b lattice parameter almost remained unchanged. At just before the dehydration reaction, the unit cell was contracted due to the dehydration. With pressure, hydromagnesite structure was maintained up to 8.7 GPa. Decomposition into magnesite MgCO3 or periclase was unobserved up to at least 21.0 GPa. With compression, the unit cell was isotopically contracted. The fit to the Birch-Murnaghan equation of state gives K0 = 32(2) and V0 = 658(4) with K’ = 4.0 (fixed). Under high-pressure and high-temperature conditions, hydromagnesite broke down into magnesite and brucite Mg(OH)2 at 200 oC and 1.2 GPa. The unit cell was isotopically expanded up to just before the breakdown. The breakdown of hydromagnesite could be caused by the dissolution by dehydration water, and subsequently magnesite and brucite were hydrothermally grown from the solution at the condition. The results obtained from this study would provide insight the CO2 geological storage.
With temperature, hydromagnesite was decomposed into periclase MgO through the poor crystalline phase, accompanied with dehydration, dihydroxylation, and decarbonation of hydromagnesite. The a and c lattice parameters were monotonously increased as increasing temperature whereas b lattice parameter almost remained unchanged. At just before the dehydration reaction, the unit cell was contracted due to the dehydration. With pressure, hydromagnesite structure was maintained up to 8.7 GPa. Decomposition into magnesite MgCO3 or periclase was unobserved up to at least 21.0 GPa. With compression, the unit cell was isotopically contracted. The fit to the Birch-Murnaghan equation of state gives K0 = 32(2) and V0 = 658(4) with K’ = 4.0 (fixed). Under high-pressure and high-temperature conditions, hydromagnesite broke down into magnesite and brucite Mg(OH)2 at 200 oC and 1.2 GPa. The unit cell was isotopically expanded up to just before the breakdown. The breakdown of hydromagnesite could be caused by the dissolution by dehydration water, and subsequently magnesite and brucite were hydrothermally grown from the solution at the condition. The results obtained from this study would provide insight the CO2 geological storage.