Experimental research of plagioclase (rock)-gas-water interaction at hydrothermal conditions for CO₂ mineralization

Experiments on CO₂-water-rock interaction at hydrothermal conditions have been performed to investigate mineral dissolution and precipitatior phenomena, mainly focusing on calcite deposition from the alteration of plagioclase and rocks during CO₂ sequestration into geothermal fields. Plagioclase from Shiga and granodiorite from the Ogachi hot/dry rock field (7 g; grain size is 0.5 to 2 mm) were independently enclosed with Kyoto tap water in a Teflon reaction container. The container is filled with CO₂ (10 MPa) or N₂ gas after evacuating and heated up to 150 °C in an electric furnace with rotation (1 rpm). After 1 to 15 days, the solutions were analyzed for their chemical compositions after filtration and the mineral surfaces were observed by using SEM-EDS. The concentration of Ca in the solutions reacted with CO₂ quickly increases within 1 day and is + ∼50 mg/L higher than those without CO₂ (with N₂ gas). The saturation index shows that the solutions with CO₂ are saturated with respect to carbonate such as calcite and aragonite during the reaction. Newly formed calcium carbonate (possibly calcite) was detected by SEM-EDS observation on the plagioclase surfaces, but the other minerals such as kaolinite were not. These results indicate that Ca can be released from rocks (silicates) easily and might be removed as CaCO₃ during CO₂ sequestration into relatively high temperature (geothermal) fields. Also, Ca-rich plagioclase has a high potential of CO₂ fixation as carbonate.