The role of H2O in the carbonation of forsterite in supercritical CO2

The effect of variable H2O content on the carbonation of forsterite in supercritical CO2 (scCO2) at 80 °C and 76 bars (7.6 MPa) was investigated by a combination of NMR, XRD, TEM and XPS. When trace amounts of H2O were included, limited reaction was observed. Below H2O saturation in scCO2, reaction products were a mixture of partially hydrated/hydroxylated magnesium carbonates and hydroxylated silica species that were mainly in an amorphous state, forming a non-resolved layer on the forsterite surface. At H2O content above saturation, where forsterite was in contact with both a CO2-saturated aqueous fluid and H2O-saturated scCO2, solid reaction products were magnesite (MgCO3) and an amorphous polymerized SiO2. Formation of these anhydrous phases implies H2O initially bound in precursor hydrated/hydroxylated reaction products was liberated, inducing further reaction. Hence, for a given fluid/mineral ratio there is a H2O threshold above which a significant portion of the H2O serves in a catalytic role where more extensive carbonation reaction occurs. Defining the role of H2O, even in low H2O content environments, is therefore critical to determining the long term impact of CO2 reactivity in the subsurface.

► H2O content dependence of forsterite carbonation in scCO2 at 80 °C and 76 bars.
► Reaction products were minimal using scCO2 alone without added H2O.
► Below H2O saturation, products were hydrated magnesium carbonates and hydroxylated silica.
► Above H2O saturation, products were MgCO3 and amorphous polymerized SiO2.
► A water threshold exists above which part of the water serves in a catalytic role.