The role of H sub(2)O in the carbonation of forsterite in supercritical CO sub(2)

The effect of variable H sub(2)O content on the carbonation of forsterite in supercritical CO sub(2) (scCO sub(2)) at 80 degree C and 76 bars (7.6 MPa) was investigated by a combination of NMR, XRD, TEM and XPS. When trace amounts of H sub(2)O were included, limited reaction was observed. Below H sub(2)O saturation in scCO sub(2), 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 H sub(2)O content above saturation, where forsterite was in contact with both a CO sub(2)-saturated aqueous fluid and H sub(2)O-saturated scCO sub(2), solid reaction products were magnesite (MgCO sub(3)) and an amorphous polymerized SiO sub(2). Formation of these anhydrous phases implies H sub(2)O initially bound in precursor hydrated/hydroxylated reaction products was liberated, inducing further reaction. Hence, for a given fluid/mineral ratio there is a H sub(2)O threshold above which a significant portion of the H sub(2)O serves in a catalytic role where more extensive carbonation reaction occurs. Defining the role of H sub(2)O, even in low H sub(2)O content environments, is therefore critical to determining the long term impact of CO sub(2) reactivity in the subsurface.