Kinetics and mechanism of forsterite dissolution at 25°C and pH from 1 to 12
Résumé
The forward dissolution rate of San Carlos forsterite Fo 91 was measured at 25°C in a mixed-flow reactor as a function of pH (1 to 12), ionic strength (0.001 to 0.1 M), ΣCO 2 (0 to 0.05 M), aqueous magnesium (10 -6 to 0.05 M) and silica (10 -6 to 0.001 M) concentrations. In CO 2-free solutions, the rates decrease with increasing pH at 1 ≤ pH ≤ 8 with a slope close to 0.5. At 9 ≤ pH ≤ 12, the rates continue to decrease but with a smaller slope of ∼0.1. Addition of silica to solution at pH above 8.8 leads to reduction of up to 5 times in the dissolution rate. Magnesium ions have no effect on forsterite dissolution rate at pH from 3 to 6 and 10 -5 < [Mg 2+] tot < 0.04 M. Aqueous carbonate ions strongly inhibit dissolution in alkaline solutions when aCO 32- > 10 -4 M. In acidic and slightly alkaline solutions, forsterite dissolution is controlled by the decomposition of a silica-rich/magnesium-deficient protonated precursor complex. This complex is formed by exchange of two hydrogen ions for a Mg atom on the forsterite surface followed by polymerization of partially protonated SiO 4 tetrahedra and rate-controlling H + penetration into the leached layer and its adsorption on silica dimers. This accounts for the observed 0.5 order dependence of dissolution rate on H + activity. In alkaline solutions, dissolution is controlled by the decomposition of Mg hydrated sites in a Mg-rich layer formed by silica preferential release. Within this conceptual model, forsterite forward rate of dissolution can be accurately described for a wide variety of solution compositions assuming two parallel reactions occurring at silica-rich and hydrated Mg surface sites: R+ (mol/cm 2/s)=2.38×10 -11 {>Si 2O-H +}+1.62×10 -10 {>MgOH 2+} where {> i} stands for surface species concentration (mol/m 2). This equation describes the weak dependence of dissolution rates on pH in alkaline solutions and the inhibiting effect of carbonate ions and dissolved silica when the hydration of surface Mg atoms with formation of >MgOH 2+ is the rate-controlling step for dissolution. It follows that the decrease of forsterite dissolution rate with increasing carbonate concentration at pH ≥ 9 in natural aquatic systems results in a decrease of atmospheric CO 2 consumption, i.e., unlike for feldspars, there is a negative feedback between pCO 2 and forsterite weathering rate. This should be taken into account when modeling the effect of mafic mineral weathering on CO 2 global balance.