Kinetics and mechanism of forsterite dissolution at 25 degrees C and pH from 1 to 12

The forward dissolution rate of San Carlos forsterite Fo(91) was measured a t 25 degrees C in a mixed-flow reactor as a function of pH (1 to 12), ionic strength (0.001 to 0.1 M), Sigma CO2 (0 to 0.05 M), aqueous magnesium (10( -6) to 0.05 M) and silica (10(-6) to 0.001 M) concentrations. In CO2-free s olutions, the rates decrease with increasing pH at 1 less than or equal to pH less than or equal to 8 with a slope close to 0.5. At 9 less than or equ al to pH less than or equal to 12, the rates continue to decrease but with a smaller slope of similar to 0.1. Addition of silica to solution at pH abo ve 8.8 leads to reduction of up to 5 times in the dissolution rate, Magnesi um ions have no effect on forsterite dissolution rate at pH from 3 to 6 and 10(-5) < [Mg2+](tot) < 0.04 M. Aqueous carbonate ions strongly inhibit dis solution in alkaline solutions when aCO(3)(2-) > 10(-4) M. In acidic and sl ightly alkaline solutions, forsterite dissolution is controlled by the deco mposition 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 S iO4 tetrahedra and rate-controlling H+ penetration into the leached layer a nd its adsorption on silica dimers. This accounts for the observed 0.5 orde r dependence of dissolution rate on H+ activity. In alkaline solutions, dis solution is controlled by the decomposition of Mg hydrated sites in a Mg-ri ch layer formed by silica preferential release. Within this conceptual mode l, forsterite forward rate of dissolution can be accurately described for a wide variety of solution compositions assuming two parallel reactions occu rring at silica-rich and hydrated Mg surface sites: R+ (mol/cm(2)/s) = 2.38 x 10(-11) {>Si2O-H+ } + 1.62 x 10(-10) { >MgOH2+} where {>i} stands for surface species concentration (mol/m(2)). This equati on describes the weak dependence of dissolution rates on pH in alkaline sol utions and the inhibiting effect of carbonate ions and dissolved silica whe n the hydration of surface Mg atoms with formation of >MgOH2+ is the rate-c ontrolling step for dissolution. It follows that the decrease of forsterite dissolution rate with increasing carbonate concentration at pH greater tha n or equal to 9 in natural aquatic systems results in a decrease of atmosph eric CO2 consumption, i.e., unlike for feldspars, there is a negative feedb ack between pCO(2) and forsterite weathering rate. This should be taken int o account when modeling the effect of mafic mineral weathering on CO2 globa l balance. Copyright (C) 2000 Elsevier Science Ltd.