Silicon Isotopes as a New Method of Measuring Silicate Mineral Reaction Rates at Ambient Temperature

Abstract

Experiments of albite dissolution under ambient temperature at pH 3 and 5 demonstrated the feasibility of using 29Si doping method to precisely measure silicate mineral dissolution rates. Initial solutions were doped with 29Si to result in an isotopic composition of 0.04% 28Si, 99.9% 29Si, and 0.06% 30Si, which compares with the Amelia and Evja albite with Si isotopic composition of 92.23% 28Si, 4.67% 29Si, and 3.10% 30Si. The isotopic contrast allows the detection of dissolution of a tiny amount of albite into the aqueous solution. Experimental results show rapid increase of 28Si abundance at the initial stage of fresh albite dissolution, reflecting reaction with high energy sites on the surface. Steady state dissolution rates were reached after 20-30 days at 22 oC. The evolution of 28Si and 29Si abundances (%) with time tightly constrains the albite dissolution rates, which are consistent with literature data. In experiments seeded with kaolinite, both albite dissolution rates and kaolinite precipitation rates were determined with a combination of Si isotopes and isotope dilution method of determining Si concentrations.The new experiments have also verified the assumptions that are necessary for the isotope doping method to be applicable. Experimental data showed no Si isotope fractionation during albite dissolution. The ion-exchange method in sample pre-treatments does not alter Si isotopic composition or silica concentrations. The errors from neglecting isotope fractionation during secondary mineral precipitation are small on the determined albite dissolution rates when the isotope doping is overwhelming. In conclusion, we now have substantial experimental data show that the 29Si doping is a new method applicable to a wide range of silicate mineral dissolution rate determinations. Particularly, our albite experimental results show promise to use isotope doping method to measure silicate dissolution rates at various saturation states and determine the rate – ΔGr relationships.