Silicon isotope fractionation during the precipitation of quartz and the adsorption of H4SiO4(aq) on Fe(III)-oxyhydroxide surfaces

Equilibrium Si isotope fractionation factors among orthosilicic acid (i.e., H4SiO4(aq)), quartz and the adsorption complexes of H4SiO4(aq) on Fe(III)-oxyhydroxide surface were calculated using the full-electron wave-function quantum chemistry methods [i.e., B3LYP/6-311G(2df,p)] with a new cluster-model-based treatment. Solvation effects were carefully included in our calculations via water-droplet method combined with implicit solvent models (e.g., PCM). The results revealed that, if it is under equilibrium conditions, heavy Si isotopes would be significantly enriched in quartz in comparison to H4SiO4(aq). However, most of the field observations suggested that quartz would have identical or even depleted delta Si-30 values compared to that of H4SiO4(aq). To explain this discrepancy between the equilibrium calculation results and the field observations, the kinetic isotope effect (KIE) associated with the formation of amorphous silica, which usually is the precursor of crystalline quartz, was investigated using quantum chemistry methods. The KIE results showed that amorphous silica would be significantly enriched in light Si isotopes during its formation. Our equilibrium fractionation results, however, matched a special type of quartz (i.e., Herkimer diamond) very well, due to its nearly equilibrated precipitation condition. Opposite to the case of precipitated quartz, a large equilibrium Si isotope fractionation (i.e., -3.0 parts per thousand) was found between the absorbed bidentate Si surface complexes (i.e., C-2 > Fe2O2Si(OH)(2)) and H4SiO4(aq). This calculated equilibrium Si isotope fractionation factor largely differed from a previous experimental result (ca. -1.08 parts per thousand). We found that the formation of transient or temporary surface complexes [e.g., V-1[Fe2OSi(OH)(3)] may have accounted for the smaller net fractionation observed. With the equilibrium and kinetic Si isotope fractionation factors provided here, the distributions and changes of Si isotope compositions in the Earth's surface systems can be better understood.