Mechanism of SeO₄^2- substitution in calcite: An XAFS study

Analysis of Se K-edge XAFS spectra of calcite grown from SeO₄^2--bearing aqueous solutions at 25°C reveals the local environment of Se incorporated in calcite. The position of the Se K-edge is consistent with oxidation state 6+, and the first shell contains four O atoms at a distance of 1.63 Å, confirming that Se is present as the tetrahedral SeO₄^2-complex ion. The second shell is split and is best fitted by approximately 2 Ca atoms at 3.17 Å and approximately 4 Ca atoms at 3.49 Å. The coordination indicated by the split second shell is consistent with substitution of the SeO₄^2- ion in the CO₃ site of the calcite, which has six nearest-neighbor Ca atoms. The large size of the SeO₄^2- ion and its tetrahedral geometry cause significant distortion of the site that is consistent with the observed Se-Ca distances. Two possible models for the local coordination of the SeO₄^2- ion are suggested that involve corner sharing of the SeO₄ tetrahedron with neighboring Ca octahedra. The results suggest that other complex anions-uparticularly the geochemically important SO₄^2- ion, which is also tetrahedral but slightly smaller than SeO₄^2--may also substitute in the CO₃ site of carbonates.