MECHANISM OF SEO42- SUBSTITUTION IN CALCITE - AN XAFS STUDY

Analysis of Se K-edge XAFS spectra of calcite grown from SeO42--bearin g aqueous solutions at 25 degrees C reveals the local environment of S e 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 Angstrom, confirming that Se is present as the tet rahedral SeO42- complex ion. The second shell is split and is best fit ted by approximately 2 Ca atoms at 3.17 Angstrom and approximately 4 C a atoms at 3.49 Angstrom. The coordination indicated by the split seco nd shell is consistent with substitution of the SeO42- ion in the CO3 site of the calcite, which has six nearest-neighbor Ca atoms. The larg e size of the SeO42- ion and its tetrahedral geometry cause significan t distortion of the site that is consistent with the observed Se-Ca di stances. Two possible models for the local coordination of the SeO42- ion are suggested that involve corner sharing of the SeO4 tetrahedron with neighboring Ca octahedra. The results suggest that other complex anions-particularly the geochemically important SO42- ion, which is al so tetrahedral but slightly smaller than SeO42--may also substitute in the CO3 site of carbonates.