A Quantum Chemical Study of Doped Aragonite and Comparison with Calcite

摘要

We have investigated the presence of foreign ions into the bulk structure and the [011] surface of aragonite using periodic ab-initio methods. Four cations isovalent to Ca~(2+) were studied: Mg~(2+), Sr~(2+), Ba~(2+) and Zn~(2+). The calculations were performed at structures (bulk, surface) that contain four and eight CaCO_3 units. Our results, at the Hartree-Fock level, show that the incorporation of those ions into aragonite depends strongly on their size. Mg~(2+) and Zn~(2+), due to their smaller size, can substitute Ca~(2+) ions in the crystal lattice while the incorporation of Sr~(2+) and Ba~(2+) into aragonite is energetically less favored. Examination of the [011] surface of aragonite revealed that the incorporation is more pronounced than in bulk structure. The incorporation of Mg~(2+) in the bulk structure of aragonite has almost the same energetic content with the corresponding one on the [104] surface of calcite. Bulk and surface incorporation exhibit different trends in aragonite and calcite. The surface incorporation is more favored in aragonite while in calcite ions prefer to be incorporated in the bulk structure.