Strong site preference of Co2+ in olivine,Co1.10Mg0.90SiO4 |
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Authors: | Subrata Ghose Che'ng Wan |
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Institution: | 1. Department of Geological Sciences, University of Washington, 98195, Seattle, Washington
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Abstract: | The crystal structure and site preference of Co2+ in a synthetic Co1.10Mg0.90SiO4 olivine have been determined from single crystal X-ray diffraction data collected on an automatic diffractometer. The R factor is 0.044 for 612 reflections. The site occupancies are: Ml site: Co 0.730±0.006; Mg 0.270; M2 site: Co 0.370, Mg 0.630. The Gibbs free energy change, ΔG° for the ion-exchange reaction between M1 and M2 sites is ?4.06 kcals/mole, assuming ideal mixing at each set of sites. This energy may be called ‘site preference energy’ of Co2+ in olivine. The strong preference of Co2+ for the M1 site can be quantitatively explained by two competing forces: preference of ions larger than Mg2+ for the M2 site and stronger covalent bonding of transition metal ions at the M1 site. For Fe2+, Mg2+, these two effects nearly neutralize each other, explaining the lack of considerable cation-ordering in Fe-Mg olivines. |
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