| Autor: |
Ghose, Subrata, Wan, Che'ng |
| Zdroj: |
Contributions to Mineralogy and Petrology; June 1974, Vol. 47 Issue: 2 p131-140, 10p |
| Abstrakt: |
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, ?G° 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. |
| Databáze: |
Supplemental Index |
| Externí odkaz: |
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