Cobalt ionic crystal radius

In aluminosilicates each high-spin trivalent transition metal ion has a larger ionic radius than the host Al3+ ion. However, cations such as Cr3+ (-249.9 kJ/mole), V3+(-182.8 kJ/mole) and Mn3+ (-150.8 kJ/mole), which acquire particularly large CFSE s in octahedral coordination (table 2.5), are induced to enter [A106] octahedra, and not five-coordinated [A105] (andalusite, yoderite) or tetrahedral [AlOJ (silhmanite) sites, by the enhanced stabilization bestowed in octahedral crystal fields. Although Co3+ ions have not been positively identified in silicate minerals, the strong enrichment of cobalt in natural and syn-... 

On the basis of ionic radius, cobalt ion should be placed between copper and cadmium ions, and thus it has a higher affinity than expected. Perhaps it substitutes the iron(II) of crystal lattice since the two ionic radii are about the same (0.84 nm). 

---