Structure and crystal-field spectra of Co₃Al₂(SiO₄)₃ and (Mg,Ni)₃Al₂(SiO₄)₃ garnet

Synthetic Co₃Al₂(SiO₄)₃ garnet and (Mg,Ni)₃Al₂(SiO₄)₃ garnet with Ni/(Ni + Mg) = 0.15-0.18 have been studied by single-crystal X-ray diffraction and optical spectroscopy. The X-ray data confirm that Co²⁺ and Ni²⁺ occupy the large distorted cubic (dodecahedral) site of the structure. Structure refinement indicates that the Ni²⁺ ion is probably displaced out of the center of this site and statically or dynamically disordered onto the 48g equipoint at (1/8,y,1/4+y). This is consistent with the lattice constant of the Ni-bearing garnet of 11.4717(7) Å being significantly larger than the lattice constant of pyrope (11.459 Å), although the ionic radius of Ni²⁺ is usually smaller than that of Mg²⁺. The lattice constant of Co₃Al₂(SiO₄)₃ garnet is 11.4597(2) Å. Because of the unusual coordination geometries of Ni²⁺ and Co²⁺, the optical spectra of the garnets are fundamentally different from those of other Co²⁺- and Ni²⁺-bearing silicates. The following crystal-field parameters were estimated from spectroscopic data: crystal-field splitting δ = 4430 cm^-1 (Co²⁺) and 4210 cm^-1 (Ni²⁺); Racah parameter B = 890 cm^-1 (Co²⁺) and 1080 cm^-1 (Ni²⁺). These data yield an exceptionally low crystal-field stabilization energy (CFSE) of Ni²⁺ in garnet of 3370 cm^-1, which explains the very low Ni contents of garnets in equilibrium with olivine. The CFSE of Co²⁺ in garnet of 5320 cm^-1 is similar to values for pyroxenes and olivine. Therefore, the partition coefficient of Co²⁺ between these phases should be close to unity, as is observed.