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.
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