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Synthetic Co 3 Al 2 (SiO 4 ) 3  garnet and (Mg,Ni) 3 Al 2 (SiO 4 ) 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 Co 2+  and Ni 2+  occupy the large distorted cubic (dodecahedral) site of the structure. Structure refinement indicates that the Ni 2+  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 Ni 2+  is usually smaller than that of Mg 2+ . The lattice constant of Co 3 Al 2 (SiO 4 ) 3  garnet is 11.4597(2) Å. Because of the unusual coordination geometries of Ni 2+  and Co 2+ , the optical spectra of the garnets are fundamentally different from those of other Co 2+ - and Ni 2+ -bearing silicates. The following crystal-field Parameters were estimated from spectroscopic data: crystal-field Splitting Δ = 4430 cm -1  (Co 2+ ) and 4210 cm -1  (Ni 2+  ); Racah parameter B = 890 cm -1  (Co 2+ ) and 1080 cm“ 1  (Ni 2+ ). These data yield an exceptionally low crystal-field stabilization energy (CFSE) of Ni 2+  in garnet of 3370 cm -1 , which explains the very low Ni contents of garnets in equilibrium with olivine. The CFSE of Co 2+  in garnet of 5320 cm -1  is similar to values for pyroxenes and olivine. Therefore, the partition coefficient of Co 2+  between these phases should be close to unity, as is observed.

Structure and crystal-field spectra of Co 3 Al 2 (SiO 4 ) 3 and (Mg,Ni) 3 Al 2 (SiO 4 ) 3 garnet