STRUCTURE AND CRYSTAL-FIELD SPECTRA OF CO3AL2(SIO4)(3) AND (MG,NI)(3)AL-2(SIO4)(3) GARNET

Synthetic Co3Al2(SiO4)(3) garnet and (Mg,Ni)(3)Al-2(SiO4)(3) garnet wi th 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 probabl y displaced out of the center of this site and statically or dynamical ly disordered onto the 48g equipoint at (1/8,y,1/4+y). This is consist ent with the lattice constant of the Ni-bearing garnet of 11.4717(7) A ngstrom being significantly larger than the lattice constant of pyrope (11.459 Angstrom), although the ionic radius of Ni2+ is usually small er than that of Mg2+. The lattice constant of Co3Al2(SiO4)(3) garnet i s 11.4597(2) Angstrom. 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 f ollowing crystal-field parameters were estimated from spectroscopic da ta: crystal-field splitting Delta = 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 ene rgy (CFSE) of Ni2+ in garnet of 3370 cm(-1), which explains the very l ow 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 oliv ine. Therefore, the partition coefficient of Co2+ between these phases should be close to unity, as is observed.