RIETVELD REFINEMENT STUDY OF THE CATION DISTRIBUTION IN (CO, MG)-OLIVINE SOLID-SOLUTION

The Co, Mg-cation ordering in the Mg2SiO2Co2SiO4 solid solution series as a function of the chemical composition has been studied by X-ray p owder diffraction methods. The structures of nine polycrystalline samp les prepared at 1200 degrees C and equally cooled to room temperature within 2 minutes have been refined by the Rietveld technique. The resu lts corroborate earlier studies showing a strong preference of Co2+ fo r the M 1 site. Considering the agreement of the powder diffraction re sults with those of single crystal studies allows the conclusion that the powder method is well suited for investigating the cation distribu tion in compounds exhibiting significant ordering effects. According t o the cation distributions derived from first rapid quench experiments , the cation order of the slowly cooled samples corresponds to an esti mated equilibrium temperature of 800 degrees C.