Crystal structure chemistry and color of doped-forsterites revisited by X-ray powder diffraction and UVNis/NIR spectroscopy

The goal of this work is to revisit the crystal chemistry of synthetic forsterite containing doping elements in the octahedral site. A number of doping elements was systematically investigated: Li+, Mn2+, Co2+, Ni2+, Zn2+, Fe2+, Cr 3+, Cu2+, Ca2+, Cd2+, Al 3+, V5+. The synthetic phases were obtained in air at high temperature (1350 °C for 3h) and environmental pressure. Results permit to revise and improve the knowledge on the crystal chemistry of forsterite and clarify the role of the doping elements in the structure modification and physical properties. The structures of the synthetic samples were analyzed either on a local and on a long range by the combination of UV/Vis/NIR spectroscopy data and Rietveld refinements. It was possible to assess the factors that rule the solubility of the doping elements in olivine, mainly their charge and ionic radius for Co2+, Ni2+, Ca2+, Cd2+, V5+, crystal field for Cu2+ and Zn 2+, kinetics and oxygen fugacity for Fe2+ and Mn 2+. The modifications of the structure were described only with respect to the distortion in the octahedral site induced by the doping element because there are no modifications of the tetrahedral polyhedra. Cation partitioning was also described and depends upon the ionic radius of the doping element. Exceptions are due to CFSE and differences in the cation-anion electronegativity. The color of the synthesized powders was predicted using the crystal field theory and corroborated by the results of the spectroscopy experiments which clearly indicate a relationship between color and structure changes.