Compositional dependence of cation distribution in some synthetic (Mg,Zn)(Al,Fe3+)2O4 spinels

Abstract

The compositional dependence of the intracrystalline cation distribution was investigated in four synthetic spinels belonging to binary solid solutions. Spinel single crystals were flux-grown in the (Mg,Zn)(Al,Fe 3+ ) 2 O 4 system, and cation distribution was determined by means of single-crystal X-ray structural refinement, electron-microprobe analysis and Mossbauer spectroscopy. Experimental data were processed and a detailed topochemical model was obtained for each sample: IV (Mg 0.76 Al 0.24 ) VI (Al 1.76 Mg 0.24 )O 4 ; IV (Zn 0.65 Mg 0.22 Al 0.13 ) VI (Al 1.87 Mg 0.13 )O 4 ; IV (Mg 0.73 Al 0.18 Fe 3+ 0.09 ) VI (Al 1.62 Mg 0.27 Fe 3+ 0.11 )O 4 and IV (Mg 0.62 Zn 0.15 Al 0.15 Fe 3+ 0.08 ) VI (Al 1.61 Mg 0.23 Fe 3+ 0.16 )O 4 . Zinc was found to occupy only the tetrahedrally coordinated site and its presence strongly influenced intersite cation partitioning. In Zn-bearing crystals both intracrystalline exchanges Mg ⟷ Al and Mg ⟷ Fe 3+ turned out to be limited. Zinc in octahedral coordination, which is rare in natural spinels, was shown to be favoured by high Fe 3+ contents and high equilibration temperatures. The cation distributions determined experimentally were found to be in close agreement with those calculated using the general thermodynamic model for spinel solid solutions by O9Neill & Navrotsky (1984).