Compositional dependence of cation distribution in some synthetic (Mg,Zn)(Al,Fe3+)(2)O-4 spinels

The compositional dependence of the intracrystalline cation distribution wa s investigated in four synthetic spinels belonging to binary solid solution s. Spinel single crystals were flux-grown in the (Mg,Zn)(Al,Fe3+)(2)O-4 sys tem, and cation distribution was determined by means of single-crystal X-ra y structural refinement, electron-microprobe analysis and Mossbauer spectro scopy. Experimental data were processed and a detailed topochemical model w as obtained for each sample: (IV)(Mg0.76Al0.24)(VI)(Al1.76Mg0.24)O-4; (IV)( Zn0.65Mg0.22 Al-0.13)(VI)(Al1.87Mg0.13)O-4; (IV)(Mg0.73Al0.18Fe0.093+)(VI)( Al1.62Mg0.27Fe0.113+)O-4 and (IV)(Mg0.62Zn0.15Al0.15Fe0.083+) (VI)(Al1.61Mg 0.23Fe0.163+)O-4. Zinc was found to occupy only the tetrahedrally coordinat ed site and its presence strongly influenced intersite cation partitioning. In Zn-bearing crystals both intracrystalline exchanges Mg <--> Al and Mg < --> Fe3+ turned out to be limited. Zinc in octahedral coordination, which i s rare in natural spinels, was shown to be favoured by high Fe3+ contents a nd high equilibration temperatures. The cation distributions determined exp erimentally were found to be in close agreement with those calculated-using the general thermodynamic model for spinel solid solutions by O'Neill & Na vrotsky (1984).