CATION DISTRIBUTION IN NATURAL ZN-ALUMINATE SPINELS

The intracrystalline cation distributions in fourteen natural Zn-alumi nate spinels were determined by means of X-ray single-crystal structur al refinement, supported for some samples by Mossbauer spectroscopy. Z inc substitutes for Mg and subordinately Fe2+ and its relevant changes in content, from 0.10 to 0.96 atoms per formula unit (apfu), are not related to variations of cell parameter. The latter is determined main ly by the substitution Fe3+ reversible arrow Al. In agreement with dat a from synthetic samples, a small but definite amount of Zn (up to O.0 6 apfu) is located in the octahedral M site. Fe2+, when present, shows a preference for tetrahedral coordination. An improved value of the t etrahedral Zn(T)-O distance (1.960 Angstrom) was obtained, integrating the set of interatomic distances used for the determination of cation distribution in spinels.