Cation distribution in natural Zn-spinels: franklinite

The intracrystalline cation distribution of nine franklinite samples from F ranklin and Sterling (New Jersey, USA) was determined by means of single-cr ystal X-ray structural refinement supported, for some samples, by Mossbauer spectroscopy. The principal substitutions observed were Al for Fe3+ on the octahedral sit e and Mn2+ for Zn on the tetrahedral site. Small amounts of Mn3+ were requi red by stoichiometry in all samples; Consequently Fe2+ was totally absent, as confirmed by room-temperature Mossbauer spectra. The latter experiments also suggested limited inversion, that is, Fe3+ in the tetrahedral site, es sentially balanced by Mn2++Zn site. in the octahedral site. To obtain consistent cation distributions in these Zn-ferrites, it was nece ssary to adopt a Zn-IV-O distance of 1.999 Angstrom, a value very different from that previously used in Zn-aluminates (1.960 Angstrom), confirming ob servations by other authors on synthetic samples.