Cation distribution in natural Zn-spinels; franklinite

The intracrystalline cation distribution in nine franklinite samples from Franklin and Sterling, New Jersey, was determined by single-crystal X-ray structural refinement supported for two samples by Moessbauer spectroscopy. The principal substitutions are Al for Fe (super 3+) in the octahedral site and Mn (super 2+) for Zn in the tetrahedral site; small amounts of Mn (super 3+) were required by stoichiometry in all samples, consequently Fe (super 2+) was absent. A limited inversion with Fe (super 3+) in the tetrahedral site, balanced by Mn (super 2+) + Zn in the octahedral site, is suggested. To obtain consistent cation distributions in these spinels, it was necessary to adopt a (super [4]) Zn-O distance of 1.999 Aa, i.e. very different from the value of 1.960 Aa previously used in Zn-aluminates, confirming observations by other authors.