Structural and spectroscopic characterization of a natural Zn-rich spinel approaching to the end-member gahnite

A natural Zn-rich spinel crystal coming from the Jemaa, Kaduna State, Nigeria, was analyzed by electron microprobe. The gahnite component (ZnAl2O4) of this sample is 94%, the remaining being dominated by a hercynite component (FeAl2O4). To our knowledge, a gahnite component of 94% is the highest value among the mineral samples studied in literature. For this reason a full characterization of this crystal was made by using single-crystal X-ray diffraction, optical absorption and Raman spectroscopies. The sample studied is an octahedral, 3 mm large single crystal with a pale blue-grey colour. The values of its main structural parameters are: unit cell parameter of 8.0850(3) Å, T-O bond distance of 1.9485(6) Å and M-O bond distance of 1.9137(3) Å. The empirical structural formula is T (Zn0:94 Fe2+ 0:04Al0:02)M(Al1:97Fe2+ 0:02Fe3+ 0:01)O4 shows a very low degree of inversion (0.02), with Zn and Al almost fully ordered in the tetrahedrally- and octahedrallycoordinated T and M sites, respectively. Raman spectra were collected over the range 100-800 cm1 Raman shift using two different laser wavelengths, 632.8 nm and 473.1 nm. A strong fluorescence was observed, but in spite of this, two Raman modes of the five Raman-active modes of the spinel phase were recorded. They are the Eg mode at 419 cm1 due to Zn in ZnO4 tetrahedra and the T2g mode at 662 cm1 due to Al in AlO6 octahedra. What observed is in excellent agreement with the literature. The optical absorption spectra were collected in the UV/VIS-NIR range 5000-30000 cm1. They show a dominant absorption band at ca. 5200 cm1 that has been assigned to a spin-allowed d d transition in Fe2+ located at the tetrahedrally coordinated T site.