Crystallographic and spectroscopic characterization of a natural Zn-rich spinel approaching the endmember gahnite (ZnAl2O4) composition

The crystal chemistry of a natural, gem-quality, blue-grey Zn-rich spinel crystal from Jemaa, Kaduna State, Nigeria, was studied using electron microprobe, single-crystal X-ray diffraction, optical absorption and Raman spectroscopies. The composition of the crystal approaches the gahnite endmember (ZnAl2O4), similar to 94 mol.%, with the remainder being dominated by a hercynite component (FeAl2O4). The unit-cell dimension is 8.0850(3) angstrom and the tetrahedral and octahedral bond distances are determined as T-O 1.9485(6) angstrom and M-O 1.9137(3) angstrom. Crystal chemical analysis resulted in the empirical structural formula (T)(Zn0.94Fe0.032+Al0.03)(M)(Al1.96Fe0.032+Fe0.013+)O-4, which shows Zn and Al almost fully ordered in the tetrahedrally and octahedrally coordinated T and M sites, respectively. Raman spectra obtained using the excitation of the blue 473.1 nm line of a Nd:YAG laser display three of the five Raman-active modes predicted for the general oxide spinel group of minerals. These are the E-g mode at 420.6 cm(-1) and the T-2g modes at 510 cm(-1) and 661 cm(-1), due to vibrations in the AlO6 octahedra. Optical absorption spectra recorded in the UV/VIS-NIR-MIR range 2000-29000 cm(-1) show a dominant absorption band at similar to 5000 cm(-1) which is caused by spin-allowed electronic d-d transitions in Fe2+ located at the T sites. The blue-grey hue exhibited by the sample is mainly due to spin-forbidden electronic transitions in Fe-T(2+) and to Fe-M(2+) <-> Fe-M(3+) intervalence charge transfer, and the poor saturation of the colour is due to the small concentration of Fe2+ and Fe3+.