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), ~94 mol.%, with the remainder being dominated by a hercynite component (FeAl2O4). The unit-cell dimension is 8.0850(3) A ˚ and the tetrahedral and octahedral bond distances are determined as TO 1.9485(6) A ˚ and MO 1.9137(3) A ˚ . Crystal chemical analysis resulted in the empirical structural formula T(Zn0.94Fe2+ 0.03Al0.03)M(Al1.96Fe2+ 0.03Fe3+ 0.01)O4, 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 Eg mode at 420.6 cm1 and the T2g modes at 510 cm1 and 661 cm1, due to vibrations in the AlO6 octahedra. Optical absorption spectra recorded in the UV/VIS-NIR-MIR range 200029000 cm1 show a dominant absorption band at ~5000 cm1 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 TFe2+ and to MFe2+ - MFe3+ intervalence charge transfer, and the poor saturation of the colour is due to the small concentration of Fe2+ and Fe3+.
Crystallographic and spectroscopic characterization of a natural Zn-rich spinel approaching the endmember gahnite (ZnAl<sub>2</sub>O<sub>4</sub>) composition / V., D'Ippolito; G. B., Andreozzi; F., Bosi; U., Halenius; L., Mantovani; Bersani, Danilo; R. A., Fregola; Mantovani, Luciana. - In: MINERALOGICAL MAGAZINE. - ISSN 0026-461X. - 77(2013), pp. 2941-2953. [10.1180/minmag.2013.077.7.05]
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