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), ~94 mol.%, with the remainder being dominated by a hercynite component (FeAl₂O₄). The unit-cell dimension is 8.0850(3) Å and the tetrahedral and octahedral bond distances are determined as T–O 1.9485(6) Å and M–O 1.9137(3) Å. Crystal chemical analysis resulted in the empirical structural formula ^T(Zn_0.94$Fe0.032+$Al_0.03)^M (Al_1.96$Fe0.032+$$Fe0.013+$⁠)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 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 ~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 ^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⁺.