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

Abstract

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(Zn0.94Fe2+ 0.03Al 0.03)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 E g mode at 420.6 cm-1 and the T2g modes at 510 cm-1 and 661 cm-1, due to vibrations in the AlO 6 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+. © 2013 The Mineralogical Society

Raman fingerprint of chromate, aluminate and ferrite spinels

“…Variation of the E g Raman peak positions with the mean cation radius in the three spinel subgroups. In the calculation of the mean cation radius, the various tetrahedral and octahedral cation radii were obtained by Andreozzi et al [36] D'Ippolito et al [42] Nestola et al [46] Bosi et al [59] Hålenius et al [60] Andreozzi and Lucchesi, [61] and Bosi et al [77] in order to take into account also the effect of the cation inversion. The size of the symbols used is equal to or larger than standard uncertainties.…”

“…[36][37][38][39][40] [41] and showed a composition closely to the magnetite (FeFe 2 O 4 ) end-member with the FeFe 2 O 4 content ranging from 50% to 98%. In addition, the Raman spectrum of a natural sample, labeled Ni8967c, closely approaching the gahnite (ZnAl 2 O 4 ) end-member, already recorded by D'Ippolito et al [42] was considered in the comparison among the different end-members. Chemical results of each sample are reported in Table 1.…”

“…In addition to the Raman spectra here measured, Fig. 3 displays the Raman spectrum of a spinel approaching the end-member gahnite (ZnAl 2 O 4 ) reported by D'Ippolito et al [42] All aluminates except gahnite exhibit all the five Raman active modes whose wavenumbers are in same sequence as in chromates…”

“…[42] Looking at the different Raman wavenumbers in the aluminates, it is possible to examine the effect of replacing the tetrahedral cation on the Raman bands and thus to assign them to internal vibrations of the AlO 6 octahedra and/or A 2+ O 4 tetrahedra. The differences in the E g , F 2g (2), and A 1g mode wavenumbers among the aluminates are approximately only 3-4%.…”

“…The peculiar peak of each end-member spinel is marked in bold type face. Raman modes values of ZnAl 2 O 4 end-member are in italics and are taken from D'Ippolito et al[42] Figure 2. Variation of the F 2g (2) mode with the tetrahedral cation radius in the chromate spinels.…”