Synthetic and natural chromium-bearing spinels: An optical spectroscopy study

Four samples of synthetic chromium-bearing spinels of (Mg, Fe2+)(Cr, Fe3+)2O4composition and four samples of natural spinels of predominantly (Mg, Fe2+)(Al, Cr)2O4composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic end-member MgCr2O4spinel was also studied at pressures up to ca. 10 GPa. In both synthetic and natural samples, chromium is present predominantly as octahedral Cr3+seen in the spectra as two broad intense absorption bands in the visible range caused by the electronic spin-allowed4A2g→4T2gand4A2g→4T1gtransitions (U- and Y-band, respectively). A distinct doublet structure of the Y-band in both synthetic and natural spinels is related to trigonal distortion of the octahedral site in the spinel structure. A small, if any, splitting of the U-band can only be resolved at curve-fitting analysis. In all synthetic high-chromium spinels, a couple of relatively narrow and weak bands of the spin-allowed transitions4A2g→2Egand4A2g→2T1gof Cr3+, intensified by exchange-coupled interaction between Cr3+and Fe3+at neighboring octahedral sites of the structure, appear at ~14,400 and ~15,100 cm-1. A vague broad band in the range from ca. 15,000 to 12,000 cm-1in synthetic spinels is tentatively attributed toIVCr2++VICr3+→IVCr3++VICr2+intervalence charge-transfer transition. Iron, mainly as octahedral Fe3+, causes intense high-energy absorption edge in near UV-range (ligand-metal charge-transfer O2-→ Fe3+, Fe2+transitions). As tetrahedral Fe2+, it appears as a strong infrared absorption band at around 4,850 cm-1caused by electronic spin-allowed5E →5T2transitions ofIVFe2+. From the composition shift of the U-band in natural and synthetic MgCr2O4spinels, the coefficient of local structural relaxation around Cr3+in spinel MgAl2O4-MgCr2O4system was evaluated as ~0.56(4), one of the lowest among (Al, Cr)O6polyhedra known so far. The octahedral modulus of Cr3+in MgCr2O4, derived from pressure-induced shift of the U-band of Cr3+, is ~313 (50) GPa, which is nearly the same as in natural low-chromium Mg, Al-spinel reported by Langer et al. (1997). Calculated from the results of the curve-fitting analysis, the Racah parameter B of Cr3+in natural and synthetic MgCr2O4spinels indicates that Cr-O-bonding in octahedral sites of MgCr2O4has more covalent character than in the diluted natural samples. Within the uncertainty of determination in synthetic MgAl2O4spinel, B does not much depend on pressure. © 2014 Springer-Verlag Berlin Heidelberg.