Synthetic and natural chromium bearing spinels: an optical spectroscopy study

Four samples of synthetic chromium-bearing spinels of (Mg, Fe2+)(Cr, Fe3+)2O4 composition and four samples of natural spinels of predominantly (Mg, Fe2+)(Al, Cr)2O4 composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic endmember MgCr2O4 spinel 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-allowed 4A2g → 4T2g and 4A2g → 4T1g transitions (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, splittingof 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 transitions 4A2g → 2Eg and 4A2g → 2T1g of 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−1 in synthetic spinels is tentatively attributed to IVCr2+ + 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−1 caused by electronic spin-allowed 5E → 5T2 transitions of IVFe2+. From the composition shift of the U-band in natural and synthetic MgCr2O4 spinels, the coefficient of local structural relaxation around Cr3+ in spinel MgAl2O4–MgCr2O4 system was evaluated as ~0.56(4), one of the lowest among (Al, Cr)O6 polyhedra 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 MgCr2O4 spinels indicates that Cr–O-bonding in octahedral sites of MgCr2O4 has more covalent character than in the diluted natural samples. Within the uncertainty of determination in synthetic MgAl2O4 spinel, B does not much depend on pressure.