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

Four samples of synthetic chromium-bearing spinels of (Mg, Fe 2+ )(Cr, Fe 3+ ) 2 O 4 composition and four samples of natural spinels of predominantly (Mg, Fe 2+ )(Al, Cr) 2 O 4 composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic end-member MgCr 2 O 4 spinel was also studied at pressures up to ca. 10 GPa. In both synthetic and natural samples, chromium is present predominantly as octahedral Cr 3+ seen in the spectra as two broad intense absorption bands in the visible range caused by the electronic spin-allowed 4 A 2 g → 4 T 2 g and 4 A 2 g → 4 T 1 g 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, 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 transitions 4 A 2 g → 2 E g and 4 A 2 g → 2 T 1 g of Cr 3+ , intensified by exchange-coupled interaction between Cr 3+ and Fe 3+ 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 IV Cr 2+ + VI Cr 3+ → IV Cr 3+ + VI Cr 2+ intervalence charge-transfer transition. Iron, mainly as octahedral Fe 3+ , causes intense high-energy absorption edge in near UV-range (ligand–metal charge-transfer O 2− → Fe 3+ , Fe 2+ transitions). As tetrahedral Fe 2+ , it appears as a strong infrared absorption band at around 4,850 cm −1 caused by electronic spin-allowed 5 E → 5 T 2 transitions of IV Fe 2+ . From the composition shift of the U-band in natural and synthetic MgCr 2 O 4 spinels, the coefficient of local structural relaxation around Cr 3+ in spinel MgAl 2 O 4 –MgCr 2 O 4 system was evaluated as ~0.56(4), one of the lowest among (Al, Cr)O 6 polyhedra known so far. The octahedral modulus of Cr 3+ in MgCr 2 O 4 , derived from pressure-induced shift of the U-band of Cr 3+ , 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 Cr 3+ in natural and synthetic MgCr 2 O 4 spinels indicates that Cr–O-bonding in octahedral sites of MgCr 2 O 4 has more covalent character than in the