Spectroscopic characterization of chromite from the Moa-Baracoa Ophiolitic Massif, Cuba

Abstract

The Cuban chromites with a spinel structure, FeCr2O4 have been studied using optical absorption and EPR spectroscopy. The spectral features in the electronic spectra are used to map the octahedral and tetrahedral co-ordinated cations. Bands due Cr3+ and Fe3+ ions could be distinguished from UV-vis spectrum. Chromite spectrum shows two spin allowed bands at 17,390 and 23,810 cm(-1) due to Cr3+ in octahedral field and they are assigned to 4A2g(F) --> 4T2g(F) and 4A2g(F) --> 4T1g(F) transitions. This is in conformity with the broad resonance of Cr3+ observed from EPR spectrum at g = 1.903 and a weak signal at g = 3.861 confirms Fe3+ impurity in the mineral. Bands of Fe3+ ion in the optical spectrum at 13,700, 18,870 and 28,570 cm(-1) are attributed to 6A1g(S) --> 4T1g(G), 6A1g(S) --> 4T2g(G) and 6A1g(S) --> 4T2g(P) transitions, respectively. Near-IR reflectance spectroscopy has been used effectively to show intense absorption bands caused by electronic spin allowed d-d transitions of Fe2+ in tetrahedral symmetry, in the region 5000-4000 cm(-1). The high frequency region (7500-6500 cm(-1)) is attributed to the overtones of hydroxyl stretching modes. Correlation between Raman spectral features and mineral chemistry are used to interpret the Raman data. The Raman spectrum of chromite shows three bands in the CrO stretching region at 730, 560 and 445 cm(-1). The most intense peak at 730 cm(-1) is identified as symmetric stretching vibrational mode, A1g(nu1) and the other two minor peaks at 560 and 445 cm(-1) are assigned to F2g(nu4) and E(g)(nu2) modes, respectively. Cation substitution in chromite results various changes both in Raman and IR spectra. In the low-wavenumber region of Raman spectrum a significant band at 250 cm(-1) with a component at 218 cm(-1) is attributed F2g(nu3) mode. The minor peaks at 195, 175, 160 cm(-1) might be due to E(g) and F2g symmetries. Broadening of the peak of A1g mode and shifting of the peak to higher wavenumber observed as a result of increasing the proportion of Al3+O6. The presence of water in the mineral shows bands in the IR spectrum at 3550, 3425, 3295, 1630 and 1455 cm(-1). The vibrational spectrum of chromite gives raise to four frequencies at 985, 770, 710 and 650 cm(-1). The first two frequencies nu1 and nu2 are related to the lattice vibrations of octahedral groups. Due to the influence of tetrahedral bivalent cation, vibrational interactions occur between nu3 and nu4 and hence the low frequency bands, nu3 and nu4 correspond to complex vibrations involving both octahedral and tetrahedral cations simultaneously. Cr3+ in Cuban natural chromites has highest CFSE (20,868 cm(-1)) when compared to other oxide minerals.

Raman spectra of selected mineral phases of the Morasko iron meteorite

“…The most typical of chromite is intense band between 680 and 730 cm −1 , resulting from A 1g vibration of CrO 6 octahedra. Reddy and Frost concluded that the positive shift of this band depends on chemical composition of chromites, namely on degree of Cr 3+ replacement by Al 3+ or Fe 3+ ions. The relatively low position (683 cm −1 ) of the discussed band observed in present work suggests that such replacement is rather small.…”

Optical properties and photocatalytic activity of nanocrystalline TiO2 doped by 3d-metal ions

“…The bands which are present in the spectra of Fe-doped samples at 1.4, 1.9 and 2.6 eV are very close to the bands of Fe 3+ in spinel [35]. So, these bands correspond to d-d transitions of Fe 3+ ions in anatase lattice.…”

Study of the Corrosion of Buried Steel Grids of Electrical Power Transmission Towers

“…The lepidocrocite is usually the initial corrosion product, and as the time of exposure to the elements longer, its transformation into goethite occurs. The most representative peaks that identifies mixtures of lepidocrocite and goethite, according to (8)(9)(10) are at 250, 380, 527, 658 and 1289 cm -1 . …”