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

Abstract

The Cuban chromites with a spinel structure, FeCr 2O 4 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 Cr 3+ and Fe 3+ ions could be distinguished from UV–vis spectrum. Chromite spectrum shows two spin allowed bands at 17390 and 23810 cm −1 due to Cr 3+ in octahedral field and they are assigned to 4A 2g(F) → 4T 2g(F) and 4A 2g(F) → 4T 1g(F) transitions. This is in conformity with the broad resonance of Cr 3+ observed from EPR spectrum at g = 1.903 and a weak signal at g = 3.861 confirms Fe 3+ impurity in the mineral. Bands of Fe 3+ ion in the optical spectrum at 13700, 18870 and 28570 cm −1 are attributed to 6A 1g(S) → 4T 1g(G), 6A 1g(S) → 4T 2g(G) and 6A 1g(S) → 4T 2g(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 Fe 2+ 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, A 1g(ν 1) and the other two minor peaks at 560 and 445 cm −1 are assigned to F 2g(ν 4) and E g(ν 2) 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 F 2g(ν 3) mode. The minor peaks at 195, 175, 160 cm −1 might be due to E g and F 2g symmetries. Broadening of the peak of A 1g mode and shifting of the peak to higher wavenumber observed as a result of increasing the proportion of Al 3+O 6. 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 ν 1 and ν 2 are related to the lattice vibrations of octahedral groups. Due to the influence of tetrahedral bivalent cation, vibrational interactions occur between ν 3 and ν 4 and hence the low frequency bands, ν 3 and ν 4 correspond to complex vibrations involving both octahedral and tetrahedral cations simultaneously. Cr 3+ in Cuban natural chromites has highest CFSE (20,868 cm −1) when compared to other oxide minerals.