EPR and XRD study of Co-bearing natural spinel from Vietnam

Blue spinel from Luc Yen district in north Vietnam are included in marble. These marbles (and the associated spinel) come from the intense metamorphism, linked to successive oroge-nies, of an ancient carbonate platform from the Paleo-Tethys Ocean. Previous chemical and spectroscopic analyses (Chauviré et al. 2015) indicated that the blue color is due to cobalt (Co2+), with some iron contribution. Chemistry by LA-ICP-MS shows that the most intense blue spinels have about 13000 ppm Fe, 2500 ppm Ni, 1200 ppm Co, 1100 ppm Cr, while Ga, V, Ti and Mn are in the range 200-400 ppm. Chauviré et al. (2015) demonstrated through UV-Vis spectra that the main absorption band between 500 and 670 nm, the dominant origin of color in these blue spinels, is composed of a series of bands at approximately 545, 550, 560, 580, 590, and 625 nm. Bands at 545, 550, 580, and 625 nm are due to cobalt (Co2+) substituting for Mg2+ in tetrahedral sites of the spinel structure while the remaining absorption bands are allocated to iron (Fe2+) in tetrahedral sites of the spinel structure. Consequently, the spectra show transmission windows between 300 and 500 nm and between 700 and 900 nm that explain the blue color. As expected, the spectra show that iron (Fe2+) and cobalt (Co2+) are the main chromophore elements. Chromium makes a significant contribution to color if the concentration is above 1000 ppm. XRD single crystal analyses on several fragments show a cell edge of about 8.0864 Å and an oxygen positional parameter close to 0.2634. Some fragments were analysed through EPR spectroscopy, at X band (~9.5 GHz) and room temperature. A complex spectrum was obtained, in which two distinct features can be identified: a sextet at g~2.0, due to the hyperfine splitting, and ascribed to the Mn(II) ion. A second set of lines appear at g values in the range 1.6 and 2.0, whose variability as a function of the crystal orientation in the magnetic field point to an ion occurring in a medium-to-strong ligand field. Only Co(II) and Cr(III) can be proposed for the final attribution.