Color mechanisms in spinel: a multi-analytical investigation of natural crystals with a wide range of coloration

Twenty natural spinel single crystals displaying colors almost representative for the entire spinel variability were investigated by electron microprobe and UV-VIS-NIR-MIR and FTIR spectroscopies. Eight of them, selected among the Fe-bearing ones, were also analyzed by X-ray diffraction, and five by Mossbauer spectroscopy to obtain information on the oxidation state and site distribution of Fe. The adopted multi-analytical approach was successful in revealing that the color displayed by aluminate spinel crystals is due to a combination of two or more minor transition elements acting as chromophore, such as V3+, Cr3+, Fe2+, Fe3+, Mn2+ and Mn3+, variably distributed in the tetrahedrally and octahedrally coordinated sites of the spinel structure. Iron-poor orange, red and magenta spinel crystals owe their color mainly to the presence of V3+ and Cr3+ at the M sites (with predominance of V3+ for orange and of Cr3+ for red color). Iron-rich pink, blue and green spinel crystals, in spite of exhibiting very different colors, have relatively similar optical absorption spectra characterized by a strong UV-edge absorption and a series of weak absorption bands in the visible range. From pink to blue and green spinel samples, color differences depend on the increase of Fe-tot (primarily) and Fe3+ contents (secondarily), which are responsible for both the intensification of UV-edge absorption and the different intensity, width and position of the prominent absorption bands occurring in the range 18,000-15,000cm(-1). The scarcity in nature of yellow spinel is explained by the rarity of conditions necessary to obtain the yellow color, such as the exclusive presence of Mn acting as a chromophore or at least the absence of Fe2+, to avoid masking of the weak electronic transitions in Fe3+, Mn2+ and Mn3+.