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 Mössbauer 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 Fetot (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 18,000−15,000 cm−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+.