Colouring mechanisms in natural spinels

Natural minerals belonging to the spinel group are actively sought as gemstones because of their intense multi-colour, high mechanical resistance and high thermal and chemical stability. Causes of colour in spinels may be different, most of them being related to transition metal ions, their valence and their coordination. However, more complex colouring mechanisms are difficult to characterize and often remain unexplained. A detailed study on the causes of colours in the spinels is lacking in literature. To fully comprehend the origin of colour, a large number of natural spinels showing colours well representative for the entire variability of colour were explored by Electron Microprobe Analysis and UV-VIS-NIR-MIR spectroscopy. From the chemical characterization, the analysed spinels exhibit a prevalent spinel s.s. (MgAl2O4) or gahnite (ZnAl2O4) component. The different showed colours do not depend on the end-member composition because, while the samples having a gahnitic composition show colours close to the blue hue, the samples having a spinel s.s. composition show all kind of colours. Hence, the various colours are due to a combination of two or more minor transition metal cations such as Cr3+, V3+, Fe2+, Fe3+ and Co2+, occupying tetrahedrally (T) and/or octahedrally (M) coordinated sites in the spinel structure. Optical spectra of about thirty natural spinels were recorded in the UV/VIS to NIR spectral range (32000–2000 cm-1). Red, orange and magenta coloured spinels show similar absorption spectra with the main absorption bands at ~25500 cm-1 and ~18500 cm-1 assigned to spin-allowed d-d transitions of Cr3+ and V3+ in the M sites. When the contents of Cr3+ predominate on the contents of V3+, the spinels appear as red, otherwise they appear as orange. The magenta spinels have considerable content of Cr and secondary amount of Fe showing a red shift of the band at ~ 18500 cm-1, with respect to the red coloured samples. The pink, blue and dark green spinels, in spite of exhibiting very different colours, show similar absorption spectra characterized by a strong UV-edge absorption at energy >> 30000 cm-1 due to the O2- → Fe2+ and O2-→ Fe3+ charge transfer transitions and a series of weak absorption bands in the visible range mainly assigned to spin-forbidden d-d transitions of Fe2+ at the T sites. The peak of maximum absorption in the range 20000 and 10000 cm-1 moves from ~18000 cm-1 for the pink spinels to ~15500 cm-1 for the green spinels (with the blue spinels showing an intermediate situation) depending on the increase of the iron total content and thus also of the Fe3+ content. In fact, the latter interacts with the Fe2+ producing the Fe2+-Fe3+ intervalence charge transfer at 15500 cm-1 in the green samples. The optical absorption bands in the range 18000-15000 cm-1 of the light blue and blue coloured spinels are also influenced by the presence of Co2+ in tetrahedrally coordinated sites.