Trace elements and color-causing role in spinel and tanzanite

Abstract

This research is aimed to investigate the significant trace elements and their color-causing roles in tanzanite and spinels (7 color varieties) by focusing on their concentrations, valencies, and site occupancy since no palpable research has been reported previously either before or after heating of both gems. According to this finding, natural tanzanites usually show strongly trichroic colors of violet, blue, and brown in different directions. However, this characteristic is easily changed to violet-blue dichroism after heat treatment. As the result, the violet-blue absorption band (centered around 450–460 nm) is obviously decreased after heating, and XAS analysis indicates the higher valency state of vanadium. Consequently, vanadium is strongly suggested as the significant coloring agent in tanzanite which is also supported by its dominant concentration.

Based on this finding, natural spinels contain trace elements which may cause various hues. Red color is attributed by the combination of significant Cr and V ratios. Magenta and purple to blue and green colors are affected by significant Fe content, whereas orange color shows contribution to significant V content. After heating, XAS indicates a greater oxidation state as well as disordering of iron and vanadium, while Cr stays consistent. Broadening of the dominant peak at around 406.2 cm-1 and the occurrence of additional small peak at around 715.2-719.8 cm-1 in Raman spectra as well as broadening of the 685 nm and poorly defined structure of additional peaks in photoluminescence spectra are significant indicators of inverse spinels (heated spinel) which strongly benefits gem-testing laboratory to recognize the heat-treated spinel.