Chemical Composition and Spectral Characteristics of Different Colored Spinel Varieties from Myanmar

With the growth of the Myanmar spinel market in recent years, spinels of colors other than red, including gray spinels, have gained increasing popularity. In this study, we performed conventional gemological, spectroscopic, and chemical analyses on the less commonly studied gray, red, pink, and purple spinels from Mogok in Myanmar to investigate their chemical composition and color mechanisms. The Raman and FTIR spectral analyses indicated that the samples contained oxides of Mg-Al end-members and that the spectral peak positions of different colors were essentially the same. According to the major, minor, and trace elements of samples determined via EPMA and LA-ICP-MS, the purple and gray samples had the most prominent Fe contents, the red spinels had the highest Cr contents, and the pink samples had high V+Cr contents, with a certain amount of Fe. The UV–visible spectra indicated that the absorption spectrum of the gray samples was predominantly influenced by the Fetot content, particularly Fe2+. The color rendering of the purple spinels was also intimately associated with Fe. The absorption spectrum of the gray spinels was weaker but more concentrated at 458 nm than that of the purple varieties. Cr3+ and V3+ in the red spinels produced broad bands near 400 nm and 540 nm, respectively, while light pink spinels exhibited Cr3+ and V3+ absorption spectra but featured an additional absorption band at 460 nm due to Fe. This study complements other research on the coloration mechanisms of multi-color spinels from Mogok, especially gray spinels.