Minor Elements and Color Causing Role in Spinel: Multi-Analytical Approaches

Abstract

Natural spinel (MgAl2O44) usually contains some minor and trace elements (e.g., Cr, Co, Fe, V) that may cause various hues. The ratios of these chromophores directly affect the color composition. The red color in spinel is attributed to the combination of significant Cr and V. Magenta and purple to blue and green colors in spinels are affected by the significant Fe concentration, whereas orange color in spinel shows the contribution of significant V content compared to Cr and Fe. After the heating experiment, advanced gemological investigation reveals some noteworthy characteristic features. X-ray absorption spectroscopy (XAS) indicates a greater change in oxidation state, as well as disordering of Fe and V. Broadening of the dominant peak at around 406 cm−1 with occurrences of additional small peaks at around 715–719 cm−1 in Raman spectra, as well as broadening of the 685 nm (R-line) and poorly defined structure of additional peaks (N-lines) in photoluminescence spectra should be significant indicators of spinel undergone heat treatment.

Purple-Violet Gem Spinel from Tanzania and Myanmar: Inclusion, Spectroscopy, Chemistry, and Color

“…Two broad bands at ~458 nm and ~476 nm may be assigned principally to spin-forbidden 6 A 1g → 4 A 1g , 4 E g transitions of isolated M Fe 3+ ions, possibly intensified by ECP interactions and by spin-forbidden transitions of T Fe 2+ [6]. The absorption around ~540 nm may be caused by spin-allowed d-d transitions 3 T 1 (F)→ 3 T 2 (F) in V 3+ at the M sites or spin-allowed electronic d-d transitions 4 A 2g → 4 T 2g (F) in Cr 3+ at the M sites [6,9,20,[36][37][38][39][40][41]. (2)…”

“…Most of the (orangy) red-pink spinels attribute their color to Cr and V concentrations [6,28]. Equally high contents of Cr and V will cause red, while lower Cr and higher V may induce a more orange hue [41]. Blue colors in spinels are mainly caused by various electronic processes in Fe and Co cations.…”

“…Blue colors in spinels are mainly caused by various electronic processes in Fe and Co cations. Other colors, magenta, purple, and green, are mainly caused by a high concentration of Fe, especially the Fe 2+ /Fe 3+ ratio [41]. Moreover, Mn is known to act as a yellow chromophore [42].…”