gems, spinel, causes of color in spinel, chromium, iron, cobalt, vanadium, spectroscopy, fluorescence

Fluorescence plays an important role in determining the color appearance of fluorescing minerals. This paper discusses the color mechanism in red spinel and the color effects of the light source as well as the background on spinel color. Excitation-Emission Matrices (EEMs) fluorescence spectroscopy was utilized to characterize the fluorescence of natural Cr-doped magnesia-alumina spinel from Myanmar. EMP, LA-ICP-MS, and optical spectroscopy were applied to study the spinel's chemical compositions and color mechanism. An X-Rite Ci-7800 spectrophotometer, which is useful for measuring colors in fluorescing minerals, was employed to acquire color data for spinel in daylight and incandescent light. The results indicated that the higher Cr/Fe value makes pinkish-red spinel have a much stronger red fluorescence effect than dark-red spinel. The two narrow absorption bands at ~25,500 cm⁻¹ (with a broad absorption band at ~24,100 cm⁻¹) and ~18,570 cm⁻¹ in the optical absorption spectrum are assigned to spin-allowed electronic d-d transitions ⁴A₂g → ⁴T₁g(F) and ⁴A₂g → ⁴T₂g(F) in Cr³⁺ at the M site. The EEMs spectra of the pinkish-red spinels show twin emissions at 706 nm excited by both ~380 nm and ~535 nm radiation, which is the key to the bright neon red color in pinkish-red spinel. The colorimetry study suggests that the strong red fluorescence produced by <460 nm radiation contributes more than 10% to lightness and chroma in pinkish-red spinel under daylight. The lightness and chroma of the spinels increase with decreasing background grayscale. The differentiation of spinel colors in dark conditions is lower than that in a bright environment.