Color–inducing elements and mechanisms in nephrites from Golmud, Qinghai, NW China: Insights from spectroscopic and compositional analyses

Gem–quality Qinghai nephrites (QN) of NW China occur in a variety of distinctive colors (white, green–white, yellow, brown, blue–violet, viridis, green, and azure–green). This study investigates the mechanisms and chemical elements that lead to coloration of the QN. Mineralogy identification indicates that the main mineral component in QN samples of all eight colors is tremolite, which accounts for over 95 vol%. Chemical composition investigations reveal elevated concentrations of Fe2+, Fe3+, Mn, Cr, and Ti in all eight colors of QN. Electron paramagnetic resonance (EPR) spectra reveal three Mn2+ centers and a rhombic Fe3+ center at the octahedral sites [M1, M2, and M3]. However, the spectrum intensities of Mn2+ and Fe3+ are not directly correlated with the concentrations of these elements. The main reason being that Mn also occurs in higher valence states, leading to minor amounts of Fe3+ substituting for Si4+ at the tetrahedral sites (T1 and T2). Under conditions of oxidation, heating to 500 and 800 °C leads to fading of some QN colors, including the yellow, brown, and blue–violet, but darkening of the green–white, viridis, green, and azure–green colors. Ultraviolet–visible spectra (UV–vis spectra) of the faded samples show that the absorption bands near 550 or 560 nm gradually decrease with heating (and may disappear completely), and the absorption bands near 437 or 450 nm are enhanced in samples where the color darkens. In detail, the yellow colors of QN are caused primarily by O2− → Fe3+ ligand–to–metal charge transfer (LMCT) and Mn3+(5Eg) → 5T2g, the blue–violet colors by Fe2+ → Ti4+ inter–valence charge transfer (IVCT), and the brown color by the electron transitions 5Eg → 5T2g of Mn3+. Color fading in response to heating is due to oxidation of Fe2+ or Mn3+, which results in reducing concentrations of these ions. Meanwhile, the darkening of the QN colors in response to heating is due to the oxidation of Fe2+, leading to increasing Fe3+ concentration. In these cases, the viridis color is caused mainly by electron transitions: Cr3+(4A2) → 4T2, Cr3+(4A2) → 4T1 + 2E, and Fe3+(6A1) → 4E + 4A1(4G); the green–white color results from the electron transition Fe3+(6A1) → 4E + 4A1(4G); and the green and azure–green colors are caused mainly by IVCT of Fe2+ → Fe3+ and Fe2+(5T2) + Fe3+(6A1) → Fe2+(5E) + Fe3+(6A1).