Spectroscopic Analysis of Nigerian Gem-grade Yellow Garnet Varieties and their Color Differences

Abstract

Background: Research on yellow garnets of different shades regarding their varieties, colors, as well as chemical states, and contents of coloring ions is somewhat lacking. Spectroscopic analyses are expected to enrich the research methods and data on yellow garnets and explore a scientific path for enhancing their colors. Methods: A series of analyses, such as X-ray diffraction, infrared spectroscopy, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and electron microprobe analyses, have been performed on the garnet crystal samples. Results: X-ray diffraction analysis and infrared spectral analysis have shown yellow and tawny garnets to be spessartite garnets; UV–visible spectral analysis has shown the yellow sample to have stronger absorption peaks at 408, 421, and 430 nm than the tawny sample. Electron microprobe and X-ray photoelectron spectroscopy analyses have shown both spessartite garnet samples to contain the same elements, including Si, O, Al, Ca, Mn, and Fe. Among them, the coloring elements, Mn and Fe, have been found to be identical in terms of type, chemical state, lattice occupancy, and coordination environment, but with different mass percentages. Conclusion: The color differences between yellow-shaded spessartite garnets from Nigeria have not been found to arise from differences in the types of coloring elements, their valance states, or coordination environments. Instead, the concentration of coloring ions Mn2+ and Fe2+ has been identified as the crucial factor. Controlling the concentrations is the key scientific direction for improving the colors of these yellow-toned spessartite garnets. conclusion: The color differences between yellow-shaded spessartite garnets from Nigeria do not arise from differences in the types of coloring elements, their valance states, or coordination environments. Instead, the concentration of coloring ions Mn2+and Fe2+ is identified as the crucial factor. Controlling the concentrations is the key scientific direction for improving the colors of these yellow-toned spessartite garnets.