The impact of mineral composition and trace metal cations on the body color of South African Sugilite Jade.

Abstract

Sugilite jade is an emerging jade material in the jewelry market with various shades of purple in appearance, and there is currently a gap in research on the blue and pink hues it possesses. The mineralogical and colorimetric characteristics of South African sugilite jade were studied using polarized light microscopy, X-ray fluorescence, X-ray powder diffraction, electron microprobe, infrared spectroscopy, and UV-Vis spectroscopy to explore the causes of its color. The primary mineralogical component of sugilite jade is sugilite, with minor constituents including pectolite-serandite, aegirine, alkaline amphibole, and alkaline feldspar. The purple hue of sugilite jade is attributed to the presence of Mn³⁺ on the A site within the crystal structure of sugilite. It has been observed that the lightness and chroma of this color are positively correlated with the Mn content. The blue color is caused by Cu²⁺ in pectolite and the pink color is attributed to Mn²⁺ in serandite. The color of sugilite is primarily influenced by its broad absorption peak in the UV-visible spectrum, which occurs between 500 and 700 nm. The intensity of this peak is directly proportional to the concentration of Mn³⁺, which determines the lightness of sugilite jade. The presence of Cu²⁺ results in a shift of the absorption peak to a higher wavelength, imparting a bluer color to the mineral.