Spectroscopic Characteristics and Coloring Mechanism of Green Kyanite

Kyanite belongs to the family of nesosilicate minerals with the standard chemical formula of Al2SiO5. Kyanite gems normally show various colors such as green, yellow, orange, brown, and colorless. Because of the controversial attribution of Cr3+ and Fe3+ in green kyanite, X-ray fluorescence (XRF), Fourier transform mid-infrared (Mid FT-IR), ultraviolet-visible light (UV-Vis), and electron paramagnetic resonance (EPR) spectroscopic analyses of green kyanite samples were conducted to understand the trace elements and coloring mechanism. Results showed that Fe3+ , Fe2+ , and Cr3+ in the samples widely exhibit isomorphism and charge transfer forms. Absorption peaks at 432 nm and 446 nm and a wide absorption hand around 600 nm belong to the substitution of an Fe-6(3-) /Cr-6(3-) with the [AlO6] octahedron. This behavior can generate significant transmission around 530 nm, which finally produces a mixed green hue. The Fe-6(3-) /Cr-6(3-) substitutions can he improved in a direction perpendicular to the c axis and lead to polychromatism of green to yellowish green. In a direction parallel to the c axis, the appearance of a blue hue is associated with the Cr3+ and [AlO6] octahedron; however, the yellow hue in green kyanite belongs to the excessive charge transfer of Fe2+-Fe3+.