Spectroscopic Characteristics and Coloring Mechanism of Green Kyanite

Xiaoxu Yan; Suwei Yue; Lü Xinyi

doi:10.3788/LOP202158.1516029

Abstract

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 band around 600 nm belong to the substitution of an

F e_{6}^{3 +}

/

C r_{6}^{3 +}

with the [AlO₆] octahedron. This behavior can generate significant transmission around 530 nm, which finally produces a mixed green hue. The

F e_{6}^{3 +}

and

C r_{6}^{3 +}

substitutions can be 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⁺.