绿色蓝晶石的宝石学特征及颜色成因分析

Spectroscopic Characteristics and Coloring Mechanism of Green Kyanite

蓝晶石化学式Al_(2)SiO_(5),属岛状硅酸盐,宝石级蓝晶石常见蓝色、绿色、黄色、橙色、褐色和无色。由于绿色蓝晶石的颜色成因存在争议,选取宝石级绿色蓝晶石,利用X射线荧光光谱(XRF)、中红外光谱(Mid FT-IR)、紫外-可见光谱(UV-Vis)和电子顺磁共振波谱(EPR)等相关谱学测试,对其致色离子配位与颜色成因进行分析。结果表明,蓝晶石样品中Fe^(2+)、Fe^(3+)和Cr^(3+)类质同象替代和离子电荷转移的方式广泛;Fe^(3+)和Cr^(3+)类质同象替代Al―O八面体,Fe^(3+)在可见光区引起432 nm和446 nm吸收峰,Cr^(3+)导致600 nm中心宽吸收带,形成530 nm附近的绿色光透过窗,而使样品呈绿色;在垂直于c轴方向,上述现象尤为明显,可使样品在该方向出现绿色-黄绿色多色性;平行于c轴方向上,Cr^(3+)与Al―O八面体位类质同象替代与蓝色调有关,过量的Fe^(2+)―Fe^(3+)电荷转移则会使蓝晶石带有黄色调。

Kyanite belongs to the family of nesosilicate minerals with the standard chemical formula of Al_(2)SiO_(5).Kyanite gems normally show various colors such as green, yellow, orange, brown, and colorless. Because of the controversial attribution of Cr^(3+)and Fe^(3+)in green kyanite, X-ray fluorescence(XRF), Fourier transform midinfrared(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 Fe^(3+), Fe^(2+), and Cr^(3+)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 Fe6^(3+)/Cr6^(3+) with the [AlO6] octahedron. This behavior can generate significant transmission around 530 nm, which finally produces a mixed green hue. The Fe6^(3+)and Cr6^(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 Cr^(3+)and [AlO6] octahedron;however, the yellow hue in green kyanite belongs to the excessive charge transfer of Fe^(2+)―Fe^(3+).