Gemological Properties And Color Mechanism Of Green Kyanite

Kyanite,a silicate mineral,is less common in gem-quality kyanite,but is widely used in the chemical industry.Gem-grade kyanite is dominated by blue kyanite,since the gem-quality green kyanite produced in Brazil has been reported since 2001,there have been reports of gem-quality green kyanite from Kenya,Tanzania,and India.The yellowish green kyanite samples of this study are said to be produced in Nepal and Tanzania,respectively.The colors are light yellow green and dark yellow green.The conventional gemological characteristics,Spectroscopic characteristics,chemical composition and color mechanismof two different green kyanite were tested and analyzed by conventional gemological testing,electron probes,X ray powder diffraction,X-ray fluorescence spectroscopy,micro-Raman spectroscopy,Fourier infrared spectroscopy,ultraviolet-visible spectrophotometer,and X-ray photoelectron spectroscopy.There are some significant differences between the two type ofyellowish green kyanite in terms of rough appearance,gemological properties,and inclusion composition.In terms of chemical composition,in addition to Si,O,Al,a large amount of Fe is present in the yellowish green kyanites.the Fe content of the deep yellowish green kyanite is 1.21 to 1.51%,which is higherthan the Fe content of the light yellowish green kyanite,which is 0.85 to 0.97%.The two different type of yellowish green cyanites have different Fe content.In the infrared and Raman spectra,both yellowish green kyanites conform to the standard kyanite spectrum.The deep yellow-green kyanite showed absorption of OH in the infrared 3700-3600 cm^-1,while the pale yellow-green did not absorb significantly,indicating that the OH was higher in the dark yellow-green.The main Raman peak of the deep yellow-green kyanite in the high-frequency region appears to shift to a high wavenumber.It is presumed that due to the increase of Fe content,the Fe-O octahedron in the kyanite crystal structure increases,makinga change in the vibration frequency of the Si-O non-bridge oxygen symmetric stretching.The UV-Vis spectra of the two yellowish green kyanites are similar,with absorption peaks at 378 nm,433 nm,and 445 nm and absorption bands centered at615 nm,all attributed to the transition of Fe³⁺.It was proved by X-ray photoelectron spectroscopy that Fe in green cyanite exists only in the form of Fe³⁺.Combined with the chemical composition test results of green kyanite,the reason why Fe³⁺makes yellowish green kyanite color is explained.In this paper,blue and yellowish green kyanite were heat-treated together.After heat treatment,the color of yellowish green kyanite did not change obviously,and the color of blue kyanite increased to light gray,indicating that it is not conducive to improving blue-blue under high temperature oxidation environment.Due to the limitation of pressure and atmospheric conditions,yellowish green cyanite can be tried under high temperature and high pressure and reducing conditions to explore the conversion relationship between yellowish green and blue kyanite.