Study On Gemological Properties And Color Mechanism Of Varicoloured Kyanite

Kyanite is a silicate mineral,which is widely used in chemical industry.In addition to blue,gem-quality kyanites are also common in blueish green,green,yellowish green and orange varieties in recent years.In this study,the conventional gemmological characteristics,Fourier infrared spectrometer,micro-Raman spectrometer,optical fiber spectrometer,ultraviolet-visible spectrophotometer,X-ray fluorescence spectrometer,electron probe microanalysis,LA-ICP-MS and other tests were carried out for kyanite samples in different colors.The chemical composition,spectroscopic characteristic and color characterization were analyzed.Color bands in blue kyanite are well developed,shown as light blue and dark blue flat bands parallel to the c-axis in the same sample.The color of green and orange kyanite is uniform.Different colors of Kyanite are bright glass luster,transparent to translucent.The refractive index ranges from 1.711 to 1.734,and the birefringence ranges from 0.015 to 0.018.The blue kyanite has strong trichromism,while the green and orange ones show weak polychromism.Under the Charles filter,most of the blue samples remain red,while most of the green samples turn red.Most of them are inert under ultraviolet fluorescence,and very few are weak red fluorescence.Cleavage lines,fluid inclusions and crystal inclusions can be seen in different colors of Kyanite.The inclusion type of blue kyanite is simple.In addition to the initial cleavage,semi euhedral colorless transparent crystal inclusions are occasionally found.The inclusions in green and orange kyanites are more abundant.Combined with Raman spectra,the transparent crystal inclusions in green kyanites are mainly crystal,and black tourmaline can also be seen.There are dark rutile minerals on the surface of the orange kyanite,and the transparent crystal inclusions in the orange kyanite are mainly zircon.Infrared and Raman spectra show that different colors of kyanite all have standard characteristic absorption peaks of kyanite.The absorption peaks belonging to OH^-1 vibration can be seen in the infrared spectra of 15 samples from 3600 to 3700 cm^-1.The main components of kyanite with different colors are Si O₂ and Al₂O₃,accounting for98～99%.Only a small amount of metal elements are substituted.Fe is the main trace element in all samples.The content of Fe in blue kyanite ranges from 200 ppm to 1000 ppm,and the highest is 1869.62 ppm.The content of Ti varies greatly.The absorption peak of UV spectrum indicates the existence of Fe³⁺and Fe²⁺-Ti⁴⁺,which is mainly caused by Fe²⁺-Ti⁴⁺ion pair and the electronic transition between Fe²⁺and Fe³⁺.The content of Fe in green kyanite ranges from 4500 ppm to14000 ppm.The absorption peak of its ultraviolet spectrum indicates the electronic transition related to Fe³⁺.The electronic transition related to Fe³⁺makes kyanite yellowish green,while Cr³⁺makes kyanite blueish green.The different concentration ratio of the two elements will directly affect the color tone.In orange kyanite,the content of Fe is up to 12812.48ppm,Mn is 2492.17ppm and 2172.41ppm.The absorption peak of ultraviolet spectrum is caused by Fe³⁺and Mn³⁺,and the electronic transition on Mn³⁺d orbital makes kyanite orange.