Chromatographic Study Based On Blue Gemstones With Pleochrois

Sapphire,cordierite and apatite are three common blue similar gemstones with pleochroism,but there are still many differences in their color quality such as hue,lightness,chroma,etc.Subjective color evaluation can not accurately quantify the specific color difference of the three gemstones,It affects the cutting processing of raw stone and the color quality classification of finished gems.The quantification of these differences through colorimetric parameters can provide a scientific theoretical basis for the rough processing and color quality grading of sapphire,cordierite and apatite,and can also be applied to the identification of sapphire,cordierite and apatite varieties,and the crystal orientation of sapphire and apatite.In this paper,the absorption spectra,chromogenic elements and color quality of sapphire,cordierite and apatite were studied and analyzed using infrared spectrometers,Raman spectrometers,X-ray fluorescence spectrometers,UV-Vis-NIR spectrometers and USB2000+fiber optic spectrometers.Got the following result:The three gemstones have different infrared absorption spectra and Raman shift peaks.The infrared absorption spectrum of sapphire has the absorption peaks of 490cm^-1,510cm^-1,600cm^-1 and a series of continuous absorption bands between 700-870cm^-1generated by Al-O bending vibration and stretching vibration.The Raman spectra of sapphire have the characteristic Raman peaks of 413cm^-1 and 697cm^-1 attributed to the O-Al-O stretching vibration,and the Raman peaks of 639cm^-1 generated by the anti-symmetric stretching vibration of O-Al-O.The infrared absorption spectrum of cordierite has 760cm^-1 absorption peak generated by the six-element ring structure vibration of cordierite,480cm^-1 absorption peak attributed to the bending vibration of[Si O₄]tetrahedron,895cm^-1,960cm^-1 and1040cm^-1 absorption peaks generated by tetrahedral stretching vibration of[Al O₄]and1130cm^-1 and 1200cm^-1 absorption peaks generated by asymmetric stretching vibration of Si-O-Si.The Raman spectra of cordierite have significant displacement peaks in the range of 1200cm^-1-1900cm^-1,including 1260cm^-1,1360cm^-1,1450cm^-1,1600cm^-1 and1870cm^-1.The infrared absorption spectra of apatite have the absorption peaks of 580cm^-1and 610cm^-1 generated by[PO₄]^3-bending vibration and the absorption peaks of1050cm^-1 and 1100cm^-1 generated by[PO₄]^3-asymmetric stretching vibration.The Raman spectra of apatite have 450cm^-1 and 755cm^-1 Raman spectra attributed to[PO₄]^3-bending vibration and 970cm^-1 and 1090cm^-1 Raman displacement peaks attributed to[PO₄]^3-asymmetric stretching vibration.The color depth of the three gemstones is related to the content of color-causing element.There is a certain positive correlation between the color depth of the sapphire and the content of the color-causing element Fe;the color depth of the cordierite has a certain positive correlation with the content of the color-causing elements Fe and Mn;The color depth of apatite is positively correlated with the total amount of Fe and rare earth elements.Sapphire,cordierite and apatite can be divided into blue,blue-purple and blue-blue colors according to the color quality level.Apatite and cordierite have the best color quality in blue tones,and Sapphire has the best color quality in blue,blue-purple tones.Sapphire,cordierite and apatite all have the best color quality in the direction perpendicular to the C-axis,that is,perpendicular to the（1000）crystal plane.In this case,the lightness value L,chroma value C*and color purity are all the best.The intensity differences between the two directions of sapphire lightness and apatite lightness reaches 52%、14.61%,the intensity differences between the two directions of sapphire chroma and apatite chroma reaches 129.02%、41.37%,the intensity differences between the two directions of sapphire color purity and apatite color purity reaches 102.39%、35.27%.The intensity differences between the two directions of cordierite lightness reaches 28.00%.Sapphire and apatite show more blue tones in the vertical C-axis direction,with an intensity difference of 143.37%and 62.13%,while cordierite shows more blue and red tones in the vertical C-axis direction.The chromaticity parameters of sapphire and apatite,such as lightness value L and chromaticity value C*,gradually change with the measurement direction.The color quality of sapphire and apatite can be generally divided into four categories in four directions:vertical C axis,60°angle with C axis,30°angle with C axis,and parallel C axis.Sapphires are classified as bright and gorgeous,bright and colorful,a little bit bright and gorgeous,a little bit grayer;apatite are classified as bright and much gorgeous,medium bright and gorgeous,a little bit grayer and gorgeous,a little bit grayer and colorful.