Gemological Characteristic And Colorimetric Study Of Yellow To Orange Red Series Garnets

This research selected 69 garnet samples from yellow to orange red.Their color was measured and quantified by x-write SP62 based on CIE1976L*a*b*color space system,to analyze quantitative relationship between composition and the influence of different standard light source,background on garnet,its color was classified and evaluated.Electron probe microanalysis(EPMA)was used to determine the subspecies of garnets.The spectral characteristics of garnets were analyzed by means of modern testing methods such as infrared spectroscopy,UV-visible spectroscopy and Raman spectroscopy.The relationship between the spectral and color was studied directly.The garnets of this color series mainly contain spessartine,grossular,and grossular-andradite by means of EPMA.Its approximate unit cell parameters are calculated based on the components.With the increase of its unit cell parameters,the wave number of the infrared absorption peak position decreases.The presence or absence of the E peak can determine whether there is a Pyrope component,and the ratio of Grossular-andradite can be judged by the prediction formula of E peak's wavenumber.Spessartine has absorption peaks of Mn²⁺and Fe²⁺.Mn²⁺controls the yellow hue of spessartine and Fe²⁺controls its orange-red hue.The higher the Mn²⁺concentration,the more orange the color of spessartine.In the grossular,the main component Ca²⁺does not contribute to the UV-visible spectrum and the chromogenic ions are trace amounts of Mn²⁺,Fe²⁺,Fe³⁺.Its hue is mainly controlled by Fe²⁺,the higher content of Fe²⁺,the more reddish of the color,while the trace Mn²⁺has control over its color weakly.Standard light source D65 is best as an evaluation light source because of the large contribution of color difference,which can show the color of garnet exactly.Owing to the increase of lightness and chroma of the garnets and make them turn yellow,standard light source A can be the exhibition light.Standard light source CWF has a significant effect on increasing the hue angle,but to reduce the chroma.The Munsell non-color background will influence the lightness L*and chroma C*of garnet,and does not change its hue significantly.As the background gray level decreases,both lightness L*and chroma C*increase,and the degree of discrimination also increases.The white background helps to distinguish the color of the garnets.The standard gold background can enhance the Lrightness L*and chroma C*of the yellow to orange red garnets.The chroma increase is the most apparent under the G4(20K)background,which is suitable as the inlay material of the garnets of this color series.With Munsell N9 as background and D65 as light source,color evaluation by K-means cluster analysis and Fisher discriminant analysis methods,orange garnets can be classified into 5 levels:fancy intense orange,fancy deep orange,fancy orange,fancy light orange,fancy dark orange.Orange Red garnets can be classified into 3levels:fancy intense orange red,fancy orange red,dark orange red.