The Study On Gemmological And Chromatic Characteristics Of Blue-green Apatites

Apatite is a common accessory rock mineral,which has been deeply studied in petrology and geochemistry,also a semi gem with rich color.In this paper,74 pieces of blue to greenblue apatites were selected to study their gemmological characteristics through basic gemmological observation,composition test(XRF and LIBS),vibration spectroscopy(Infrared and Raman)and Ultraviolet-visible spectrum analysis.The relationship between color and composition,structure of apatite was established through color quantization and cluster analysis,and the quantitative classification was established based on Munsell.All the samples are fluorapatite.There are a lot of isomorphic substitution inside apatite.It is suggested that the weak double peaks of 1446cm^-1and 1476cm^-1in infrared sepectrum,the double peaks of 1500cm^-1and 1520cm^-1in Raman spectrum can be seen as the weak indication of CO₃^2-in apatite,and the weak absorption of 518cm^-1in infrared and the combination of 1007cm^-1in Raman can be seen as the substitution indication of SO₄^2-for PO₄^3-.There are three main causes of apatite color:Rare earth ions,Rransition metal ions and color centers(mainly F-type ones).The color quantization showed that L*?[41.91,65],C*?[19.86,35.96]and h⁰?[192.9°,224.5°].The hue angle of the sample was close to the center of blue-green.The samples were divided into 5 groups by cluster analysis.The color parameters of apatite are more affected by the composition than the structure.Rare earth elements play an important role in the coloration of apatite,Pr³⁺has inhibitory effect on the lightness,the higher the total REE content,the more obvious the green tone of apatite color,Ce³⁺has inhibitory effect on the blue tone,Y³⁺can improve the chromaticity.According to Munsell's classification,the hue of apatite samples is blue-green-blue,and the lightness and chromaticity are divided into five levels.The chromaticity of the samples ranges from level 1 to level 4,and the lightness ranges from dark to bright.