Spectral Characteristics and Color Genesis of Yellow-Green Prehnite
Color is one of the important optical properties of gemstones,and it is also an important parameter affecting their quality and price.As a common jade variety,the complex structure,color ions,valence states and coordination of yellow-green prehnite have led to great controversy about its color genesis.In this paper,the species,valence,and coordination state of the main chromogenic ions of two typical yellow-green prehnite samples(light yellow-green sample LYG and dark yellow-green sample DYG)were investigated using modern analytical methods such as Fourier infrared absorption spectroscopy(FTIR),ultraviolet-visible spectroscopy(UV-Vis),laser-exfoliation-plasma mass spectrometry(LA-ICP-MS)and Mössbauer spectroscopy,which provides a scientific explanation for the color genesis of yellow-green prehnite.UV-Vis analysis indicates that both sample LYG and sample DYG showed obvious absorption bands at～425 and～585 nm,which were related to the electron leap of Feoct3+and the charge transfer of Fech2+-Feoct3+,respectively.Ultrafine parameter analysis of Mössbauer shows that the IS of Fe3+in both sample LYG and sample DYG is 0.34 mm·s-1,and the QS values are 0.22 and 0.35 mm·s-1,respectively,both consistent with the characterization of Fe3+in the octahedral ligand of prehnite.Sample LYG(IS=1.08 mm·s-1)and sample DYG(IS=1.07 mm·s-1)have similar IS values for Fe2+,both of which are consistent with the characterization of Fe2+in octahedral coordination.However,the QS value of Fe3+in sample DYG(QS=2.78 mm·s-1)is significantly higher than that of the light yellow-green sample LYG(QS=1.12 mm·s-1),suggesting that Fe21 is in the distorted octahedron in the former structure.The content of Fe and the proportion of Fe2-in sample DYG(4.04 wt％;11.88％)are higher than those of the light yellow-green sample LYG(3.55 wt％;5.27％),suggesting that the yellow-green color of prehnite may be related to the content of Fe and the proportion of Fe2+.The contents of V in sample LYG and the dark yellow-green sample DYG are 633 and 1 810 μg·g-1,respectively,indicating that V may contribute to the yellow-green color of prehnite.Still,its content is significantly lower than Fe,so the contribution to the color is much lower than Fe3+and Fe2+.The present study not only identified the main chromogenic ion species(Fe)of yellow-green prehnite,but also determined the valence states(Fe3+and Fe2+)and coordination states(octahedral coordination)of the chromogenic ions,which provides a solid theoretical basis for the quantitative study of the color of yellow-green prehnite.
Ion speciesValenceCoordination stateChromogenic mechanism
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