摘要
颜色是宝石重要的光学性质之一,也是影响宝石品质及价格的重要参数.黄绿色葡萄石作为一种常见的玉石品种,其结构、致色离子的种类、价态及配位的复杂性,导致前人关于其致色成因存在较大的争议.采用傅里叶红外吸收光谱(FTIR)、紫外-可见光分光光谱(UV-Vis)、激光剥蚀-等离子体质谱(LA-ICP-MS)及穆斯堡尔谱(Mossbauer)等现代分析测试手段,对2个典型的黄绿色葡萄石样品(浅黄绿色样品LYG和深黄绿色样品DYG)的主要致色离子种类、价态及配位状态进行了深入研究,为黄绿色葡萄石的致色成因提供科学的解释.UV-Vis分析结果表明,样品LYG和样品DYG均出现明显的～425和～585 nm处的吸收带,分别与Feoct3+的电子跃迁和Fech2+-Feoct3+的电荷转移有关.Mössbauer超精细参数分析结果显示,样品LYG和样品DYG中Fe3+的IS均为0.34 mm·s-1,QS值分别为0.22和0.35 mm·s-1,均符合葡萄石八面体配位中Fe3+的特征.样品LYG(IS=1.08 mm·s-1)和样品DYG(IS=1.07 mm·s-1)的Fe2+具有相近的IS值,均符合八面体配位中Fe2+的特征.然而,样品DYG中Fe2+的QS值(QS=2.78 mm·s-1)明显高于样品LYG(QS=1.12 mm·s-1),说明前者结构中Fe2+处于畸变的八面体中.样品DYG结构中Fe的含量和Fe2+的比例(4.02 wt％;11.88％)均高于样品LYG(3.55 wt％;5.27％),表明黄绿色葡萄石的颜色可能与Fe的含量和Fe2+的比例有关.样品LYG和样品DYG中V的含量分别为633和1 810μg·g-1,指示V可能对黄绿色葡萄石的颜色有贡献,但含量明显低于Fe,所以V对颜色的贡献远低于Fe3+和Fe2+.该研究不仅查明了黄绿色葡萄石的主要致色离子种类(Fe),而且确定了致色离子的价态(Fe3+和Fe2+)和配位状态(八面体配位),为黄绿色葡萄石颜色的定量研究提供了有益的基础素材.
Abstract
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.
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