黄绿色葡萄石的谱学特征及颜色成因

Spectral Characteristics and Color Genesis of Yellow-Green Prehnite

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-ICPMS)及穆斯堡尔谱(Mössbauer)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~585nm处的吸收带,分别与Fe^(3+)oct的电子跃迁和Fe^(2+)ch-Fe^(3+)oct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Fe^(3+)的IS均为0.34mm·s^(-1),QS值分别为0.22和0.35mm·s^(-1),均符合葡萄石八面体配位中Fe^(3+)的特征。样品LYG(IS=1.08mm·s^(-1))和样品DYG(IS=1.07mm·s^(-1))的Fe^(2+)具有相近的IS值,均符合八面体配位中Fe^(2+)的特征。然而,样品DYG中Fe^(2+)的QS值(QS=2.78mm·s^(-1))明显高于样品LYG(QS=1.12mm·s^(-1)),说明前者结构中Fe^(2+)处于畸变的八面体中。样品DYG结构中Fe的含量和Fe^(2+)的比例(4.02wt%;11.88%)均高于样品LYG(3.55wt%;5.27%),表明黄绿色葡萄石的颜色可能与Fe的含量和Fe^(2+)的比例有关。样品LYG和样品DYG中V的含量分别为633和1810μg·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Fe^(3+)和Fe^(2+)。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(Fe^(3+)和Fe^(2+))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。

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~425and~585nm,which were related to the electron leap of Fe^(3+)octand the charge transfer of Fe^(2+)ch-Fe^(3+)oct,respectively.Ultrafine parameter analysis of Mössbauer shows that the IS of Fe^(3+)in both sample LYG and sample DYG is 0.34mm·s^(-1),and the QS values are 0.22and 0.35mm·s^(-1),respectively,both consistent with the characterization of Fe^(3+)in the octahedral ligand of prehnite.Sample LYG(IS=1.08 mm·s^(-1))and sample DYG(IS=1.07mm·s^(-1))have similar IS values for Fe^(2+),both of which are consistent with the characterization of Fe^(2+)in octahedral coordination.However,the QS value of Fe^(3+)in sample DYG(QS=2.78mm·s^(-1))is significantly higher than that of the light yellow-green sample LYG(QS=1.12mm·s^(-1)),suggesting that Fe^(2+)is in the distorted octahedron in the former structure.The content of Fe and the proportion of Fe^(2+)in sample DYG(4.04 wt%;11.88%)are higher than those of the light yellow-green sample LYG(3.55wt%;5.27%),suggesting that the yellow-green color of prehnite may be related to the content of Fe and the proportion of Fe^(2+).The contents of V in sample LYG and the dark yellow-green sample DYG are 633and 1810μg·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 Fe^(3+)and Fe^(2+).The present study not only identified the main chromogenic ion species(Fe)of yellow-green prehnite,but also determined the valence states(Fe^(3+)and Fe^(2+))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.