水热法合成蓝绿色绿柱石的宝石学及光谱学特征

Gemological and Spectroscopy Characteristics of Synthetic Blue-Green Beryl by Hydrothermal Method

针对新出现在市场上的一种水热法合成蓝绿色绿柱石,运用LA-ICP-MS、红外光谱、拉曼光谱、紫外-可见光谱进行系统研究,旨在获得其宝石学及谱学特征,探讨颜色成因,为检测机构鉴定该合成宝石提供参考数据。结果表明,样品折射率为1.570~1.576,与天然绿柱石相近,内部含特征的水波纹状生长纹理,可作为主要鉴定特征之一。LA-ICP-MS分析表明,该合成绿柱石化学成分相对单一,主要致色元素为Cr和Ti,还含有微量的V,碱金属含量极低。紫外-可见光谱主要显示Cr的吸收峰,结合LA-ICP-MS测试,认为其蓝绿色调主要由Cr和Ti共同导致。其中绿色调主要由Cr致色,微量的V可能也对绿色调有所影响。钛则致紫色,与绿色叠加形成样品具有的蓝绿色调,具体的致色机理有待进一步研究。在2000~4000 cm^(-1)的红外光谱中,以3700 cm^(-1)为中心的宽吸收带吸收强烈,归属于两种类型通道水的基频振动及其耦合;2449,2615,2746,2813,2885和2983 cm^(-1)处吸收峰,均为Cl-引起;3108和3299 cm^(-1)的较强吸收峰由NH 4+所致。在4000~8000 cm^(-1)的近红外吸收光谱中,为合成绿柱石通道水的合频和倍频振动区。其中,Ⅰ型水的合频振动所致的5275 cm^(-1)处强吸收峰、伴随5106和5455 cm^(-1)处较强吸收峰,及Ⅰ型水倍频振动所致的7143 cm^(-1)强吸收峰,可作为样品是水热法合成绿柱石的重要鉴定特征,且对于鉴定较厚的刻面宝石尤为重要。天然绿柱石中相应的这两处吸收峰强度较弱甚至不存在。样品的拉曼光谱和标准绿柱石的拉曼光谱一致。685 cm^(-1)峰的半高宽为7.1~7.3 cm^(-1),小于8.5 cm^(-1),可作为水热法合成绿柱石的又一鉴定特征。

In this paper,for a new blue-green hydrothermal synthetic beryl on the market,the systematic research is conducted by using LA-ICP-MS,IR spectrum,Raman spectrum,UV-Vis absorption spectrum to obtain the gemology and spectroscopic characteristics and to analyze the causes of colour,and provide reference data for testing institutions to identify such synthetic gems.The results show that the sample’s refractive index is 1.570~1.576,which is similar to natural beryl.All sample have a characteristic water ripple growth texture inside,which can be used as one of the main identification characteristics of this synthetic beryl.LA-ICP-MS analysis showed that the synthetic beryl’s chemical composition is relatively single,the main chromogenic elements are Cr and Ti,and also contains trace amounts of V,and the alkali metal content is extremely low.The UV-Vis spectrum mainly shows the absorption peak of Cr,combined with the LA-ICP-MS test results,it is believed that Cr and Ti mainly cause the blue-green tone.The Cr mainly causes the green tone,and trace V may also affect the green tone.Titanium causes purple,superimposed with green to form the blue-green hue of the sample.The specific color mechanism needs to be further studied.In the infrared spectrum of 2000~4000 cm^(-1),the broad absorption band centered at 3700 cm^(-1) absorbs strongly,which belongs to the fundamental frequency vibration and coupling of two channel water types.Peaks at 2449,2615,2746,2813,2885,2983 cm^(-1) are caused by Cl-;The strong absorption peaks of 3108 and 3299 cm^(-1) are caused by NH 4+.The near-infrared absorption spectrum of 4000~8000 cm^(-1),it is the combined frequency and frequency doubling vibration area of the channel water in synthetic beryl.Among them,the strongest absorption peaks at 5275 cm^(-1),accompanied by the stronger absorption peaks at 5455 and 5106 cm^(-1) caused by the combined frequency vibration of type I water,and the strong absorption peak at 7143 cm^(-1) caused by frequency doubling vibration of type I water can be used as an important identification feature of hydrothermal synthetic beryl and is particularly important for the identification of thick faceted gemstones.The corresponding absorption peaks in natural beryl are weak or even absent.The Raman spectrum of the sample is the same as that of standard beryl.The half-height width of the 685 cm^(-1) peak is 7.1~7.3 cm^(-1),less than 8.5 cm^(-1),which can be used as another identification feature of this hydrothermal synthetic beryl.