Gemmological Characteristic of Hydrothermal Synthetic Paraíba-Colour Beryl
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摘要: 近期国际珠宝市场中出现一种Tairus水热法合成绿柱石，其颜色与含Cu、Mn的绿蓝色锂电气石(商业名称Paraíba碧玺)较为接近。采用常规宝石学测试手段，并结合电子探针、傅里叶变换红外光谱、紫外-可见吸收光谱等分析测试方法，就水热法合成Paraíba色绿柱石的化学成分、谱学特征及颜色成因等问题展开研究。结果表明，水热法合成Paraíba色绿柱石的颜色主波长为487.7～490.2 nm，明度为17.1%～31.5%，彩度为47.8%～93.7%，折射率、双折射率和密度值均比天然绿柱石略高，内部具典型的微波状、阶梯状、交叉状、紊乱状及近平行生长纹理；化学成分以贫碱富铜、铁含镍为特征；六方环结构通道中Ⅰ型水分子和Ⅱ型水分子并存，其合频振动致特征近红外吸收谱带分别位于5 450、5 127、5 270 cm-1处；此外还有一定量的矿化剂组分存在。最后对水热法合成Paraíba色绿柱石的呈色机理一并给予了探讨。Abstract: The appearance of a kind of synthetic beryl introduced into the market by Tairus Created Gems recently, is very similar to copper and manganese-bearing greenish blue elbaite tourmaline (referred to as “Paraíba” in the trade). Thus, the chemical compositions, spectrum characteristics and colouring mechanism of hydrothermal synthetic Paraíba-colour beryl samples were researched by using the conventional gemmological methods and modern analytical techniques including electron probe micro-analyzer (EPMA), Fourier transform infrared spectroscopy (FTIR) and ultraviolet-visible absorption spectrometry (UV-Vis). The results showed that the dominant wavelength, brightness and saturation (CIE 1931) of hydrothermal synthetic Paraíba-colour beryl samples are 487.7-490.2 nm, 17.1%-31.5% and 47.8%-93.7% respectively. The refractive index and birefringence are 1.584-1.589 and 0.005, and density is 2.771-2.789 g/cm3, which are slightly higher than natural beryl. The gemmological microscope investigations revealed that hydrothermal synthetic Paraíba-colour beryl exhibits typical growth patterns such as micro-wave, stair-step, cross, disorder and subparallel ones. EPMA analysis showed that the hydrothermal synthetic Paraíba-colour beryl is characterized by poor alkali (the content of K2O and Na2O content is generally below 0.2%), rich copper (the content of CuO is higher than 5.00%) and iron (the content of FeOT is 1.76%-3.00%), and nickel-bearing (the content of NiO is 0.03%-0.15%). The FT-MIR spectra defined the vibration areas of silica tetrahedron group in six-membered rings. The symmetry and asymmetry stretching modes of O—Si—O and Si—O—Si mainly appear at 970, 1 246 cm-1 respectively. The FT-NIR analysis indicated that Type-Ⅰ and Type-Ⅱ water molecules are coexisting within the ring channel of hydrothermal synthetic Paraíba-colour beryl, and the absorption bands relating to the combination modes of type-Ⅰ and type-Ⅱ H2O mainly appear at 5 450, 5 127 and 5 270 cm-1. Besides, a certain amount of mineralizing agent used to synthetize beryl hydrothermally locates within the ring channel, and absorption bands appearing at 2 736 cm-1 and 2 450 cm-1 relating to chloridion are considered to be one of the identification characteristics of hydrothermal synthetic Paraíba-colour beryl. The UV-Vis spectra and EPMA results showed that the Paraíba-colour of hydrothermal synthetic beryl is mainly attributed to Cu2+, Fe2+, Fe3+ and Ni2+ in the crystal structure. The Fe3+ with 6A1g→4Eg(4D) and 6A1g→4Eg+4A1g(4D) d—d electron transition absorption bands appear at 370 nm and 426 nm. The Ni2+ with 3A2g→3T1g(3P) d—d electron transition absorption band appears at 460 nm. A combination of the Cu2+ with 2E→2T2(2D), the Fe3+ with 6A1g→4T2g(4G), the Fe2+ with 5T2g→5Eg(5D) d—d electron transition and Fe2+—Fe3+ charge transfer absorption bands result in the strong absorption between 613-638 nm and 1 500 nm.
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Keywords:
- synthetic beryl /
- hydrothermal method /
- FTIR /
- colouring mechanism
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