Mineralogical and Spectral Characteristics of Turquoise and Its Common Imitations
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摘要:
绿松石在全世界有着悠久的历史，对于中国文化的起源与发展有着巨大影响。而到了现代，绿松石因其颜色、质地、历史寓意以及宗教韵味而在全世界风靡，绿松石的造假手段也在不断更新，绿松石市场的健康发展受到冲击。本研究选取来自矿区、厂家与可信渠道购得的绿松石及其相似品，通过这些样品研究天然绿松石样品与绿松石仿制品的矿物谱学特征，以期系统性地分析天然绿松石与绿松石仿制品的谱学特征差异，并为鉴定提供科学依据。本文主要通过常规宝石学测试、红外光谱、拉曼光谱、紫外-可见光谱与能谱分析等现代测试技术对天然绿松石及其仿制品样品进行系统的矿物谱学研究, 得出以下结论。
(1) 天然绿松石样品的密度2.45~2.75 g/cm3，折射率1.61~1.63，w(Al2O3)为33.84%~34.03%，w(P2O5)为29.97%~33.69%，w(Fe2O3)为2.60%~3.25%，w(CuO)为8.19%~9.13%。
(2) 结构水、结晶水与磷酸根基团的振动频率决定了天然绿松石的红外吸收光谱与拉曼光谱。其中, 由结构水的伸缩振动致红外吸收峰出现在3 508 cm-1和3 463 cm-1处，拉曼光谱谱峰则出现在3 496、3 471、3 448 cm-1处；弯曲振动致红外吸收峰出现在841 cm-1和784 cm-1处，拉曼光谱谱峰出现在812 cm-1处。结晶水的伸缩振动致红外吸收峰出现在3 287 cm-1和3 086 cm-1处，拉曼光谱谱峰出现在3 265 cm-1和3 069 cm-1处；弯曲振动致红外吸收峰出现在1 652 cm-1处，拉曼光谱出现在1 618 cm-1处。磷酸根基团的伸缩振动致红外吸收峰出现在1 112 cm-1和1 060 cm-1处，拉曼光谱出现在1 159、1 102、1 039、937 cm-1处；弯曲振动致红外吸收峰出现在648、569、483 cm-1处，拉曼光谱出现在643、589、547、470、414 cm-1处。天然绿松石的紫外-可见光谱发现有4个吸收峰，分别是位于267 nm处由O2--Fe3+的电荷转移所引起的吸收峰，位于430 nm处由Fe的水合离子中Fe3+的d—d电子跃迁所引起的吸收峰，位于670nm与799 nm由Cu的水合离子中Cu2+的d—d电子跃迁所引起的两个吸收峰。
(3) 绿松石仿制品品种繁多，市场常见的品种主要包括三水铝石、重晶石、菱镁矿、方解石及多种矿物混合物5大类，其与天然绿松石鉴别较为有效且快速的方法是红外吸收光谱。天然绿松石及其仿制品的鉴别可以通过3 600~3 000 cm-1和1 200~900 cm-1范围内的红外吸收峰与绿松石的红外光谱进行比较[1-2]，且紫外-可见光谱中是否有Fe的水合离子中Fe3+的d—d电子跃迁所引起的430 nm处的吸收峰[3-4]和化学成分中是否含有Cu和Fe这两种绿松石的致色元素作为辅助证据。
Abstract:This article mainly used modern testing techniques such as conventional gemmological tests, infrared spectrometer, Raman spectrometer, ultraviolet visible spectrometer, and energy spectrum analysis to systematically study the mineralogical and spectra characteristics of the turquoise and its imitations purchased from mining areas, manufacturers, and trusted channels. We have come to the following conclusion. The density of natural turquoise is between 2.45-2.75 g/cm3, and the refractive index is between 1.61-1.63. The vibration frequencies of structural water, crystalline water, and phosphate groups determine the infrared absorption spectra and Raman spectra of natural turquoise. The infrared spectrum tests show the IR absorption peaks around 3 508, 3 463, 3 287, 3 086, 1 652, 1 112, 1 060, 841, 784, 648, 569 cm-1 and 483 cm-1; Raman spectrum tests show the peaks around 3 496, 3 471, 3 448, 3 265, 3 069, 1 618, 1 159, 1 102, 1 039, 937, 812, 643, 589, 547, 470 cm-1 and 414 cm-1; The absorption peaks appear near 267, 430, 670 nm and 799 nm in the UV visible spectrum.
There are many varieties of turquoise's imitation products, commonly seen in the market mainly including trihydrate alumina, barite, magnesite, calcite and mineral mixtures. The most effective and rapid method for distinguishing them from natural turquoise is infrared absorption spectrum of turquoise in the range of 3 600-3 000 cm-1 and 1 200-900 cm-1. The presence of an absorption peak at 430 nm in the UV visible spectrum and the presence of Cu and Fe of colouring elements in turquoise can be used as auxiliary evidence.
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Keywords:
- turquoise /
- imitation /
- infrared spectroscopy /
- Raman spectroscopy
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图 2 五种绿松石仿制品的红外吸收光谱Figure 2. Infrared absorption spectra of five turquoise imitations -
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图 2 五种绿松石仿制品的红外吸收光谱
Figure 2. Infrared absorption spectra of five turquoise imitations
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