Gemmological Characteristics of Green Opals from Brazil
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摘要: 近期国内珠宝市场上出现一种产自巴西的绿欧泊,该类绿欧泊表现出的宝石矿物学特征与传统的欧泊有着明显差异。采用常规宝石学测试手段,并结合电子探针、X射线粉末衍射、扫描电子显微镜、红外吸收光谱、紫外吸收光谱等测试分析方法,就其化学成分、物相、微结构、谱学特征及颜色成因等问题展开初步研究。结果表明,巴西绿欧泊的主要矿物组成为蛋白石,含不等量的α-方石英和α-鳞石英等次要矿物,整体呈不规则胶束状结构,局部微波纹理发育;该类欧泊主要由直径约10~30nm的SiO2球粒聚集而成;由H2O和M-OH倍频振动致近红外吸收谱带位于7 000cm-1处,由H2O合频振动致近红外吸收谱带分别位于5 200,5 612cm-1处,由M-OH与[SiO4]合频振动致近红外吸收谱带位于4 600~4 100cm-1范围内。笔者还对巴西绿欧泊的呈色机理一并给予了探讨。Abstract: The gemmological characteristics of green opals found in Brazil, are quite different from those of traditional opals, and are investigated by using conventional gemmological testing methods and modern analytical techniques including electron microprobe (EPMA), Xray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectrometer (FTIR) and ultraviolet-visible absorption spectrometer (UV-Vis).The chemical compositions, mineral phase, microstructure, spectrum characteristics and colouring mechanism are also preliminarily researched.The results show that refractive index of the green opal samples from Brazil is 1.42-1.45, and density is 2.10-2.28g/cm3.The polarizing microscope investigations reveal that the green opal samples from Brazil are of irregular micelle structure and microwave-like texture, which indicate the heterogeneity of mineral component and distribution.The XRD analyses show that the green opals from Brazil are mainly composed of amorphous SiO2, α-cristobalite andα-tridymite.The SEM images show random accumulation of SiO2 nanoparticles about 10-30nm in diameter, most of which are assembled to form aggregates with diameters from 100nm to 400nm.It is then inferred that the uniform size, irregular shape and disordered arrangement of SiO2 aggregates may result in unobvious play-of-color effect in green opals.The FT-MIR spectra characterize the vibration areas of Si-O groups.The asymmetry and symmetry stretching modes of Si-O mainly appear at 1 100, 795cm-1 respectively.The bending vibration mode of Si-O appears at 475cm-1. The FT-NIR spectra defines the vibration areas of different water species in green opals.The first overtone of H2O and M-OH stretching modes mainly appear at 7 000cm-1, and the H2O combination mode appears at 5 200, 5 612cm-1.The absorption bands relating to the combination modes of M-OH and[SiO4]are between 4 100cm-1 and 4 600cm-1, which indicate that the water in green opals may occur in the form of planar water and structural water.The UV-Vis spectra and EPMA results show that the colour of green opals is mainly attributed to Fe3+, Fe2+ and Cr3+, and part Cr3+ impure ions tend to combine with H2O into[Cr (H2O) 6]3+hydrated ions.The author further divides green opals form Brazil into Fe bearing ones and Cr bearing ones according to their UV-Vis characteristics and chemical compositions.The Fe3+ with 6A1 →4E (4D), 6A1 →4E+4A1 (4D) and 6A1 →4T2 (4G) d-delectron transition absorption bands in Fe bearing opals appear at 373, 450and 635nm.The Cr3+ with 4A2g →4T1g (4F), 4A2g →4T2g (4F) and 4A2g →2Eg (2G) d-delectron transition absorption bands in Cr bearing ones appear at 420, 608-620and 684nm respectively.
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Keywords:
- green opals /
- microstructure /
- FTIR /
- colouring mechanism /
- Brazil
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