Prase Opal from Tanzania
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摘要: 坦桑尼亚绿色蛋白石是最近出现在市场上的宝石品种，其鲜艳的苹果绿色引起大家的广泛关注。采用常规的宝石学研究方法对坦桑尼亚绿色蛋白石样品的宝石学特征进行测试分析，运用电子探针（EPMA）、X射线粉末衍射（XRD）、X射线荧光光谱（XRF）、红外光谱（IR）及紫外－可见－近红外吸收光谱（UV－Vis－NIR）等测试分析方法对坦桑尼亚绿色蛋白石样品的矿物组成、微量元素、振动光谱及呈色机理等问题进行了研究。结果表明，坦桑尼亚绿色蛋白石的主要矿物组成为蛋白石，并含有一定量的α－方石英和α－鳞石英。绿色蛋白石的褐黄色围岩风化较严重，主要由蛋白石、针铁矿组成，含有一定量的α－方石英与α－鳞石英，以及少量的磁铁矿、铬铁矿、硬锰矿。X射线荧光光谱结果表明，坦桑尼亚绿色蛋白石的主要元素为Si，过渡族金属元素主要为Ni和Fe，还有少量的Cr、Cu等元素；围岩主要含有Si，Fe元素，其较蛋白石样品Fe、Mg、Al、Cr、Ca等元素质量分数较高，而Ni、Si等元素稍低。样品测试显示为蛋白石红外光谱，区别于玉髓的红外光谱。紫外－可见－近红外吸收光谱表明，坦桑尼亚绿色蛋白石主要在400，650nm处有吸收带，在近红外区域可测到其各种形式的水的吸收谱带。绿色蛋白石的致色元素主要为Ni和Fe，Ni是其产生绿色的主要原因，而Fe的存在使其颜色略带黄色调。Abstract: Recently there appears a new gem from Tanzania in the market,which is called prase opal.The colour of prase opal is bright apple-green and it rises people's concern.This paper mainly analyzes the gemmological and mineral characteristics of prase opal form Tanzania.With the help of electron microprobe analysis (EPMA), X-ray power diffraction(XRD) X-ray fluorescence spectrometer (XRF), infrared spectroscopy and ultraviolet-visible-near infrared spectroscopy, the mineral components, trace elements, vibrational spectrum and colour mechanism of prase opal are analyzed, The results indicate that the main mineral components of prase opal are amorphous silica and a certain amount of low-tridymite, and low-cristobalite. The host rock of prase opal consists of amorphous silica, goethite and low-tridymite, low-cristobalite, with also a small number of magnetite, chromite and psilomelane distributing sporadically. Moreover the host rock has been weathered seriously,which shows earthy luster. X-ray fluorescence spectrometer results indicate that the main element of prase opal is Si, and there are also some transition elements such as Ni and Fe with also a bit of Cr and Cu. Because of the existence of water, the loss on ignition of prase opal are approximate 6%. It also shows that the main elements of host rock are Si and Fe with some amount of Mg,Al, Cr, Ca and Ni,which are different from those of the prase opal. Compared with the element contents of prase opal, the host rock contains more Fe, Mg, Al, Ca, Cr and less Ni. Moreover, the LOI of host rock is more, about 7%. The infrared spectrum of the prase opal proves that the samples are opal not chalcedony again in spite of partly weak micro-fibrous phenomenon observed by optical microscope. UV-Vis-NIR spectrum shows broad bands at 400,650 nm in the visible region. It also shows some bands which caused by various existent form of water. Prase opal contains trace elements including Ni and Fe. The octahedrally coordinated divalent nickel cause three clearly typical d-d bands, and two bands which from 3A2g(F) → 3T1g(F)(389 nm)and 3A2g(F) → 3T1g(P) (654 nm)are responsible for the green colour of the prase opal. Moreover, the presence of Fe3+ modify the colour to be more yellowish green.
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Keywords:
- prase opal /
- colouring mechanism /
- Tanzania
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