山东类达碧兹蓝宝石的宝石学特征及其热处理研究

Gemmological Characteristic and Heat Treatment of Trapiche-Like Sapphire from Shandong Province

  • 摘要: 山东昌乐蓝宝石因产量高、结晶颗粒大、晶形完整、品种繁多一直受到广泛关注1-2,但关于其特殊品种类达碧兹蓝宝石的研究较少,现象成因仍没有定论3-4。此外,山东蓝宝石颜色偏深,透明度较差,价格较低,如何热处理改色提高其美学、市场价值一直是研究的热点5-6。目前前人的研究对于加热过程中蓝宝石的包裹体、谱学特征的变化缺乏完整、系统地记录与总结。因此,本研究利用常规宝石学仪器、拉曼光谱仪、红外光谱仪、激光剥蚀等离子体质谱仪、扫描电镜、X射线能谱仪、显微紫外-可见-近红外吸收光谱仪以及高温管式炉对山东类达碧兹蓝宝石样品进行了宝石学特征及其热处理改色等方面的研究工作。山东类达碧兹蓝宝石原石样品的表面通常覆盖一层黑色的被膜1,常有明显的熔蚀凹坑,垂直c轴的切面上具有六方环状色带以及“达碧兹”现象,即一个蓝褐-蓝黑色的六边形“核”以及六条银色纤维状的“臂”(图 1)。“臂”和扇形区域的矿物组成均为刚玉,因而属于类达碧兹蓝宝石,且其内部包裹体丰富,“核”“臂”及扇形区域的包裹体各不相同。深色的“核”主要是大量褐色、黑色不规则片状、针状或点状的包裹体密集分布所致,“臂”由大量平行于其延伸方向的薄膜流体包裹体、微小裂隙以及愈合裂隙组成7。化学成分结果表明,该类山东蓝宝石样品是典型的玄武岩型成因8-11,“核”区域中Fe和Ti元素含量较其他区域更高,色带与母岩浆中Fe和Ti元素含量周期性变化有关,其微量元素特征较容易区别于世界上大部分其他产地玄武岩型蓝宝石;红外光谱结果表明,山东类达碧兹蓝宝石样品均有-OH振动吸收峰(3 696、3 624、3 309 cm-1),表明其未经过热处理;显微紫外-可见-近红外吸收光谱表明,山东类达碧兹蓝宝石样品中Fe2+-Ti4+对与蓝色深浅变化直接相关,而Fe3+的致色能力较弱,这些致色离子几乎在所有可见光范围内都有选择性吸收,总吸收量大导致山东蓝宝石颜色发暗。热处理实验在氧化环境下进行,设置温度范围为1 000~1 600 ℃,温度间隔为100 ℃。该实验可以使蓝宝石样品的颜色变浅,但伴随着灰色调,同时类达碧兹现象、色带随温度的升高也逐渐减弱。包裹体的颜色和种类、红外透射光谱和红外反射光谱也在不同温度下发生了相应的变化。因此,热处理(1 000 ℃及以上)山东达碧兹蓝宝石样品的鉴定可以从内、外部特征及光谱学特征来入手。

     

    Abstract: The sapphire from Changle, Shandong Province have received widespread attention for the high yield, large crystalline grains, complete crystal shapes, and diverse varieties1-2. However, there are fewer studies on the trapiche-like sapphire, a special variety, and the cause of its phenomenon is still inconclusive3-4. In addition, The value of the sapphire from Shandong Province is inexpensive because of the dark colour and poor transparency. The research hotspot on improving the colour and market value of the sapphire from Shandong Province has been discussed. However, the features of inclusions and spectral characteristics of sapphire from Shandong Province during the heating process were not thoroughly documented or summarized in the earlier studies5-6. Thus, in this study, the gemmological characteristics and heat treatment of trapiche-like sapphires from Changle, Shandong Province were studied using standard gemmological instruments, Raman spectroscopy, infrared spectroscopy, laser ablation-inductively coupled plasma-mass spectrometry, scanning electron microscopy-energy dispersive spectrometer, ultraviolet visible near-infrared spectroscopy and high-temperature tube furnaces.The surface of trapiche-like sapphire samples from Shandong Province typically have visible erosion pits and a black film1. There is a hexagonal colour zoning and trapiche phenomenon on the cross section perpendicular to the c-axis, that is a combination of the blue-black hexagonal "core" and six silvery fibrous "arms"(Fig. 1). The mineral componets of the "arm" and fan-shaped areas is corundum, indicating that it should be trapiche-like sapphire with many kinds of different inclusions. The dark "core" is mainly caused by a large number of brown, black irregular flake-shape, needle-shape or point-shape inclusions that are densely dispersed. The "arms" is composed of a significant amount of thin-film fluid inclusions, fine fissures, and healing fissures that are parallel to the direction of the extension7.The results of chemical composition show that the sapphire samples from Shandong Province are typical basalt-type genesis8-11. The contents of Fe and Ti elements is the highest in the "core" area. Colour bands are related to the cyclic fluctuations in the elemental composition of Fe and Ti in the primary magma. It is easier to distinguish the trace element characteristics between sapphire samples from Shandong Province and the sapphires with mostly various basalt origins in the world. The IR spectra of the trapiche-like sapphire samples from Shandong Province all have OH-related infrared absorption peaks at 3 696, 3 624, 3 309 cm-1, indicating that they had not undergone heat treatment. The micro-UV-Vis-NIR absorption spectra show that Fe2+-Ti4+ pair is closely linked to the shift in blue hue, and Fe3+ is a weak chromophore. In short, sapphire samples from Shandong Province have deeper colour because of the selective absorption of these chromogenic ions in practically all visible light ranges, as well as their high total absorption.The temperature range of the heat treatment in oxidation circumstances is 1 000-1 600℃, with the temperature step of 100℃. That experiment can make the colour of sapphire samples lighter with producing gray tones. Additionally, as the temperature increased, the trapiche-like phenomenon and colour band progressively faded. The colour and types of inclusions, as well as variations in infrared transmission and reflection spectra, have changed at different temperatures. Therefore, the identification of heated sapphire (heating to 1 000 ℃ or above) can be identified by internal and external characteristics as well as spectroscopic features.

     

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