Gemmological and Spectroscopic Characteristics of Synthetic Alexandrite by the Horizontally Oriented Crystallisation Method
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摘要:
目前市场上合成变石中以提拉法合成产品为主，水平定向结晶法(HOC法)合成变石产品相对少见，其宝石学与谱学特征的研究资料仍有待补充。本文选择8颗来自俄罗斯厂家的HOC法合成变石样品为研究对象，采用折射仪等常规宝石学仪器、激光剥蚀电感耦合等离子体质谱仪、能量型色散X射线荧光能谱仪、紫外-可见光光谱仪、荧光光谱仪、拉曼光谱仪和傅里叶变换红外光谱仪对该类样品进行测试分析，探究其包裹体特征，微量元素组成、红外光谱、紫外-可见光光谱和荧光光谱等特征及与天然变石的差异。放大观察结果显示，HOC法合成变石样品的典型内含物为大量定向拉长空洞以及簇状金属包裹体。化学成分测试显示含有致色元素Cr和V，Cr含量较天然变石偏高；Fe元素含量极低，Mg、Ti、Ga等元素含量较天然变石低，且具有异常高的Mo含量，推测为金属Mo坩埚残余所致。紫外-可见光光谱仪测试结果显示为典型Cr谱，在橙黄光区与紫光区有两个宽的吸收带，645、656 nm处具有肩峰，680 nm处具有弱吸收峰。三维荧光光谱显示HOC法合成变石具有678、680 nm处强荧光峰以及690、696 nm处弱荧光峰，均为Cr元素所致，不具有天然变石中Ti和O元素的配合物所致460~550 nm处的发光中心。红外吸收光谱显示不具有天然变石特有的2 160、2 402 cm-1处吸收峰以及3 000~3 500 cm-1范围内与水有关的吸收。
Abstract:Cumently, the market is dominated by synthetic alexandrite synthesized by the czochralski method, while the synthetic alexandrite synthesized by the Horizontally Oriented Crystallisation (HOC) method is relatively rare, and the study of its gemmological and spectroscopic characteristics still needs to be supplemented. In this paper, 8 synthetic alexandrite by HOC method from a Russian manufacturer were selected as the research objects and tested and analyzed using conventional gemmological instruments such as refractometers, laser-ablation inductively coupled plasma mass spectrometer (LA-ICP-MS), energy dispersive X-ray fluorescence spectrometer (EDXRF), UV-Vis spectrometer, fluorescence spectrometer, Raman spectrometer, and Fourier transform infrared (FTIR) spectrometer, to explore its inclusion characteristics, trace elements compositions, infrared spectrum, UV-Vis spectrum, fluorescence spectrum, and other characteristics to dirstinguish it from natural alexandrite. The magnified observation results show that the typical inclusions of the synthetic alexandrite samples by HOC method are a large number of oriented elongated voids and clustered metal inclusions. Chemical composition tests show the presence of the colour-causing elements Cr and V. The Cr content is higher than that of natural alexandrite. Fe element content is very low; the content of Mg, Ti, Ga and other elements are lower than the natural alexandrite, with unusual high Mo content, presumably due to the Mo crucible residue. UV-Vis spectrometer test results show a typical chromium spectrum, with two broad absorption bands in the orange-yellow and violet regions, shoulder peaks at 645 nm and 656 nm, and a weak absorption peak at 680 nm. Three-dimensional fluorescence test concluded that HOC method synthetic alexandrite has strong fluorescence peaks at 678 nm and 680 nm, and weak fluorescence peaks at 690 nm and 696 nm, which are all caused by Cr element, and does not have the luminescence center at 460-550 nm caused by Ti and O element in natural alexandrite. The infrared absorption spectrum shows that it does not have the absorption peaks at 2 160 cm-1 and 2 402 cm-1, which are unique to natural alexandrite, and there is no obvious water related absorption between 3 000-3 500 cm-1.
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图 1 HOC法合成变石样品: (a)led灯箱光源下；(b)钨丝灯光源下
Figure 1. Synthetic alexandrite samples by the HOC method: (a)led light source; (b) tungsten light source
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图 2 提拉法合成变石(上排)与天然变石(下排)样品：(a)led灯箱光源；(b)钨丝灯光源
Figure 2. Synthetic alexandrite samples by the czochralski method (top) and natural alexandrite samples (bottom): (a) led light source; (b) tungsten light source
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图 3 HOC法合成变石样品内部特征：(a-d)定向拉长空洞; (e-f)金属包裹体
Figure 3. Internal characteristics of synthetic alexandrite samples by the HOC method: (a-d)oriented elongated voids; (e-f)metal inclusions
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图 4 HOC法合成变石样品XRF测试结果
Figure 4. XRF test results of synthetic alexandrite samples by HOC method
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图 5 合成变石与天然变石样品的紫外-可见光光谱
Figure 5. UV-Vis spectra of synthetic and natural alexandrite samples
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图 6 HOC法合成变石和天然变石样品的荧光光谱: (a), (d)样品H-2;(b), (e)样品H-3;(c), (f)样品N-2 (图d, e, f分别为a, b, c的局部放大)
Figure 6. Fluorescence spectra of the HOC method synethetic alexandrite and natural alexandrite samples: (a), (d)sample H-2;(b), (e)sample H-3;(c), (f) sample N-2 (Fig.d, e, f are partial enlargements of a, b, c, respectively)
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图 7 HOC法合成变石和天然变石样品的光致发光光谱
Figure 7. The photoluminescence spectra of HOC method synthetic alexardrite and natural alexandrite samples
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图 8 HOC法合成变石和天然变石样品的红外透射光谱
Figure 8. Infrared transmission spectra of HOC method synthetic alexandrite and natural alexandrite samples
表 1 变石样品的基本宝石学性质
Table 1 Basic gemmological characteristics of alexandrite samples
特征 天然变石 HOC法合成变石 提拉法合成变石 颜色(Led灯箱光源下) 蓝绿色 带紫色调的蓝紫色 蓝紫色 颜色(钨丝灯光源下) 淡紫红色 紫红色 紫色 多色性 蓝绿-黄绿-紫红 蓝绿-黄绿-紫红 蓝绿-黄绿-紫红 相对密度 3.55~3.69 3.66~3.83 3.66~3.71 折射率 1.740 ~1.755 1.741 ~1.750 1.746 ~1.755 荧光 长波：弱红色 长波：强红色 长波：强红色 短波：惰性 短波：暗红色 短波：暗红色 
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表 2 合成和天然变石样品的LA-ICP-MS测试数据
Table 2 The test results of LA-ICP-MS of synthetic and natural alexandrite samples
/μg·g-1 样品号 Mg Ti Fe V Cr Ga Mo W H-1 - - - 517 3 441 - 8.33 0.339 H-2 11.4 34 36 610 4 567 - 12.80 0.480 H-3 - 27 46 483 1 962 0.19 10.80 0.620 H-4 - - - 554 4 442 - 7.93 0.357 H-5 - - - 535 3 465 - 10.78 0.358 N-1 90 468 3 033 796 1 048 221 - - N-2 120 480 2 022 736 1 303 185 0.36 0.028 N-3 162 480 3 655 790 1 595 180 0.30 - N-4 144 366 14 544 170 1 583 208 - 0.069 N-5 4.8 600 6 922 258 1 732 207 - 0.073 C-1 1.8 30 - 375 1 936 - 3.27 - C-2 90 29 - 342 2 338 - 0.74 - 注：-表示低于检出限。
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