Gemmological and Spectroscopic Characteristics of Natural and Synthetic Fluorites
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摘要:
萤石既是一种重要的工业矿物，也是珠宝、矿物晶体市场的重要品种。近年来，随着宝石首饰市场需求的多样化，萤石也被广泛用于制作刻面宝石、手串、手镯等珠宝饰品。着手于天然及合成萤石的鉴定需求，本文对天然萤石及合成萤石样品进行显微镜观察、飞秒激光剥蚀等离子体质谱(fs-LA-ICP-MS)、X射线荧光光谱(XRF)、紫外-可见吸收光谱(UV-Vis)、红外光谱(FTIR)及光致发光光谱(PL)测试，获取了不同样品的包裹体、光学性质、化学成分及谱学特征。结果表明：(1)合成萤石中无明显包裹体，而天然萤石中有大量气液包裹体；(2)合成紫色及无色萤石中掺杂的Nd及Eu元素的质量分数分别达775.13×10-6~832.63×10-6和15 173.88×10-6~16 674.97×10-6，显著高于天然萤石，且可以被XRF检出而易于鉴别；(3)合成萤石的紫外-可见光谱中可以显著观察到由掺杂的稀土元素导致的吸收峰和荧光峰，而天然萤石的紫外-可见光谱并无明显的吸收峰或荧光峰；(4)红外光谱显示，天然萤石样品中有大量的CO32-、CO2、羟基和水相关的振动峰，而合成萤石则没有，是区分萤石是否天然的重要特征；(5)光致发光光谱则表明，合成萤石因各稀土元素含量均较天然萤石高，不同价态的多种稀土元素均会产生荧光峰，导致其光谱较天然萤石复杂，且合成无色萤石掺杂大量Eu, Eu跃迁峰强度显著高于天然萤石，可用于对萤石天然性的辅助鉴别。
Abstract:Fluorite is not only an important industrial mineral, but also a major species in the jewelry and mineral collection market. In recent years, with the diversification of demand in the gemstone jewelry market, fluorite has been widely used to make faceted gemstones, beads, bracelets, and other forms of jewelry. Fluorite is a common and easy to be synthesized material. Synthetic fluorite sometimes appears in the mineral collections and gem markets. Based on the identification demand of natural and synthetic fluorite, through microscopic observation, femtosecond laser ablation plasma mass spectrometer (fs-LA-ICP-MS), X-ray fluorescence spectrometer (XRF), ultraviolet-visible absorption spectrometer (UV-Vis), infrared spectrometer (FTIR), and photoluminescence spectrometer (PL), inclusion, optical properties, chemical compositions, and spectroscopy characteristics of natural fluorite and synthetic fluorite samples were obtained. Microscopic observation shows that there are no obvious inclusions in synthetic fluorite samples, while natural fluorite samples contain a large amount of gas-liquid inclusions. By fs-LA-ICP-MS, the Nd and Eu contents in synthetic violet fluorite and colourless fluorite samples reach 775.13×10-6-832.63×10-6 and 15 173.88×10-6-16 674.97×10-6, respectively, significantly higher than those in natural fluorite samples, such amount of Nd and Eu can be detected by XRF and makes it easy for identification. In UV-Vis spectra, synthetic fluorite samples shows obvious absorption and fluorescence peaks caused by doped REE, while natural fluorite samples show nothing obvious in their spectra. FTIR shows a large number of vibration peaks related to CO32-, CO2, hydroxyl groups, and water in natural fluorite samples, while synthetic fluorite samples do not, which is a useful character to distinguish whether fluorite is natural or not. The PL spectrum indicates that synthetic fluorite has a higher content of various REE than that in natural fluorite, and multiple REE in different valence states produce different fluorescence peaks, resulting in a more complex PL spectrum than that in natural fluorite. Moreover, the Eu2+ transition peak intensity of synthetic colorless fluorite samples doped with a large amount of Eu is significantly higher than that of natural fluorite, which can be used to assist in the identification of fluorite's naturalness.
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Keywords:
- synthetic fluorite /
- REE doping /
- fs-LA-ICP-MS /
- UV-Vis /
- FTIR /
- PL
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图 1 自然光下及长波紫外线(365 nm)下萤石样品的荧光特征
Figure 1. Fluorite samples and their fluorescence characteristics under long wave (365 nm) ultraviolet light
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图 2 天然萤石样品FPB和样品FB中大量的气液两相包裹体
Figure 2. A large number of gas-liquid inclusions in natural fluorite samples FPB and FB
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图 4 天然和合成萤石样品的紫外-可见吸收光谱
Figure 4. UV-Vis spectra of natural and synthetic fluorite samples
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图 5 天然和合成萤石样品的红外光谱
Figure 5. Infrared spectra of natural and synthetic fluorite samples
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图 6 萤石样品的光致发光光谱：(a)样品SFPA; (b)样品SFA; (c)样品FPA; (d)样品FA
Figure 6. PL spectra of fluorite samples: (a)sample SFPA; (b)sample SFA; (c)sample FPA; (d)sample FA
表 1 天然和合成萤石样品的常规宝石学特征
Table 1 Conventional gemmological characteristics of natural and synthetic fluorite samples
样品编号 颜色 重量/ct 折射率 相对密度 长波紫外光下荧光 短波紫外光下荧光 消光现象 SFPB 紫色 12.20 1.434 3.23 弱蓝紫色 无 全暗 SFB 无色 12.11 1.432 3.20 强蓝白色 强蓝白色 全暗 FPB 紫色 17.35 1.432 3.22 弱蓝紫色 无 异常消光 FB 无色 15.72 1.431 3.22 弱白色 无 异常消光 
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表 2 合成萤石和天然萤石样品的微量元素含量
Table 2 Contents of trace elements of synthetic fluorite samples and natural fluorite samples
×10-6 样品编号 Na Mg Al Si K Ga Ge As Sr La Ce Pr Nd Sm Eu SFA-1 413.79 61.72 17.48 1 571.08 16.15 - - - 107.68 3.86 6.67 0.83 4.08 0.93 829.39 SFA-2 106.74 27.93 5.77 1 229.87 - 0.01 - 0.71 105.58 3.77 6.61 0.78 3.98 0.90 799.84 SFA-3 72.36 33.53 6.62 1 132.80 0.99 0.07 0.69 - 101.78 3.69 6.08 0.79 4.02 0.94 818.13 SFA-4 53.49 20.84 9.69 1 222.98 - - 0.01 - 105.97 3.71 6.47 0.80 3.84 0.93 832.63 SFA-5 67.79 14.29 4.48 1 104.69 - 0.04 0.05 0.47 99.91 3.48 6.17 0.78 3.79 0.97 831.88 SFA-6 470.29 110.29 33.58 2 812.78 9.96 0.03 - 0.38 100.41 3.51 5.92 0.75 3.79 0.93 800.87 SFA-7 476.05 109.16 33.54 2 195.70 21.49 - 0.76 - 95.43 3.50 6.16 0.73 3.73 1.05 794.68 SFA-8 322.23 83.07 26.75 1 856.62 13.23 - 0.10 - 94.58 3.31 5.83 0.76 3.72 1.05 797.28 SFA-9 440.17 110.77 30.65 2 379.68 12.94 0.05 - 0.65 102.26 3.50 5.95 0.78 3.59 1.03 775.13 SFA-10 762.84 186.29 48.09 3 264.27 16.46 0.04 - - 97.18 3.59 6.10 0.77 3.80 0.92 784.13 SFPA-1 - 4.19 0.90 895.46 - 48.99 113.45 75.78 133.30 7.56 13.69 2.49 15 461.72 1.21 0.41 SFPA-2 - 3.84 - 781.46 - 49.71 114.32 71.71 135.73 7.16 13.32 2.61 16 221.28 1.13 0.31 SFPA-3 8.67 2.32 - 401.44 - 47.57 106.81 74.14 133.09 6.80 13.14 2.55 15 173.88 1.11 2.19 SFPA-4 - 3.66 0.10 564.06 - 48.94 113.72 76.00 134.08 6.79 13.06 2.49 15 703.65 1.16 0.31 SFPA-5 - 3.02 - 229.56 - 46.58 113.61 75.06 132.12 7.21 13.79 2.48 15 756.14 1.03 0.36 SFPA-6 21.30 3.10 0.06 892.70 9.84 47.93 103.53 66.58 130.00 7.14 13.46 2.42 16 526.04 1.14 0.33 SFPA-7 - 1.54 - 527.29 - 48.53 103.77 72.44 128.96 7.36 13.53 2.51 16 211.71 1.26 0.30 SFPA-8 20.34 3.07 - 456.11 - 48.35 108.17 69.92 127.80 7.07 13.34 2.59 16 438.53 1.06 0.31 SFPA-9 - 2.69 1.67 392.68 - 49.98 115.50 72.08 131.80 7.11 13.42 2.57 16 674.97 1.31 0.28 SFPA-10 - 3.02 - 327.00 - 49.39 106.73 72.77 127.03 6.70 12.67 2.57 15 405.02 1.14 0.31 FA-1 321.54 0.62 3.90 465.82 9.35 0.06 - 0.64 7.83 0.02 0.01 - 0.01 0.01 - FA-2 - 2.29 - 387.39 2.57 - - 1.52 8.49 0.21 0.12 0.03 0.22 0.10 0.03 FA-3 22.66 - 4.30 266.35 5.55 0.02 - 0.13 7.91 0.11 0.08 0.02 0.16 0.07 0.02 FA-4 37.54 3.05 - 923.25 - 0.07 0.62 0.33 8.18 0.06 0.03 0.01 0.10 0.08 0.01 FA-5 116.05 1.04 - 7.94 25.04 - - - 7.94 0.01 - - 0.01 0.01 - FA-6 108.38 - - - 28.84 0.06 1.16 0.09 8.08 0.25 0.19 0.04 0.35 0.14 0.05 FA-7 27.59 3.92 0.82 - - - 0.11 - 9.41 0.02 0.02 - 0.02 0.03 0.01 FA-8 - - - 1 905.02 0.66 - - - 8.63 0.03 0.03 0.01 0.07 0.04 0.01 FA-9 - 0.62 2.33 - 8.79 0.07 - 0.37 7.94 0.15 0.11 0.03 0.19 0.12 0.04 FA-10 - 3.15 2.25 - 5.62 - 0.24 0.92 8.75 0.17 0.14 0.03 0.19 0.10 0.05 FPA-1 - - - 1 013.90 0.70 0.07 0.18 0.09 80.82 2.36 3.97 0.46 1.67 0.44 0.08 FPA-2 - 1.17 - 361.72 - 0.04 0.17 0.69 87.71 1.34 2.27 0.23 1.04 0.23 0.06 FPA-3 - 1.16 0.55 876.27 - - 1.03 - 86.43 2.07 3.65 0.43 1.69 0.47 0.14 FPA-4 10.20 - 0.38 253.75 - - 0.81 - 88.89 2.45 4.09 0.48 2.00 0.44 0.10 FPA-5 63.69 7.16 - - - - - 0.82 87.92 5.06 8.42 1.01 3.72 0.81 0.19 FPA-6 - - 3.38 - 1.03 - 0.36 - 72.77 2.30 3.74 0.40 1.57 0.36 0.09 FPA-7 - 1.75 - - 4.01 - 0.77 1.40 93.52 0.33 0.72 0.13 0.63 0.53 0.23 FPA-8 66.18 5.48 0.83 526.72 11.89 0.12 1.04 - 64.70 5.29 8.52 0.96 3.68 0.66 0.13 FPA-9 - 0.80 - 1 073.19 - 0.15 0.22 0.95 75.65 1.68 2.70 0.30 1.28 0.28 0.05 FPA-10 36.65 3.58 2.85 - 4.42 0.03 - 0.01 78.07 3.06 5.02 0.58 2.13 0.43 0.08 注：“-”表示低于检测限
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