Chemical Composition and Spectroscopic Characteristic of Tourmaline with Usambara Effect
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摘要:
Usambara(乌桑巴拉)效应作为一种罕见的光学效应，在电气石上得到了很好的体现，然而对具有该效应的电气石谱学特征研究仍有待完善。本文以一批具有Usambara效应的电气石为研究对象，利用常规宝石学测试仪器、激光剥蚀电感耦合等离子体质谱仪、红外光谱、显微紫外-可见光谱仪以及荧光光谱仪研究其谱学特征。结果表明，这批具有Usambara效应的电气石样品体色均为深绿色，将多片样品重叠，经光源透射，可见透射区域整体颜色变化为红色，符合Usambara效应；激光剥蚀电感耦合等离子体质谱结果表明，该批样品基本属于镁电气石，其微量元素中含有较高的Cr含量；红外光谱显示3 569cm-1处有强的镁电气石特征吸收峰；紫外-可见吸收光谱表明，具有Usambara效应的电气石样品的颜色与Cr的自旋允许电子d-d跃迁以及弱自旋禁阻吸收密切相关；荧光光谱结果显示，可见光波段中有3处激发光源可使样品产生强烈红色荧光, 该荧光由Cr3+多重禁阻跃迁所导致。
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关键词:
- Usambara效应 /
- 电气石 /
- 谱学特征 /
- 颜色成因
Abstract:As a rare optical effect, Usambara effect has been well recorded in tourmaline, but knowledge on the chemical and spectroscopic characteristics of tourmaline with this effect is still to be improved. In this paper, a batch of tourmaline showing Usambara effect is studied using conventional gemmological testing instruments, laser ablation inductively coupled plasma mass spectrometer, infrared spectroscopy, micro-UV-Visible spectrometer and fluorescence spectrometer.The experimental results show that the colour of these tourmaline samples with the Usambara effect is dark green. When multiple samples are overlapped and transmitted by a light source, the overall colour of the visible transmission area changes to red, which is consistent with the Usambara effect. The results of laser ablation inductively coupled plasma mass spectrometry indicate that these samples belong to magnesium tourmaline with high Cr content as trace element. The infrared spectra show a strong absorption peak of magnesium tourmaline at 3 569 cm-1. The ultraviolet-visible absorption spectra show that the colour of the Usambara effect tourmaline samples is closely related to the spin-permitted electron d-d transition and the weak spin-forbidden absorption of Cr. The results of fluorescence spectra show that there are three luminescent centers in the visible range caused by Cr3+ multiple forbidden transitions, which produce strong red fluorescence.
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Keywords:
- Usambara effect /
- tourmaline /
- spectroscopic characteristic /
- colour origin
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图 1 2 850 K光纤灯照射下电气石样品呈现出的Usambara效应: (a)透射光单独照射样品均呈现绿色调；(b)光源透过重叠在一起的4片样品呈现红色调
Figure 1. Tourmaline samples showing Usambara effect under transmission light of 2 850 K: (a)separate slices show green tone; (b)overlapped slices show red tone)
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图 2 电气石样品的测试点位分布图
注：d为样品厚度
Figure 2. Testing point distribution map of tourmaline samples
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图 3 电气石样品各测试点在X、Y、Z和W位点中的元素投点占位分布三元图
Figure 3. Triple plot of element loading distribution at X, Y, Z, and W sites at each testing point of tourmaline samples
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图 4 电气石样品的红外反射光谱(a)和红外透射光谱(b)
Figure 4. Infrared reflection spectra (a) and infrared transmission spectra (b) of tourmaline samples
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图 5 电气石样品的显微紫外-可见吸收光谱
Figure 5. Ultraviolet-visible absorption spectra of the tourmaline samples
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图 6 电气石样品的透射率及透射窗
Figure 6. The transmission rate and transmission windows of the tourmaline samples
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图 7 电气石样品TS-1的三维荧光光谱、特征激发和发射光谱
Figure 7. 3D fluorescence spectra, characteristic excitation spectra, and emission spectra of the tourmaline sample TS-1
表 1 电气石样品中测试点的化学成分
Table 1 Chemical compositions of the tourmaline samples' testsing points
TS-1-A TS-1-B TS-1-C TS-2-A TS-2-B TS-2-C TS-3-A TS-3-B TS-4-A TS-4-B 主要氧化物/% B2O3 10.68 10.66 10.68 10.68 10.66 10.65 10.66 10.56 10.73 10.62 Na2O 1.53 1.61 1.85 1.62 1.84 1.57 1.58 1.92 1.72 1.78 MgO 11.68 11.50 11.22 11.15 11.17 11.48 11.20 10.67 11.47 11.07 Al2O3 30.44 30.68 30.81 30.97 31.00 30.62 29.72 29.22 32.01 31.35 SiO2 38.02 38.26 38.41 38.29 38.18 37.92 38.29 38.13 38.55 38.20 CaO 2.87 2.74 2.28 2.56 2.13 2.69 2.66 1.97 2.30 2.30 Cr2O3 4.22 3.97 4.16 4.14 4.43 4.48 5.16 6.81 2.73 4.02 微量元素/10-6 Li 17.59 19.38 20.89 18.97 22.00 17.68 17.94 20.42 28.47 19.18 K 385.65 313.77 435.03 303.95 397.82 356.63 366.40 393.92 330.22 474.86 Ti 1 220.96 1 280.66 1 402.69 1 500.30 1 601.93 1 260.74 1 273.57 1 304.43 1 084.88 1 364.05 V 480.72 491.46 510.78 507.53 521.89 521.42 452.46 559.28 271.21 402.11 Mn 20.27 18.41 18.77 19.69 18.35 18.63 19.23 17.55 17.95 19.07 Fe 232.37 247.47 223.28 224.57 234.88 208.52 271.52 258.29 165.09 202.91 Co - - - - - - - - - - Ni* - 5.17 - 5.51 2.40 3.33 - - 1.82 3.50 Cu - - - - - - - - - - 注：*表示该行数据属于半定量；“-”表示低于检测限 
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表 2 电气石样品测试点的经验结构公式及所属电气石亚种类型
Table 2 Empirical structural formula for testing points of tourmaline samples and their corresponding types of tourmaline subspecies
样品号 化学式 亚种 TS-1-A (Na0.47Ca0.49K0.01□0.04)(Mg1.60Al0.82Cr0.53V0.01Ti0.02)3(Al4.85Mg1.15)6(Si6O18)(BO3)3(OH)3.27O0.73 Uvite Ca-Mg-O root name TS-1-B (Na0.49Ca0.46K0.01□0.04)(Mg1.58Al0.84Cr0.50V0.01Ti0.03)3(Al4.87Mg1.13)6(Si6O18)(BO3)3(OH)3.21O0.79 Dravite Oxy-dravite TS-1-C (Na0.57Ca0.39K0.01□0.04)(Mg1.52Al0.85Cr0.52V0.01Ti0.03)3(Al4.88Mg1.12)6(Si6O18)(BO3)3(OH)3.20O0.80 Dravite Oxy-dravite TS-2-A (Na0.50Ca0.43K0.01□0.06)(Mg1.53Al0.86Cr0.52V0.01Ti0.03)3(Al4.90Mg1.10)6(Si6O18)(BO3)3(OH)3.18O0.82 Dravite Oxy-dravite TS-2-B (Na0.56Ca0.36K0.01□0.07)(Mg1.52Al0.86Cr0.55V0.01Ti0.03)3(Al4.90Mg1.10)6(Si6O18)(BO3)3(OH)3.28O0.72 Dravite Oxy-dravite TS-2-C (Na0.48Ca0.46K0.01□0.06)(Mg1.57Al0.83Cr0.56V0.01Ti0.02)3(Al4.87Mg1.13)6(Si5.99Al0.01O18)(BO3)3(OH)3.29O0.71 Dravite Oxy-dravite TS-3-A (Na0.49Ca0.45K0.01□0.05)(Mg1.44Al0.78Cr0.65V0.01Ti0.03)3(Al4.79Mg1.21)6(Si6O18)(BO3)3(OH)3.14O0.86 Dravite Oxy-dravite TS-3-B (Na0.59Ca0.34K0.01□0.06)(Mg1.28Al0.75Cr0.86V0.01Ti0.03)3(Al4.74Mg1.26)6(Si6O18)(BO3)3(OH)3.16O0.84 Dravite Oxy-dravite TS-4-A (Na0.52Ca0.39K0.01□0.08)(Mg1.66Al0.93Cr0.34V0.01Ti0.02)3(Al4.98Mg1.02)6(Si6O18)(BO3)3(OH)3.31O0.69 Dravite Oxy-dravite TS-4-B (Na0.54Ca0.39K0.01□0.05)(Mg1.54Al0.89Cr0.50V0.01Ti0.03)3(Al4.94Mg1.06)6(Si6O18)(BO3)3(OH)3.22O0.78 Dravite Oxy-dravite 注：数据结果仅显示两位小数；种类命名两列分别参考Tindle等和Henry等[5-6]
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