Spectral Characteristic of Gem Grade Clinohumite
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摘要:
斜硅镁石是一种稀有的黄色至橙色的硅酸盐矿物，近年来市面上开始出现宝石级的斜硅镁石，而国内鲜有相关的宝石学研究。为补充该宝石的研究空白，选取三种不同色调(黄色、橙黄色和橙红色)的斜硅镁石进行常规宝石学测试，结合电子探针、激光拉曼光谱、傅里叶变换红外光谱和紫外-可见光谱分析技术，对斜硅镁石的化学成分、光谱特征及颜色成因等进行初步研究。结果表明，斜硅镁石的折射率为1.629~1.668，具有较强的双折射(DR：0.031~0.036)，相对密度为3.14~3.27，短波紫外光下呈中等黄色-橙色荧光；部分样品有明显聚片双晶，包裹体形态丰富，并发现金红石包裹体；斜硅镁石含有主量元素Si、Mg和Ti，微量元素Ca、Fe、K和V，结合紫外-可见光谱推测颜色与Fe和Ti密切相关；拉曼光谱和红外光谱都显示主要峰位与SiO4四面体、MgO6八面体和-OH的振动有关，最强拉曼位移为830 cm-1和862 cm-1，最强红外光谱反射峰在925~935 cm-1附近，都与SiO4振动有关。
Abstract:Clinohumite is a rare yellow to orange silicate mineral.In recent years, gem grade clinohumite has appeared on the market, but there are few related gemmological studies in China.To supplement the research gap of this gemstone, clinohumite samples of three different colour tones (yellow, orange-yellow and orange-red) were selected for conventional gemmological tests. Combined with electronic probe, laser Raman spectrometer, Fourier transform infrared spectrometer, and UV-Vis spectrometer, the chemical compositions, spectral characteristics, and mechanism of coloration were preliminarily studied.The results show that RI of clinohumite is 1.629-1.668, with a strong birefringence(DR: 0.031-0.036).Their relative density ranges from 3.14 to 3.27, and medium yellow to orange fluorescence can be observed under shortwave ultraviolet light. Some samples have obvious twin crystal patterns and rich inclusion morphology. Rutile inclusions were found in gemstone grade clinohumite.Clinohumite contains main elements Si, Mg, and Ti, as well as trace elements Ca, Fe, K, and V. Combined with UV-Vis spectra, it is speculated that the colour is closely related to Fe and Ti.Laser Raman spectra and infrared spectra are mainly related to vibrations of SiO4 tetrahedron, MgO6 octahedron and -OH.The strongest Raman shifts (830 cm-1 and 862 cm-1) and the strongest infrared spectral reflection peaks (around 925-935 cm-1) both related to SiO4 vibration.
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Keywords:
- clinohumite /
- infrared spectrum /
- Raman spectrum /
- UV-Vis spectrum
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致谢: 本项目由上海市高校优秀青年教师培养基金(晨光计划)(项目号：AASH2106)、上海建桥学院博士项目基金资助，向基金委员会致以感谢！
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图 3 Diamond ViewTM下斜硅镁石样品的荧光特征
Figure 3. Fluorescence characteristics of clinohumite samples under Diamond ViewTM
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图 4 斜硅镁石样品的包裹体：(a-b)短柱状和长针状包裹体；(c-d)六方薄片状的固体包裹体；(e-f)无色的晶体包裹体和白色的絮状包裹体；(g-h)粒状黑色固体包裹体
Figure 4. Inclusions in clinohumite samples: (a-b)short columnar and long needle-shaped inclusions; (c-d)hexagonal sheet-like solid inclusions; (e-f)colourless crystal inclusions and white flocculent inclusions; (g-h)granular black solid inclusions
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图 5 斜硅镁石样品在100~1 200 cm-1(a)和100~4 000 cm-1(b)范围的拉曼光谱
Figure 5. Raman spectra of clinohumite samples between 100—1 200 cm-1(a) and 100—4 000 cm-1(b)
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图 8 斜硅镁石样品指纹区400~1 500 cm-1的红外光谱
Figure 8. Infrared reflection spectra of fingerprint region of clinohumite samples between 400-1 500 cm-1
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图 9 斜硅镁石在3 000~3 800 cm-1 (a)和4 000~8 000 cm-1 (b) 的红外光谱
Figure 9. Infrared reflection spectra of clinohumite samples between 3 000—3 800 cm-1 (a) and 4 000—8 000 cm-1 (b)
表 1 斜硅镁石样品的常规宝石学特征
Table 1 Conventional gemmological characteristics of clinohumite samples
样品 颜色 光泽 折射率 双折射率 相对密度 紫外荧光 多色性 编号 LW SW C1 金黄色 玻璃光泽 1.629~1.660 0.031 3.14 惰性 中等黄色 中等—浅黄、黄色 C2 金黄色 玻璃光泽 1.629~1.660 0.031 3.22 惰性 中等黄色 中等—浅黄、黄色 C3 橙黄色 玻璃光泽 1.630~1.661 0.031 3.27 惰性 中等橙色 中等—黄色、橙红色 C4 橙黄色 玻璃光泽 1.632~1.668 0.036 3.20 惰性 中等橙色 中等—黄色、橙红色 C5 橙红色 玻璃光泽 1.632~1.668 0.036 3.23 惰性 中等橙色 中等—黄色、橙黄色、棕红色 C6 橙红色 玻璃光泽 1.633~1.668 0.035 3.23 惰性 中等橙色 中等—黄色、橙黄色、棕红色 
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表 2 斜硅镁石样品的化学成分定量分析
Table 2 Quantitative analysis of chemical compositions of clinohumite samples
氧化物 C1 C2 C3 C4 C5 C6 K2O/% 0.02 0.02 0.02 0.02 — 0.03 CaO/% 0.02 0.02 0.02 0.01 0.01 0.03 TiO2/% 2.13 1.93 2.36 2.76 2.63 2.63 MgO/% 55.74 56.86 55.75 55.18 56.03 55.02 SiO2/% 38.36 38.85 38.42 38.16 38.50 38.21 FeOT/% 0.06 0.05 0.27 0.22 0.22 0.22 Cr2O3/% 0.01 0.03 0.03 0.01 — 0.01 F/% 1.87 2.01 2.06 1.60 1.80 2.04 Total/% 97.43 98.93 98.05 97.27 98.44 97.34 元素阳离子个数 K 0.000 4 0.000 5 0.000 4 0.000 4 0 0.000 7 Ca 0.000 3 0.000 4 0.000 4 0.000 1 0.000 2 0.000 6 Ti 0.027 4 0.024 4 0.030 1 0.035 5 0.033 5 0.033 9 Mg 1.419 8 1.426 2 1.410 9 1.407 7 1.412 3 1.402 6 Fe 0.000 9 0.000 7 0.003 8 0.003 1 0.003 1 0.003 1 Cr 0.000 2 0.000 4 0.000 3 0.000 1 0 0.000 1 Si 0.655 3 0.653 6 0.652 1 0.653 0 0.650 9 0.653 5 N 2.210 9 2.222 6 2.217 3 2.215 9 2.226 3 2.205 1 注：表中N为M阳离子个数∶Si个数；不包括H2O；斜硅镁石中H2O质量分数的理论值为3%;FeOT包括FeO和Fe2O3；—表示低于检测限
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