Mineralogical Characteristics and Colouration Mechanism of Blue Opals from Peru
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摘要: 近年来蓝色蛋白石的研究仅限于矿物成分及致色机理，并未对其化学成分、红外光谱、拉曼光谱等开展较为深入的分析。本文在前人的研究基础上，通过傅里叶变换红外光谱（FTIR）、X射线粉晶衍射（XRD）、电子探针分析（EMPA）、紫外可见分光光谱、拉曼光谱等技术对样品的振动光谱、官能团表征、矿物组成及呈色机理进行研究。研究结果表明：蓝色蛋白石的主要组成矿物为非晶态蛋白石，且振动光谱与天然蛋白石存在一定程度的频率位移。EMPA分析结果显示蓝色蛋白石主要元素为Si和Cu，且紫外可见分光光谱表征为742 nm附近一吸收强度较高的宽谱带。综合电子探针和紫外可见吸收光谱的测试结果得出，蓝色蛋白石的致色元素为Cu，在其内部呈典型平面正方形结构的[Cu2+（H2O）4]2+，且Cu含量与其蓝色的体色存在一定的正相关性，即随着Cu含量增加蓝色体色更加浓艳。Abstract: In recent years, the study of blue opal has been limited to the mineral composition and coloration mechanism. However, the chemical composition, infrared spectrum, and Raman spectrum of the blue opal have not been studied in details. Combined with previous research, Fourier Transform Infrared Spectroscopy (FTIS), X-ray Diffraction (XRD), Electron Microprobe Analyzer (EMPA), and UV Vis Absorption Spectroscopy (Uv-Vis) and Raman Spectroscopy were used to investigate vibration spectra, functional group characterization, mineral composition, and coloring mechanism of blue opal samples. Results show that the main mineral composition of blue opal is amorphous opal and the vibrational spectra show some peak frequency shifts relative to natural opal. EMPA shows that the main elements of blue opals are Si and Cu, whereas the Uv-Vis spectrum shows a broad band with higher absorption intensity near 742 nm. Combined EMPA and Uv-Vis spectrum results further indicate that the coloration element for the blue color of the opal is Cu, with the typical planar square structure of[Cu2+(H2O)4]2+. Moreover, the blue color and the Cu content are positively correlated, and with the increasing Cu content the blue color is more intense.
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表 1 蓝色蛋白石的红外吸收光谱特征与归属
Table 1. Representation and attribution of FTIR of blue opal
波数(cm-1) 归属 1230(br) νas(C—O—C) 1167(m)，1100(s), 1029(m-s) νas(Si—O—Si) 793(w) νs(Si—O—Si) 700(w)，649(w) νs(Si—C) 510(m), 476(m)，478(m) γ(Si—O—Si) 注：w=弱，sh=肩吸收，m=中等，br=宽吸收，s=强；νas=不对称伸缩振动，νs=对称伸缩振动；β=面内弯曲振动，γ=面外弯曲振动，δ=变形振动，δs=对称变形振动。 
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表 2 蓝色蛋白石化学成分分析结果
Table 2. Chemical analysis of blue opals measured by EMPA
样品编号 SiO2(%) CuO(%) FeO(%) Na2O(%) MgO(%) Al2O3(%) Cr2O3(%) NiO(%) 合计(%) OP-1 68.637 25.698 0.001 0 0.123 0.025 0.025 0 94.509 OP-2 71.563 23.209 0.236 0 0.097 0.375 0.002 0 95.481 OP-3 71.397 24.61 0.104 0 0.098 0.178 0 0 96.387 OP-4 70.572 22.439 0.251 0.001 0.104 0.451 0.007 0 93.824 AVG 70.542 23.989 0.148 0 0.106 0.257 0.009 0 -
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