巴基斯坦宝石级榍石谱学研究

Study on Spectroscopy of Sphene From Pakistan

榍石作为一种稀有宝石,具有良好的宝石学特性以及出色火彩,在欧美、日本、印度等国备受欢迎。随着近几年国内外珠宝市场的交流增加,中国某些地区也开始流行这种外观炫目的宝石。但国内有关宝石级榍石研究较少,认知依然停留在折射率、色散等基本矿物学性质上,对其光谱学特征、颜色成因等方面研究依然存在空白。在对巴基斯坦黄绿色榍石进行成分分析基础上,归纳总结其红外光谱,拉曼光谱及紫外-可见光光谱的特征。通过对榍石样品进行激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)测试,发现该批次次样品主要元素含量稳定,其中TiO_(2)含量为35.42 Wt%,MnO和Cr_(2)O_(3)含量非常低(分别为~0.04 Wt%和0.01 Wt%),因此该批次榍石样品颜色主要与含量较高的Fe相关。在红外振动的基频波段400~1200 cm^(-1)范围内可观察由Si—O和Ti—O振动引起的吸收峰,同时在6800 cm^(-1)处观察到表明Fe^(2+)存在的吸收宽带。拉曼光谱测试结果(45~1500 cm^(-1))与前人研究基本一致,3354 cm^(-1)处的弱拉曼峰,推测与Fe相关。通过对比定向切割样品GS-1在5个不同方向上的拉曼光谱结果,可发现不同结晶面上拉曼峰位置基本不变,但部分振动的相对强度存在差异。紫外-可见光光谱分峰拟合结果表明,14461 cm^(-1)处吸收可能由八面体内Fe^(2+)的d—d禁阻跃迁产生,而15887,16781和17781 cm^(-1)三个吸收峰则与八面体内Fe^(3+)的d—d禁阻跃迁相关。该研究的创新性主要体现在以下两个方面:(1)系统性总结宝石级榍石红外光谱、拉曼光谱及紫外-可见光光谱特征,并结合成分测试结果进行分析;(2)通过相关软件对榍石样品紫外-可见光光谱测试结果进行分峰拟合,根据各谱峰出现位置,相对强度和峰形特征,结合前人总结规律,对吸收峰进行归属,提出Fe可能是导致宝石级黄绿色榍石颜色形成的原因。

As a precious gem,sphene features sound characteristics of gemology and outstanding brilliance,enjoying great popularity in Europe,United State,Japan and India.As the communication between domestic and overseas jewelry market has been increasing in recent years,some Chinese regions are also witnessing increased popularity of such a gem with a brilliant appearance.However,domestic research on gem-level sphene is quite a few,with relevant recognition being limited to refractive index,chromatic dispersion and other basic properties of mineralogy.There is a big gap in terms of the study on its spectroscopy feature and color formation reasons.On the basis of analyzing the constituent analysis of yellow-green sphene,this paper summarizes characteristics of its infrared spectroscopy,Raman spectrum and ultraviolet-visible spectrum.According to the LA-ICP-MS test on sphene samples,it finds that the major element content of samples of this batch is stable.Among them,the content of TiO_(2) is 35.42 Wt%,and the content of MnO and Cr_(2)O_(3) is quite low(~0.04 Wt%and 0.01 Wt%respectively).Therefore,this batch’s color of sphene sample is mainly related with Fe,the element of high content.The scope of fundamental frequency 400~1200 cm^(-1) of infrared vibration could witness the absorption peak resulting from Si—O and Ti—O vibration.Meanwhile,the absorption width that existed in Fe^(2+)could be found at 6800 cm^(-1).The test result of the Raman spectrum(45~1500 cm^(-1))is basically in line with previous studies.It is speculated that the weak Raman spectrum is related to Fe.On the basis of the comparison of Raman spectrum results of cutting sample GS-1 in 5 different directions,it finds that the location of Raman spectrum in different crystal faces keeps unchanged,but the relative strength of some vibrations varies.As shown by the ultraviolet-visible spectrum peak-differentiating and imitating,the absorption at 14461 cm^(-1) might be derived from the d—d forbidden transition of Fe^(2+)within octahedron,but the three absorption peaks of 15887,16781 and 17781 cm^(-1) have resulted from the d—d forbidden transition of Fe^(3+)within octahedron.Innovations of this research can be summarized into two aspects as below:(1)it systematically summarizes characteristics of infrared spectroscopy,Raman spectrum and ultraviolet-visible spectrum of gem-grade sphene and makes analyzes based on the constituent test result;(2)Relevant software is applied to make peak-differentiating and imitating on the ultraviolet-visible spectrum of sphene samples.Absorption peaks are assigned in accordance with locations of spectrum peaks,relative strength,peak shape characteristics and principles summarized by previous researchers.In the end,it raises that Fe might be the cause for the color development of gem-grade yellow-green sphene.