马达加斯加南部紫色蓝宝石的宝石学及谱学特征

Gemmological and Spectroscopic Characteristics of Purple Sapphire from Southern Madagascar

  • 摘要: 一直以来,蓝宝石都因其瑰丽的颜色和特殊的光学效应深受广大消费者的喜爱,然而目前对于紫色蓝宝石的系统研究较少。本文选取马达加斯加南部紫色蓝宝石为研究对象,利用常规宝石学仪器、显微拍照、偏光显微镜、拉曼光谱仪、傅里叶变换红外光谱仪、激光剥蚀等离子质谱仪和显微紫外-可见-近红外光谱仪研究其宝石学及谱学特征。结果显示,马达加斯加南部紫色蓝宝石具有深浅不一的紫色,原石表面有三角形生长丘、凹坑、石英晶体和结晶程度较差的石墨;放大观察发现内部存在大量锆石包裹体及其与磷灰石、独居石、钛铁矿的组合,个别样品中发现锐钛矿、叶蜡石;结合LA-ICP-MS和显微紫外-可见吸收光谱测试结果发现:(1)当Fe含量在4 000~5 300 μg/g,Cr含量在300~500 μg/g时,显示377、388 nm和450 nm处清晰的Fe谱和560 nm附近的Cr谱,蓝宝石样品呈浅紫色,(2)当Fe含量低于1 600 μg/g,Cr含量低于200 μg/g时,显示较不明显的Fe谱和Cr谱,蓝宝石样品呈浅粉紫色,(3)当Fe含量低于1 600 μg/g,Cr含量高于1 500 μg/g时,显示清晰的Cr谱,以及少量的Fe2+-Ti4+离子对产生的560 nm附近的吸收叠加在Cr谱上,蓝宝石样品呈鲜艳的紫色,样品还具有低Ti、V、Mg、Ga的特点,且Cr/Ga、Fe/Mg、Ga/Mg的比值均偏向变质岩型成因蓝宝石的地球化学特征;红外透射光谱显示样品均具有3 309 cm-1处微弱的吸收峰;通过紫外荧光测试,可以初步判断元素的富集情况。

     

    Abstract: Sapphire, due to its magnificent colour and special optical effect, is loved by many consumers. However, there are few systematic studies on purple sapphires to date. The gemmological and spectroscopic characteristics of purple sapphire samples from southern Madagascar were investigated using conventional gemmological instruments, photomicrograph, polarizing microscope, Raman spectrometer, Fourier transform infrared spectrometer, laser ablation inductively coupled plasma mass spectrometer, and microscopic UV-Vis-NIR spectrometer. The results showed that the purple sapphires from southern Madagascar have different shades of purple, and there are triangular mounds, pits, quartz crystals and poorly crystallized graphite on the raw stone surface. Magnified observation found that there are lots of zircon inclusions and their combinations with apatite, monazite and ilmenite, and anatase and pyrophyllite are found in individual samples. Combining with the results of LA-ICP-MS and micro-UV-Vis absorption spectra, it showed that: (1)when the Fe content is between 4 000 μg/g and 5 300 μg/g and the Cr content is between 300 μg/g and 500 μg/g, the spectrum at 377, 388 nm and 450 nm reveals clear Fe spectrum and the spectrum near 560 nm reveals clear Cr spectrum, and the samples show light purple; (2)When the Fe content is less than 1 600 μg/g and the Cr content is less than 200 μg/g, the characteristic spectra of Fe and Cr are less obvious, and the samples show light pinkish purple; (3)When the Fe content is lower than 1 600 μg/g and the Cr content is higher than 1 500 μg/g, a clear spectrum of Cr is displayed, and a small amount of Fe2+-Ti4+ ion pairs produce absorption near 560 nm superimposed on the Cr spectrum, and the samples show vivid purple. The samples also have the characteristics of low contents of Ti, V, Mg and Ga, and the Cr/Ga, Fe/Mg and Ga/Mg ratios are more consistent with the geochemical characteristics of metamorphic sapphires. Infrared trans mission spectra show that all the samples had a weak absorption peak at 3 309 cm-1. The enrichment of elements can be preliminarily identified by UV fluorescence observation.

     

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