Nondestructive Identification of Mineral Inclusions by Raman Mapping: Micro-Magnetite Inclusions in Iridescent Scapolite as Example
YE Xu1, QIU Zhi-li1, 2*, CHEN Chao-yang3, ZHANG Yue-feng1
1. School of Earth Science and Engineering, Sun Yat-sen University，Guangdong Provincial Key Lab of Geological Processes and Mineral Resource Survey, Guangzhou 510275, China
2. Guangxi Key Laboratory of Exploration for Hidden Metallic Ore Deposits, College of Earth Sciences, Guilin University of Technology, Guilin 541006, China
3. Gemmological Institute，China University of Geosciences，Wuhan 430074，China
Abstract：Using Raman spectroscopy to nondestructively identify tiny inclusions in minerals is a significant problem in mineralogy and gemology. Iridescent scapolite is a kind of scapolite with special inclusions. Its inclusions present various spectral color under reflected light. In this research, magnetite inclusion in iridescent scapolite was nondestructively identified by hyperfield microscope, EPMA, Micro-Raman Spectrometer and XRD. Raman spectroscopy mapping technology was innovatively used. The microscopic characteristics indicate that the inclusions may be related to solid exsolution. The tiny inclusions grow parallelly and form a structure that is similar to the reflection grating, leading to iridescence under reflected light. The results of EPMA show that the end-member composition of iridescent scapolite is Ma68.2—69.7Me30.3—31.8, belonging to the dipyre subgroup. According to Raman spectra of some inclusions, there is a weak peak at 661 cm-1. This peak, which does not appear in all testing locations, is easy to be ignored due to the low signal-to-noise ratio. In order to further study the assignment on this peak, the Raman mapping testwas carried out, and the mapping image was made based on the relative intensity of the peak range at 630～680 cm-1. The result confirmed that the weak Raman peak at 661 cm-1 appeared in most of the inclusion positions. This Raman peak can be assigned to the vibration mode A1g of magnetite. Then we confirmed that the needle-like inclusions with iridescent effect contained smaller magnetite inclusions. According to the results of XRD, a diffraction peak at 2.51 Å, which belongs to the (311) crystal surface of the magnetite. It was detected in the sample containing many inclusions. It also furtherpro of the Raman mapping results are credible. According to the above experiments, Raman mapping technology may be an effective auxiliary means to identify tiny inclusions in minerals and gemstones. This study innovatively proposes that if the Raman signal of inclusions in minerals is weak, the effectiveness of the signal can be judged by combining the Raman mapping results with the distribution characteristics of inclusions. At the same time, it provides a new research idea and method for the nondestructive identification of inclusions in minerals.
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