Thermally induced phase transition of troilite during Micro-Raman spectroscopy analysis

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Xiaoguang Li, Yi Chen, Xu Tang, Lixin Gu, Jiangyan Yuan, Wen Su, Hengci Tian, Huiqian Luo, Shuhui Cai, Sridhar Komarneni

Icarus
Available online 10 October 2022, 115299

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“Highlights

• Troilite is sensitive to laser heat in both vacuum and ambient air conditions.
• Troilite may served as the origins of lunar iron oxides and nano metallic iron.
• The effect of high temperature needs to be considered when evaluating the remanent magnetization.”

“Troilite is one type of FeS polymorph formed under reducing environmental conditions. However, its phase transition by laser heating during Raman analysis has not been investigated in detail. This study focuses on identifying changes to Raman spectra of troilite resulted by laser heating during Raman analysis so as to determine optimized analytical conditions for characterizing iron sulfides. We comfirm that iron sulfides exposed in air are easily transformed to magnetite and hematite after a high-power laser (> 200 mW/μm2 for pyrite and > 14 mW/μm2 for troilite) irradiation. Troilite crystal structure is also broken easily by laser (>12 mW/μm2) under the vacuum conditions due to the volatilization of S and Fe, possibly inducing the formation of nanophase metallic iron. Therefore, iron sulfides are expected to be sensitive to laser heating. Here, we have confirmed the laser heating effect through a set of heating experiments from ambient temperature to 500 °C with various laser powers. Our results suggest that Raman analysis for troilite should be performed with a low laser power of <1.50 mW (12 mW/μm2) both in air and vacuum environments. The heating effects on troilite phase transition can be responsible for the formation of magnetite, hematite, and nanophase metallic iron in lunar samples. The thermally induced phase transition of troilite observed in this study is important because it undoubtedly modifies both the redox state and magnetic property of extraterrestrial samples and would trigger a misleading interpretation of planetary evolution.”

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