Thermally induced phase transition of troilite during micro-raman spectroscopy analysis

• Published in: Icarus (New York, N.Y. 1962) Vol. 390; p. 115299
• Main Authors: Li, Xiaoguang, Chen, Yi, Tang, Xu, Gu, Lixin, Yuan, Jiangyan, Su, Wen, Tian, Hengci, Luo, Huiqian, Cai, Shuhui, Komarneni, Sridhar
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 15-01-2023
• Subjects: Laser irradiation; Magnetite; Nanophase metallic iron; Raman spectroscopy; Troilite; Magnetite; Troilite; Laser irradiation; Raman spectroscopy; Nanophase metallic iron
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2022.115299

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.
•Troilite is sensitive to laser heat in both vacuum and ambient air conditions.•Troilite could transform to iron oxides or nano metallic iron during heating.•High-temperature effect must be considered when evaluating the remanent magnetization.