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

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

Research output: Contribution to journalArticlepeer-review

Abstract

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.

Original languageEnglish (US)
Article number115299
JournalIcarus
Volume390
DOIs
StatePublished - Jan 15 2023

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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