¹Xiaoguang Li¹Yi Chen,²Xu Tang,²Lixin Gu,¹Jiangyan Yuan,¹Wen Su,³Hengci Tian,⁴Huiqian Luo,¹Shuhui Cai,⁵Sridhar Komarneni
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115299]
¹State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
²Electron Microscope Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
³Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
⁴Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁵Department of Ecosystem Science and Management and Materials Research Institute, 204 Energy and the Environment Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
Copyright Elsevier

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.