Structural and Vibrational Investigation of Römerite Under Icy Satellite and Martian Temperature Conditions



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In the ongoing study of planetary and satellite compositions, spectral data collected from planetary surfaces are often compared to laboratory spectra of minerals collected under Earth’s atmospheric conditions. However, temperature can have a significant effect on the underlying physical phenomena that give rise to the spectral features of a mineral. Here, the mixed valence hydrous sulfate römerite, Fe^{2+}Fe^{3+}(SO4) · 14H2O is used to study these effects. In their investigations of Jupiter’s icy satellites, researchers have proposed hydrous sulfates may be a principal constituent of their surfaces, and on Mars, hydrous sulfates have been identified. Thus, these discoveries necessitate a thorough analysis of hydrous sulfates under temperature conditions relevant to these planetary bodies. Here, single crystal X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, and Mössbauer spectroscopy are used to characterize römerite under low-temperature conditions relevant to icy satellites and Mars. Single crystal X-ray diffraction measurements were taken at temperatures ranging from 100-300K, including a reverse temperature series. The evolution of römerite’s unit cell parameters, atomic positions, and bond properties with temperature was observed. Low-temperature Fourier-transform infrared spectroscopy measurements were performed ranging from 20-300K. These results are compared with other hydrous sulfate phases. Complementary to infrared spectroscopy data, Raman spectroscopy data were collected over a temperature range from 153-296K. Approximate band assignments for infrared and Raman spectra are made, and changes in band width, position, and intensity with temperature are discussed. We observe a strong relationship between temperature and the distortion of water in the structure of römerite, which is evaluated through changes in band width and peak position with temperature. Mössbauer measurements were collected from 4-295K and are still under review, but room temperature Mössbauer measurements of römerite are included in this study. Strong displays of temperature dependence in spectra and data from multiple methods confirms that when performing laboratory comparisons of hydrous sulfates to planetary spectra, temperature-relevant laboratory data should be used to optimize accuracy. The distortion of water molecules in the römerite structure also highlights the importance of studying all hydrous minerals at temperature conditions relevant to planetary bodies of interest.



Mineralogy, Planetary Science, Jupiter, Mars, Römerite, X-ray diffraction, Mössbauer Spectroscopy, Icy Satellites, Infrared Spectroscopy, Raman Spectroscopy