Time-resolved remote Raman study of minerals under supercritical CO 2 and high temperatures relevant to Venus exploration

Abstract

We report time-resolved (TR) remote Raman spectra of minerals under supercritical CO 2 (approx. 95 atm pressure and 423 K) and under atmospheric pressure and high temperature up to 1003 K at distances of 1.5 and 9 m, respectively. The TR Raman spectra of hydrous and anhydrous sulphates, carbonate and silicate minerals (e.g. talc, olivine, pyroxenes and feldspars) under supercritical CO 2 (approx. 95 atm pressure and 423 K) clearly show the well-defined Raman fingerprints of each mineral along with the Fermi resonance doublet of CO 2 . Besides the CO 2 doublet and the effect of the viewing window, the main differences in the Raman spectra under Venus conditions are the phase transitions, the dehydration and decarbonation of various minerals, along with a slight shift in the peak positions and an increase in line-widths. The dehydration of melanterite (FeSO 4  · 7H 2 O) at 423 K under approximately 95 atm CO 2 is detected by the presence of the Raman fingerprints of rozenite (FeSO 4  · 4H 2 O) in the spectrum. Similarly, the high-temperature Raman spectra under ambient pressure of gypsum (CaSO 4  · 2H 2 O) and talc (Mg 3 Si 4 O 10 (OH) 2 ) indicate that gypsum dehydrates at 518 K, but talc remains stable up to 1003 K. Partial dissociation of dolomite (CaMg(CO 3 ) 2 ) is observed at 973 K. The TR remote Raman spectra of olivine, α-spodumene (LiAlSi 2 O 6 ) and clino-enstatite (MgSiO 3 ) pyroxenes and of albite (NaAlSi 3 O 8 ) and microcline (KAlSi 3 O 8 ) feldspars at high temperatures also show that the Raman lines remain sharp and well defined in the high-temperature spectra. The results of this study show that TR remote Raman spectroscopy could be a potential tool for exploring the surface mineralogy of Venus during both daytime and nighttime at short and long distances.