Deep-Ultraviolet Raman Spectra of Mars-Relevant Evaporite Minerals under 248.6 nm Excitation

We report the first deep-ultraviolet (DUV) Raman spectra for a number of evaporite minerals that are relevant to lacustrine and fluvial environments found on Earth and Mars, and show that DUV Raman can provide detailed information on elemental composition and crystal structure. The minerals included three borates, eight carbonates, and seven sulfates, with each class of mineral exhibited very distinct spectra under 248.6 nm excitation, dominated by the various internal vibrations of the borate, carbonate and sulfate oxyanions. The spectra were found to shifted to lower wavenumbers compared to positions reported in the literature using longer excitation wavelengths. Within each class, mineral end members differing only in their metallic cation could be distinguished by the position of the dominant vibrational mode and the relative intensities of the minor modes, ascribed to the electrostatic impact of the metallic cation on the vibrational behavior of the oxyanion. There was also evidence that DUV Raman can reveal minor metallic components even if they are not apparent in XRD, as two of three calcite (CaCO3) samples exhibited a shoulder on the dominant peak that we assigned to a Mg-rich perturbation. UV absorption by Fe ions was a major factor in determining measurable signal, with Fe-rich minerals exhibiting weak/undetectable spectra. Understanding the spectra of these evaporite minerals will be essential to interpreting and identifying complex mineral samples using this technique, and represents an important addition to the spectral standards library of DUV Raman spectroscopy.