Ab initio simulations and experimental Raman spectra of Mg2SiO4 forsterite to simulate Mars surface environmental conditions

In this study, we present the full Raman vibrational spectrum of forsterite (Mg2SiO4), computed from first principles, employing a hybrid HF/density functional theory Hamiltonian (WC1LYP) as implemented in the CRYSTAL14 code, at static equilibrium and at the temperatures of 0, 300 and 1000 K. The simulations are compared with the available literature data, confirming the accuracy of the calculations, and to experimental Raman spectra taken at room temperature on a natural sample of forsterite (Mg1.76 Fe0.22 SiO4), to test the effect of compositions on the reliability of a comparison between a computed spectrum for the end member and the experimental spectrum on a real sample having a slightly different compositions. The comparison with the experimental data at room temperature shows a very good agreement (an average discrepancy of 7 cm-1), and it allows a reliable symmetry assignment of Raman signals to specific vibrational modes. Spectra are also simulated by changing the mass of the nuclei for each of the six symmetry-independent species within the unit to quantify the contributions of each element to the overall vibration. The aim is not only to relate the major experimental peaks to specific structural features but also to link them to the Raman shifts observed because of both temperature and composition variation. Moreover, so as to foresee the possible response of Raman spectra to the different environmental conditions occurring on planetary surfaces, i.e. Mars, we calculate full Raman spectra at 0, 300 and 1000 K including zero point vibrational effects, within the limit of the quasi-harmonic approximation. These results may be useful to widen a Raman database and provide new clues to improve the interpretation of data acquisitions during the 2020 ExoMars mission, which will carry on board a Raman Laser Spectrometer.