Mechanochemical synthesis of (Mg1−x Fe x )2SiO4 olivine phases relevant to Martian regolith: structural and spectroscopic characterizations

Abstract

To fabricate metals from the base materials for future Mars exploration, synthesis of representative olivine phases and their structural and spectroscopic characterizations are of crucial importance. Using mechanochemical technique that mimics the mechanical weathering, a complete solid solution of (Mg1−x Fe x )2SiO4 has been synthesized to investigate the associated crystal chemical properties. X-ray powder diffraction data Rietveld analysis confirms that each polycrystalline sample crystallizes in space group Pbnm. The average crystallite size ranges between 80(1) nm and 223(4) nm. Each lattice parameter increases with increasing Fe-content due to the larger Fe2+ radius than that of Mg2+, following Vegard’s rule. For a given nominal chemical composition, substitution of Mg with Fe at the M1-site (4a: 0,0,0) is preferred to the M2-site (4c: x,y,¼). As a consequence, the average Fe-content lies slightly below the equivalence line for x = 0.2–0.8, indicating that the Fe/Mg ratio in the amorphous scattering content is most likely greater than unity. Characteristic Raman spectral features of the olivines have been explained in terms of the chemical composition (x). Simple regression models are demonstrated based on both X-ray diffraction and Raman spectroscopic data for the calculation of Mg/Fe in olivines. Diffuse reflectance UV/Vis spectra RATD analysis shows each olivine phase possesses direct band-gap between 3.38(3) eV and 4.90(3) eV. This study could keep valuable information to relevant databases for future human missions on Mars, in particular, for precise estimation of the representative olivines from the remote X-ray diffraction and spectroscopic data.