Raman spectroscopy and structural study of baryte-hashemite solid solution from pyrometamorphic rocks of the Hatrurim Complex, Israel

A number of the baryte, BaSO₄, - hashemite, BaCrO₄, solid solution compounds were synthesized previously. In this study, Raman spectra of naturally occurring phases belonging to the baryte-hashemite series from the pyrometamorphic rocks of the Hatrurim Complex were investigated. The Raman spectrum of natural hashemite, obtained for the first time, shows the position of the fundamental bands for the chromate anion vibrations. The bands related to the stretching vibrations (ν₁, ν₃) occur at 864 cm⁻¹ and in 871–909 cm⁻¹ regions, whereas the bending vibrations (ν₂, ν₄) are visible in the 346–360 cm⁻¹ and 400–422 cm⁻¹ range, respectively. Received results allowed to observe a gradual shift of bands in baryte-hashemite solid solution as a consequence of the substitution by different cations. The position of bands depends on the Cr/S ratio in analysed samples, and it is determined by differences in atomic mass, and ionic radii between Cr⁶⁺ and S⁶⁺, which affect changes in the strength and length of bonds. The occupancy of the same atomic position by two different cations enables to notice variations of polyhedra geometry, and unit cell parameters despite that baryte and hashemite are isostructural and crystallize in the same Pnma space group. We also confirm that the immobilization of the toxic (CrO₄)²⁻ ion in the baryte structure may occur directly without oxygen state reduction, we propose to using a baryte-hashemite solid solution as a reservoir for the incorporation of Cr as an environmental pollutant.