Rietveld Refinement and Vibrational Spectroscopic Study of U-Bearing Natural Celestine

This study presents a comprehensive inquiry into the structural refinement and Raman spectroscopy of natural barium strontium sulfate, characterized as Sr_0.967(2)Ba_0.030(2)U_0.0022(6)Y_0.0012(6)SO₄. The crystal structure of orthorhombic sulfate celestine is meticulously refined using Rietveld methods with conventional powder X-ray diffraction data, optimizing the structural model within space group Pnma (Z = 4). The ultimate reliability factors, namely χ² = 5.6, R_F = 3.6, and R_Bragg = 4.65, confirm the outstanding quality of the refinement process. These factors serve as indicators of the degree to which the refined crystal structure in alignment with the experimental data. In this case, the values suggest a high level of accuracy and precision in the refinement, reinforcing the reliability of the obtained structural model for the celestine crystal. The resulting unit cell parameters are reported as a = 8.372(9) Å, b = 5.357(6) Å, c = 6.877(7) Å, and V = 308.42(7) ų. The average <M-O> Vinograd et al. (Appl. Geochem. 89, 59–74, 2018) distance is 2.832(6) Å, while the corresponding average <S-O> Baker and Bloomer (Geochim. Cosmochim. 52, 335–339, 1988) distance is 1.497(4) Å. This investigation confirms the isostructural nature of the celestine solid solution with pure SrSO₄ and barite (BaSO₄). Chemical analysis through X-ray fluorescence provides the empirical formula for celestine as (Sr_0.979Ba_0.013U_0.001Y_0.005)SO₄. Additionally, the application of Vegard’s law for unit cell parameters consistently aligns with the chemical formulas obtained from Rietveld refinement and X-ray fluorescence analysis, underscoring the reliability of Vegard’s law in predicting compound composition. Distinctive vibrational modes in natural celestine are unveiled through Raman spectroscopy, revealing a composition diverging from typical celestine. The observed shift in Raman bands is ascribed to the incorporation of Ba, U, and Y into the celestine lattice, resulting in a solid solution denoted as Sr_1-x (Ba, U, Y)ₓSO₄. The chemical formula derived through the application of Vegard’s law for ν₁(SO₄) is reported as Sr_0.977 Ba_0.023SO₄. This formula closely corresponds to the formulation presented earlier.