Single-crystal elasticity of celestine at ambient conditions

Natural sulfates, particularly the barite-celestine (Ba, Sr)SO4 solid solution, are important mineral repositories for radionuclides such as 226Ra derived from nuclear waste, making their thermodynamic and mechanical properties a subject of recent research interest. Despite this importance, the elastic properties of celestine (SrSO4) remain poorly constrained with previously reported bulk and shear moduli differing by as much as 47% and 38%, respectively, and individual single-crystal elastic moduli showing even greater discrepancies. To address these uncertainties, we determined the single-crystal elasticity of natural near-endmember celestine using Brillouin spectroscopy. The best-fit single-crystal elastic moduli are: C11 = 105.1(3) GPa, C22 = 96.8(3) GPa, C33 = 124.1(3) GPa, C44 = 15.0(1) GPa, C55 = 40.4(2) GPa, C66 = 29.0(1) GPa, C12 = 51.1(2) GPa, C13 = 47.4(3) GPa, and C23 = 37.8(5) GPa. Voigt-Reuss-Hill averaging of the single-crystal elastic moduli yields an adiabatic bulk modulus (Ks) of 66.2(4) GPa, and a shear modulus (G) of 28(2) GPa. This study reports the most accurate single-crystal elastic properties of celestine under ambient conditions to date, proving new reference values for this technologically and geologically important mineral.