Elasticity of natural aragonite samples by Brillouin spectroscopy

The single-crystal elastic moduli of three natural aragonites, CaCO₃, have been determined at ambient conditions by Brillouin spectroscopy. The samples contain different amounts of Sr ranging from 0.1 mol% to 1.5 mol% and cover the majority of the compositional range of natural aragonites. Brillouin spectroscopy can resolve changes in elasticity produced by small differences in the Sr content in aragonite, however, these changes are at the limit of the resolving power of our measurements. Our results are in good agreement with the full tensor of a Sr-bearing natural aragonite determined by Brillouin spectroscopy and with the tensor obtained by DFT calculations. The decrease we measured of the aggregate adiabatic bulk modulus, \(K_S\), with increasing Sr content is qualitatively in good agreement with the softening observed in previously measured isothermal bulk modulus, \(K_T\), of synthetic CaCO₃-SrCO₃ solid solutions and with the value of \(K_S\) determined for SrCO₃. Our study provides the first full tensor on nominally pure (end-member) aragonite and places contraints on its dependence on Ca/Sr substitutions at levels observed in natural samples. Furthermore, we compare the elastic tensor of aragonite and calcite, the two main CaCO₃ polymorphs observed at ambient conditions, and describe their differences based on the arrangement of the polyhedral structural units of the two polymorphs. In particular, the different arrangement of the Ca-O polyhedra leads to a smaller compressibility of aragonite along the c-axis, higher compressibilities along the a- and b-axes and an overall lower bulk modulus (\(-\)9.7%) with respect to that of the less dense calcite.