Elasticity and High-Pressure Compressibility of Apatite

Abstract

Apatite exhibits pronounced compositional variability at the channel anion site, yet experimental constraints on how this variability affects elasticity remain limited. Here we report combined Brillouin light scattering and single-crystal X-ray diffraction measurements of the elasticity of two natural single-crystal fluorapatites (Fap) with distinct channel-anion compositions (Ca9.98Na0.01Sr0.01)(P5.80S0.07Si0.08C0.03)O24F1.96Cl0.05OH0.03 (F1.96-Fap) and (Ca9.84Na0.01Sr0.01)(P5.73S0.10Si0.12Ti0.01C0.04)O24F1.89Cl0.05OH0.07 (F1.89-Fap). Five independent elastic coefficients were determined at ambient conditions and evaluated together with static compression data to constrain both elastic moduli and equation-of-state parameters. All the independent elastic coefficients decrease with increasing non-F channel-anion substitution, with the most pronounced reduction observed in C33. Correspondingly, the bulk modulus shows a measurable decrease, whereas the shear modulus and shear-wave velocity show minor changes within the investigated compositional range, where the adiabatic bulk modulus decreases from 95.8(4) GPa for F1.96-Fap to 93.7(5) GPa for F1.89-Fap, while the shear modulus decreases from 47.7(3) to 47.5(3) GPa. These results suggest that channel-anion chemistry is an important factor influencing the elasticity and anisotropy of apatite. The quantified compositional dependence established here provides a basis for predicting the elastic behavior of apatite-group minerals and has implications for interpreting the mechanical and seismic properties of volatile-bearing phosphates in Earth and planetary interiors.