Elasticity of Amorphous Calcium Carbonate at High Pressure and Its Dependence on the H2O Content: A Brillouin Scattering Study to 20 GPa

Sound velocities and elastic properties of three synthetic samples of amorphous calcium carbonate (ACC), with different water contents up to 18 wt%, have been determined at high pressures, up to 20 GPa, by Brillouin spectroscopy. The experimental results show a smooth pressure-dependence of both longitudinal, vp, and transverse, vs, velocities in compression and decompression. Our results do not show any anomaly at 10 GPa, where a structural phase transition has been reported for a hydrous ACC. Instead, the anomalous behaviour of the isentropic bulk modulus, KS, and its isothermal pressure derivative, KS ', obtained in this study, with respect to the isothermal bulk modulus, KT, suggest that ACC undergoes a continuous structural evolution upon compression. The presence of 18 wt% H2O leads to a lowering of the bulk modulus up to 39% and of the shear modulus up to 38%. The values of the bulk and shear moduli for anhydrous ACC are Ka = 54(2) GPa, and Ga = 22(1) GPa, respectively. If 10 wt% CaCO3 is present in subducted carbonated oceanic crust, the phase transition from aragonite to anhydrous ACC, observed at pressures between 4 and 10 GPa and T>1000 K, would lead to a seismically detectable discontinuity in the acoustic velocities (-2.8% for vp, -4.4% for vs at 4.8 GPa).