Single-crystal elasticity of diaspore, AlOOH, to 12 GPa by Brillouin scattering

The high-pressure elasticity of diaspore (AlOOH) has been determined by Brillouin spectroscopy to 12 GPa in diamond anvil cells. Experiments were carried out using a 16:3:1 methanol-ethanol-water mixture as pressure medium, and ruby as pressure standard. Acoustic velocities were measured in three roughly orthogonal planes at ambient and eight elevated pressures. The nine individual elastic stiffness constants of the orthorhombic crystal were obtained by fitting the velocity data to Christoffel's equation. Aggregate elastic moduli and pressure derivatives were calculated from the C_ijs by fits to Eulerian finite strain equations, yielding: K_S0 = 152 (1) GPa, G₀ = 117.2(5) GPa, (∂ K_S / ∂ P)_T0 = 3.7 (1), (∂ G / ∂ P)₀ = 1.5 (1) for the Voigt-Reuss-Hill average. All individual C_ijs increase with pressure but C₂₃ and C₅₅ exhibit anomalously low pressure derivatives. From calculated linear compressibilities, the a-axis is the most compressible. The b-axis becomes the least compressible axis at high pressures. Over the examined pressure range, the azimuthal P-wave anisotropy decreased from 22% to 16%, while the azimuthal S-wave anisotropy increased from 15% to 21%. Both volume and axial compression curves calculated using our Brillouin results are in good agreement with the results from static compression studies. High-pressure sound velocities in diaspore exceed those of other hydrous minerals as well as many anhydrous phases relevant to Earth's upper mantle.