Sound velocities in shock-synthesized stishovite to 72 GPa

Stishovite (rutile-type SiO2) is the archetype of dense silicates and may
occur in post-garnet eclogitic rocks at lower-mantle conditions. Sound
velocities in stishovite are fundamental to understanding its mechanical
and thermodynamic behavior at high pressure and temperature. Here, we use
plate-impact experiments combined with velocity interferometry to
determine the stress, density, and longitudinal sound speed in stishovite
formed during shock compression of fused silica at 44 GPa and above. The
measured sound speeds range from 12.3(8) km/s at 43.8(8) GPa to 9.8(4)
km/s at 72.7(11) GPa. The decrease observed at 64 GPa reacts a decrease in
the shear modulus of stishovite, likely due to the onset of melting. By 72
GPa, the measured sound speed agrees with the theoretical bulk sound speed
indicating loss of all shear stiffness due to complete melting. Our sound
velocity results provide direct evidence for shock-induced melting, in
agreement with previous pyrometry data.