Determination of elastic constants of a single-crystal topaz and their temperature dependence via sphere resonance method

•Elastic constants of a single-crystal topaz were determined.•All the elastic constants decrease linearly with increasing temperature.•Seismic velocities in topaz, olivine and garnet are compared at 10 GPa.•Topaz will show distinctly high velocity at the slab-mantle interface.

Water and halogens in ocean floor sediments transported by a descending slab might play important roles in geodynamic processes. Imaging subducted sediments through seismological observations requires a thorough understanding of elastic properties of sediment origin hydrous minerals. Topaz is a sediment origin hydrous mineral, which is formed at the depth of 250-350 km on a cold subducting slab. We determined elastic constants and their temperature derivatives of a natural single-crystal of topaz (Al1.97SiO4(F1.56, OH0.42)) at the temperature from 271.5 to 312.7 °K by using the sphere-resonance method. Elastic constants at an ambient temperature (T = 291.9 °K) are C11 = 281.21(1) GPa, C22 = 346.23(9) GPa, C33 = 294.99(9) GPa, C44 = 108.49(1) GPa, C55 = 132.47(1) GPa, C66 = 130.32(1) GPa, C12 = 121.48(3) GPa, C13 = 80.94(3) GPa and C23 = 81.77(2) GPa. Since our sample [Al2SiO4(F1.56,OH0.42)] was relatively rich in fluorine, only small differences in elastic constants can be seen between our sample and fluorine end member. Elastic constants of OH-rich topaz should be experimentally investigated to understand the influence of F-OH substitution on elasticity of topaz. All the elastic constants decrease linearly with increasing temperature. The temperature derivatives are dC11/dT = −0.014(3) GPa/°K, dC22/dT = −0.010(7) GPa/°K, dC33/dT = −0.021(5) GPa/°K, dC44/dT = −0.011(1) GPa/°K, dC55/dT = −0.016(2) GPa/°K, dC66/dT = −0.0101(2) GPa/°K, dC12/dT = −0.0041(6) GPa/°K, dC13/dT = −0.001(2) GPa/°K and dC23/dT = −0.002(1) GPa/°K. The isotropic seismic velocities in topaz are distinctly higher than those in olivine at 10 GPa and 300-1400 °K. There should be a strong velocity contrast between the overlying mantle and the thin sediment-origin layer at the depth around 300 km. A seismological technique like the receiver function technique should be applied to detect a thin layer of topaz in a cold subduction zone.