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

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 (Al_1.97SiO₄(F_1.56, OH_0.42)) 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(F_1.56,OH_0.42)] 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.