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 (Al1.97SiO4(F-1.56, OH0.42)) at the temperature from 271.5 to 312.7 degrees K by using the sphere resonance method. Elastic constants at an ambient temperature (T = 291.9 degrees K) are C-11 = 281.21(1) GPa, C-22 = 346.23(9) GPa, C-33 = 294.99(9) GPa, C-44 = 108.49(1) GPa, C-55 = 132.47(1) GPa, C-66 = 130.32(1) GPa, C-12 = 121.48(3) GPa, C-13 = 80.94(3) GPa and C-23 = 81.77(2) GPa. Since our sample [Al1.97SiO4(F-1.56, 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 dC(11)/dT = 0.014(3) GPa/degrees K, dC(22)/dT = 0.010(7) GPa/degrees K, dC(33)/dT= 0.021(5) GPa/degrees K, dC(44)/dT = 0.011(1) GPa/degrees K, dC(55)/dT= 0.016(2) GPa/degrees K, dC(66),/dT = 0.0101(2) GPa/degrees K, dC(12)/dT = 0.0041(6) GPa/degrees K, dC(13)/dT = 0.001(2) GPa/degrees K and dC(23)/dT = 0.002(1) GPa/degrees K. The isotropic seismic velocities in topaz are distinctly higher than those in olivine at 10 GPa and 300-1400 degrees 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. (C) 2017 Elsevier B.V. All rights reserved.