Topaz is an aluminosilicate mineral phase stable in the hydrothermally altered pegmatitic rocks and also in subducted sedimentary lithologies. In nature, topaz often exhibits solid solution between fluorine and hydrous end members. We investigated elasticity of naturally occurring single crystal topaz (Al2SiO4F1.42(OH)0.58) using Resonant Ultrasound Spectroscopy. We also explored the temperature dependence of the full elastic constant tensor. We find that among the various minerals stable in the Al2O3-SiO2-H2O ternary system, topaz exhibits moderate elastic anisotropy. As a function of temperature, the sound velocity of topaz decreases with dVPdT and dVSdT being −3.10 and −2.30 × 10−4 km/s/K. The elasticity and sound velocity of topaz also vary as a function of OH and F content. The effect of composition (xOH) on the velocity is equally important as that of the effect of temperature. We also note that the Debye temperature (ΘD) of topaz at room temperature condition is 910 K and decreases at higher temperature. The Debye temperature shows positive correlation with density of the mineral phases in the Al2O3-SiO2-H2O ternary system.

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Scientific reports