Single crystal elasticity of natural topaz at high-temperatures

Sumudu Tennakoon,Ye Peng,Sergio Speziale,Mainak Mookherjee,Tiglet Besara,Luis Andreu,Fernando Rivera,Geeth Manthilake

doi:10.1038/s41598-017-17856-3

Abstract

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 frac{{boldsymbol{d}}{{boldsymbol{V}}}_{{boldsymbol{P}}}}{{boldsymbol{d}}{boldsymbol{T}}} and frac{{boldsymbol{d}}{{boldsymbol{V}}}_{{boldsymbol{S}}}}{{boldsymbol{d}}{boldsymbol{T}}} 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 ({{boldsymbol{x}}}_{{boldsymbol{O}}{boldsymbol{H}}}) on the velocity is equally important as that of the effect of temperature. We also note that the Debye temperature ({{boldsymbol{Theta }}}_{{boldsymbol{D}}}) 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.

Highlights

IntroductionAs a function of temperature, the sound velocity of topaz decreases with dVP and dVS being −3.10 and −2.30 × 10−4 km/s/K
The shear- elastic constants differ from the first principles calculations[11] by 4%, 13%, and 8% respectively. This is likely related to (a) difference in the temperature, i.e., first principles simulations were at static conditions (0 K) whereas, our results are at room temperatures, (b) the first principles results were for the topaz-OH, i.e., hydroxyl end member whereas our study is conducted on a natural crystal of topaz with enrichment in the fluorine content with a stoichiometry of Al2SiO4F1.42(OH)0.58 and (c), first principles methods often uses approximations that underestimates or off-diagonal components (C12, C13, C23), (c) shear elastic constants (C44, C55, C66), and (d) the aggregate elastic moduli (KH and GH) of topaz experiments on topaz crystal[13] with as a xOH
The sound velocity anisotropy AVP = 11.9 % is smaller than the previous experimental study AVP(topaz-F)[10] by ~0.8 % but it is greater than the AVP11 determined by first principles simulations by ~3.1 %

Summary

Introduction

As a function of temperature, the sound velocity of topaz decreases with dVP and dVS being −3.10 and −2.30 × 10−4 km/s/K. The Debye temperature shows positive correlation with density of the mineral phases in the Al2O3-SiO2-H2O ternary system. Experimental studies on the Al2O3-SiO2-H2O ternary system that represents subducted hydrated sediments revealed end-member topaz-OH (Al2SiO4(OH)2) as a likely phase stable up to a pressure of around ~12 GPa6–9. More recently pressure dependence of the full elastic constant tensor of topaz-OH has been explored using first principles method based on density functional theory[11]. The temperature dependence of a natural topaz has been explored up to 312 K13. We explore the effect of temperature on the full elastic tensor of a natural topaz up to 973 K, temperature relevant for the Earth’s interior

Methods

Results

Discussion

Conclusion