Thermal Equation of State of Natural F-Rich Topaz up to 29 GPa and 750 K

Subducting oceanic sediments and crusts, originating from the Earth’s surface and descending into its deep interior, are important carriers of volatiles. The volatiles have significant effects on materials cycling and the dynamic evolution of the subduction zones. A simplified Al 2 O 3 -SiO 2 -H 2 O (ASH) ternary system models the relationship of minerals in the hydrated and alumina-silica rich sedimentary layer. Topaz Al 2 SiO 4 (F,OH) 2 is an important mineral in the ASH system and comprises two volatiles: H 2 O and fluorine (F). In this study, the thermoelasticity of a natural F-rich topaz was investigated using synchrotron-based single-crystal X-ray diffraction combined with diamond anvil cells up to 29.1 GPa and 750 K. The pressure-volume-temperature data were fitted to a third-order Birch-Murnaghan Equation of state with V 0 = 343.15(7) Å 3 , K 0 = 166(1) GPa, K 0 ′= 3.0(1), ( ∂K 0 / ∂T ) P = −0.015(9) GPa/K and α 0 = 3.9(5) × 10 −5 K −1 . The isothermal bulk modulus increases with the F content in topaz, and the various F contents present significant effects on its anisotropic compressibility. Our results further reveal that the isothermal bulk modulus K 0 of the minerals in ASH system increases with density. F and H contents in hydrous minerals might greatly affect their properties (e.g., compressibility and stability), providing more comprehensive constraints on the subduction zones.