Crystal structure, thermal expansivity, and elasticity of OH‑chondrodite: trends among dense hydrous magnesium silicates

Abstract

We report the structure and thermoelastic properties of OH-chondrodite. The sample was synthesized at 12 GPa and 1523 K, coexisting with hydroxyl-clinohumite and hydrous olivine. The Fe content Fe/(Fe + Mg) is 1.1 mol%, and the monoclinic unit-cell parameters are as follows: a = 4.7459(2) Å, b = 10.3480(7) Å, c = 7.9002(6) Å, α = 108.702(7)°, and V = 367.50(4) Å3. At ambient conditions, the crystal structure was refined in space group P21/b from 1915 unique reflection intensities measured by single-crystal X-ray diffraction. The volume thermal expansion coefficient was measured between 150 and 800 K, resulting in αV = 2.8(5) × 10−9 (K−2) × T + 40.9(7) × 10−6 (K−1) − 0.81(3) (K)/T2, with an average value of 38.0(9) × 10−6 (K−1). Brillouin spectroscopy was used to measure a set of acoustic velocities from which all 13 components (Cij) of the elastic tensor were determined. The Voigt–Reuss–Hill average of the moduli yields for the adiabatic bulk modulus, KS0 = 117.9(12) GPa, and for shear modulus, G0 = 70.1(5) GPa. The Reuss bound on the isothermal bulk modulus (KT0) is 114.2(14) GPa. From the measured thermodynamic properties, the Grüneisen parameter (γ) is calculated to be 1.66(4). Fitting previous static compression data using our independently measured bulk modulus (isothermal Reuss bound) as a fixed parameter, we refined the first pressure derivative of the bulk modulus, KT′ = 5.5(1). Systematic trends between H2O content and physical properties are evaluated among dense hydrous magnesium silicate phases along the forsterite–brucite join.