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

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 α_V = 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 (C_ij) of the elastic tensor were determined. The Voigt–Reuss–Hill average of the moduli yields for the adiabatic bulk modulus, K_S0 = 117.9(12) GPa, and for shear modulus, G₀ = 70.1(5) GPa. The Reuss bound on the isothermal bulk modulus (K_T0) 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, K_T′ = 5.5(1). Systematic trends between H₂O content and physical properties are evaluated among dense hydrous magnesium silicate phases along the forsterite–brucite join.