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) Å 3 . At ambient conditions, the crystal structure was refined in space group P 2 1 / 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)/ T 2 , 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, G 0 = 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 H 2 O content and physical properties are evaluated among dense hydrous magnesium silicate phases along the forsterite–brucite join.