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 modu-lus, 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 ther-modynamic properties, the Grüneisen parameter (γ) is calculated to be 1.66(4). Fitting previous static compression data using our independently measured bulk modulus (iso-thermal 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 sili-cate phases along the forsterite–brucite join. Keywords Chondrodite · Dense hydrous magnesium silicates · Crystal structure · Thermal expansivity · Bulk modulus · Grüneisen parameter