Elasticity of phase-X at high pressure

Using a plane-wave pseudopotential method, we calculate the equation of state and elasticity of phase-X [(K,Na)(2)Mg2Si2O7] up to 30 GPa, which encompasses its experimentally observed stability field. We find that the pressure-volume results for anhydrous phase-X are well represented by a third order Birch-Murnaghan formulation, with K-0 = 132 GPa, K'(0) = 4.4 and V-0 = 284.6 angstrom(3) for the potassium (K) end member and K-0 = 128 GPa, K'(0) = 4.0 and V-0 = 265.9 angstrom(3) for the sodium (Na) end member. The full elastic constant tensor reveals significantly larger stiffness along the [001] direction than in the (100) plane. We attribute such stiffness to the 180(0) angle of Si-O-Si units and their orientation parallel to the [001] direction. Assuming ideal behavior between Na and K phase-X, the shear modulus softens with sodium content as G = [84 - 0.14 x Na(wt%)] GPa. Citation: Mookherjee, M., and G. Steinle-Neumann (2009), Elasticity of phase-X at high pressure, Geophys. Res. Lett., 36, L08307, doi: 10.1029/2009GL037782.