Compression data for iron were collected using X-ray diffraction on samples under hydrostatic pressure. Stress inhomogeneity effects were eliminated by the use of an encapsulated liquid pressure medium. Pressures were determined on the basis of both NaCl and ruby fluorescence methods; no significant difference was observed when both were used at the same time. Compression data for a gasketed sample of γ–Fe2SiO4 + NaCl were also reported. Comparisons of both data sets were then made with existing hydrostatic and non-hydrostatic data for materials compressed in the diamond cell. The bulk modulus (K0) of iron reported here was found to be 1.64±0.07 Mbar if the pressure derivative of bulk modulus (K0) was set at 4.0. The difference observed between bulk moduli calculated from hydrostatic and nonhydrostatic data is attributed to the presence of stress anisotropy in the latter case, and is apparently related to the shear strength of the sample. The discrepancies between the bulk moduli obtained under hydrostatic and non-hydrostatic conditions for α–Fe, α–Fe2SiO4, γ–Fe2SiO4, Fe2O3, Fe3O4, and SiO2 appear to increase with increasing shear strength.

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