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The solid solution (Mg,Fe)O is a major phase in Earth's lower mantle, coexisting with silicate perovskite (Mg,Fe)SiO₃ or possibly a pure silica (SiO₂) phase. However physical property data for the magnesiowüstite solid solution is sparse and inconsistent. We have synthesised a suite of single-crystals of (Mg,Fe)O with 6, 15, 27, 36, 56, and 75 mol% total Fe by the interdiffusion of Fe and Mg between magnesiowüstite powders and single-crystal periclase (MgO) at 1450°C and 10^-7 atmospheres oxygen fugacity (see Annual Report 1998). The unit cell parameters of these samples were measured at room pressure by X-ray diffraction and were found to vary linearly with Fe content up to Fe/(Fe+Mg) ˜ 0.40, whereas samples with higher Fe contents fall below the trend (Fig. 3.1-2). Mössbauer spectroscopy reveals that about 12% of the total Fe in these samples is ferric iron, Fe³⁺, compensated by magnesium vacancies, with the smaller unit-cell parameters reflecting this nonstoichiometry.

Fig. 3.1-2: Plot of the cell-parameter against Fe-content for single crystal (Mg,Fe)O.

We selected samples with 27, 56, and 75% total Fe for single-crystal, hydrostatic compression experiments in order to determine how the elastic properties vary with composition. Isothermal volume-pressure data were collected to about 10 GPa in a diamond anvil cell using quartz as an internal pressure standard, and were fit with a three parameter, Birch-Murnaghan third-order equation of state to obtain the bulk modulus of each sample and its pressure derivative. The latter is invariant with composition, but the bulk moduli decrease significantly with increasing Fe content (Fig. 3.1-3) in a non-linear fashion. It is not yet clear whether this non-linear variation is the result of the compositional variation alone, or is influenced by the nonstoichiometry at higher Fe contents.

Fig. 3.1-3: Plot of the bulk-modulus against Fe-content for single-crystal (Mg,Fe)O. The range of common literature values for end-member phases periclase (MgO) and wüstite (Fe1-XO) are indicated by thick solid lines, joined at their mid-points by a dashed line.