The effect of iron concentration on the compressibility of (Fe,Mg)(Al,Si)O3 perovskite.

Recent studies have indicated that the substitution of Al into (Mg,Fe)(Al,Si)O3 perovskite is coupled to ferric Fe and that a significant portion of the Fe in the lower mantle perovskite is, therefore, likely to be in the ferric state. The substitution of trivalent cations into silicate perovskite can occur either by coupled substitution onto both Mg and Si sites or by substitution onto the Si site charge balanced by the creation of an oxygen vacancy. The results of previous studies are inconclusive as to the effect of these substitutions on elastic properties. To resolve these issues we have investigated the effect of bulk iron concentration on equation of state properties of Al bearing magnesium silicate perovskite using single crystal x-ray diffraction in a diamond anvil cell. Three single crystals with the compositions, (Mg2+0.93,Fe3+0.034,Fe2+0.035,Al3+0.012)(Si4+0.89,Al3 +0.096)O2.96 (Mg2+0.872,Fe3+0.088,Fe2+0.042,Al3+0.007)(Si4+0.892,Al 3+0.108)O3 (Mg2+0.782,Fe3+0.192,Fe2+0.048,Al3+0.031)(Si4+0.863,Al {3+}0.137)O2.95 were synthesized using a multianvil apparatus at 25 GPa and 2000°C. Ferric iron concentrations were determined using Mössbauer and Electron energy loss spectroscopic techniques. The single crystals were compressed to 9 GPa at ambient temperature. The compression data were fitted to a third order Birch Murnaghan equation of state. The bulk modulii of the three crystals decreases with increasing Fe content from 243(3) GPa to 240(2) GPa to 234(2) GPa. In addition, increasing the Fe content raises the K'.