Complete substitution of Fe2+ by Mg in Fe4O5: The crystal structure of the Mg2Fe2O5 end-member

Abstract

The crystal structure of a novel Mg 2 Fe 2 O 5 oxide synthetized at 15 GPa and 1550 °C has been determined by means of single-crystal X-ray diffraction. This compound is isostructural with Fe 4 O 5 and can be considered as the other end-member of a solid solution between these two oxides involving the substitution of Fe 2+ for Mg. The resulting unit-cell lattice parameters a = 2.8889(4), b = 9.7282(4), and c = 12.5523(7) A are smaller than those of Fe 4 O 5 . Mg and Fe 3+ cations are found to be disordered among the three crystallographic sites of the Mg 2 Fe 2 O 5 structure, although preference of Mg for the trigonal prism coordination (M3) is observed. Substitution of Mg into the Fe 4 O 5 structure reduces the octahedral distortion of both the M1 and M2 sites. Like Mg, Cr has recently been found to substitute into Fe 4 O 5 , so that Fe 3+ /∑Fe can vary from 0 to 1.0 in the Mg-Cr-Fe oxides system. Substitution of both Mg and Cr in Fe 4 O 5 also makes this phase more relevant for bulk compositions expected in the Earth’s transition zone and deep upper mantle. M 4 O 5 phases having the CaFe 3 O 5 -type structure, therefore, need to be considered as a new addition to the phase relations of several simple oxide systems at pressure and temperature conditions at which the spinel-structured phases become unstable.