Calculated role of aluminum in the incorporation of ferric iron into magnesium silicate perovskite

An atomistic computer simulation study was undertaken to address the role of Al in the substitution of ferric iron into magnesium silicate perovskite, MgSiO 3 . Calculated substitution and vacancy energies at 0 K were used to consider two different substitution mechanisms: (a) Fe 3 and Al independently charge balanced by vacancies or by a similar cation in another site, or (b) Fe 3 and Al charge balancing each other. Our results show that it is preferential by 1 to 2 eV for Fe 3 to substitute into an Mg site charge balanced by an Al substitution into an Si site. This is increasingly favorable if the Fe 3 and Al substitute into adjacent sites. The coupled substitution occurs because it is energetically unfavorable for Al to enter the dodecahedral site.