An atomistic computer simulation study has been undertaken to determine the energies associated with different mechanisms of Fe3+ incorporation into forsterite and wadsleyite, and to determine why Fe3+ substitutions into some mantle minerals (such as forsterite) are less favorable than in others. We have also compared our results to previous results obtained for perovskite. In all three phases, the most favorable substitution mechanism is Fe3+ entering both the Mg and Si sites. In forsterite, the energy of incorporation is 2 eV less favorable than the same mechanism in perovskite and 1 eV less favorable than in wadsleyite. These differences are due to significantly different energies for the substitution of Fe3+ into the Si site in each of these silicates. This substitution was most favorable in perovskite (octahedral site), less favorable in wadsleyite (tetrahedral site) and least favorable in forsterite (also tetrahedral). The energy difference between forsterite and wadsleyite was found to be the result of structural effects. The linked tetrahedra in wadsleyite are able to distort more easily to accommodate Fe3+ than the isolated SiO4 tetrahedra in forsterite.