The electronic properties of two spinels Fe3O4 and Fe2SiO4 are studied by the density functional theory. The local Coulomb repulsion U and the Hund exchange J between the 3d electrons on iron are included. For U = 0, both spinels are half-metals, with the minority t2g states at the Fermi level. Magnetite remains a metal in a cubic phase even at large values of U. The metal-insulator transition is induced by the X3 phonon, which lowers the total energy and stabilizes the charge-orbital ordering. Fe2SiO4 transforms to a Mott insulating state for U > 2 eV with a gap δg ∼ U. The antiferromagnetic interactions induce the tetragonal distortion, which releases the geometrical frustration and stabilizes the long-range order. The differences of electronic structures in the high-symmetry cubic phases and the distorted low-symmetry phases of both spinels are discussed.