One of Pauling's examples is olivine, M2SiO4, where M is a mixture of Mg2+ at some sites and Fe2+ at others. The structure contains distinct SiO4 tetrahedra which do not share any oxygens (at corners, edges or faces) with each other. The lower-valence Mg2+ and Fe2+ cations are surrounded by polyhedra which do share oxygens.