containing different cations Edit
Structure of olivine. M (Mg or Fe) = blue spheres, Si = pink tetrahedra, O = red spheres.
In a crystal containing different cations, those of high valency and small coordination number tend not to share polyhedron elements with one another.[8] This rule tends to increase the distance between highly charged cations, so as to reduce the electrostatic repulsion between them.
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.