Paulings rules

A coordination polyhedral forms around each cation; CN (number of nearest neighbouts) of the cation is deremined by the radius ratio RR=Rc/Ra

The strenth of each electrostatci valence bond reaching a cation = the charge of the cation divided by the number of anions which it coordinated evb= cation charge/ CN

THUS cations that form stronger bonds are small and have a high charge

The existence of edges, and particularly of faces, common to coordination polyhedra decreases the stability of ionic structures.

MINERALS WANT TO AVOIDTHIS AS CATION WILL BE CLOSER TO ONE ANOTHER WHICH IS UNFAVOURABLE

IN structures with differently charged cations, the highly charged cation minimises the number of anions that they share between polyhedra so that the high charged cations can be kept a maximum distance apart

Implication: low charfed cations are more likely to share two or more anions on edges or faces of their coordination polyhedra ( e.g. perovskite (CaTiO₃)

The number of essentially different kinds of constituesnt in a crystal tends to be small