Structural Elements of Soil Minerals

A coordinated polyhedron of anions is formed around each cation, the distance being determined by the sum of the ionic radii of both and the coordination number by the radius ratio. 

In a stable coordination structure, the total strength of the valence bonds of an anion with all neighboring cations is eqnal to the anion charge. 

The existence of edges, and particniarly of faces, conunon to the anion polyhedra in a coordinated strncture decreases its stability. This effect is larger for cations with high valency and small coordination number. 

In a crystal containing different cations, those of high valency and small coordination number tend not to share polyhedron elements with each other. 

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

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