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An important concept related to the formation of coordination polyhedra is electrostatic valency (EV). In a stable coordination structure, the total strength of all the bonds that reach a cation from all neighboring anions is equal to the charge on the cation. This is another way of saying that the positive charge on the cation is neutralized by the electrostatic component of the bonds between it and its nearest neighbor anions. Similarly, every anion in the structure is surrounded by some number of nearest neighbor cations to which it is bonded, and the negative charges on each anion are neutralized by the electrostatic component of the bonds between it and its nearest neighbor cations. For a cation of charge Z bonded to a number of nearest neighbor anions (CN), the electrostatic valency of each bond is given by the charge of the cation divided by the number of nearest neighbors to which it is coordinated:

For example, in the case of the silica tetrahedron (SiO₄)⁻⁴ each Si⁺⁴ cation is coordinated with four O⁻² anions (Figure 2.20). The electrostatic valency of each bond is given by EV = Z/CN = +4/4 = +1. What this means is that each bond between the coordinating silicon ion (Si⁺⁴) and the coordinated oxygen ions (O⁻²) balances a charge of +1. Another way to look at this is to say that each bond involves an electrostatic attraction between ions of opposite charge of one charge unit. Since there are four Si–O bonds, each balancing a charge of +1, the +4 charge on the silicon ion is fully neutralized by the four nearest neighbor anions to which it is bonded. However, although the +4 charge on the coordinating silicon ion is fully satisfied, the −2 charge on each of the coordinated ions is not. Since each has a −2 charge, a single bond involving an electrostatic attraction of one charge unit neutralizes only half their charge. They must attract and bond to one or more additional cations, with an additional total electrostatic valency of one, in order to have their charges effectively neutralized. So it is that during mineral growth, cations attract anions and anions attract additional cations of the appropriate charge and radius which in turn attract additional anions of the appropriate charge and radius as the mineral grows. In this manner minerals retain their essential geometric patterns and their ions are neutralized as the mineral grows. In the following section we will introduce the major mineral groups and see how their crystal chemistry forms the basis of the mineral classification.

Figure 2.19 Common coordination polyhedra: (a) cubic closest packing, (b) cubic, (c) octahedral, (d) tetrahedral, (e) triangular, (f) linear.

Source: Wenk and Bulakh (2004). © Cambridge University Press.

Table 2.6 Variations in ionic radius (in angstroms) with coordination number (CN) for some common cations.

Ion	CN = 4	CN = 6	CN = 8
Na+1	0.99	1.02	1.18
K+1		1.38	1.51
Rb+1		1.52	1.61
Cs+1		1.67	1.74
Mg+2	0.57	0.72
Al+3	0.39	0.48
Si+4	0.26	0.40
P+5	0.17	0.38
S+6	0.12	0.29