Silicates and other polynuclear complexes

Only when sufficient partners are available for each central ion to realize the maximum coordination does the chemical formula give the correct coordination number. In other cases the coordinated ions will belong to more than one central ion. 

When we also take account of the cations which do not belong to the complex ion, we can formulate the following rule  

When we make as many lines of force start from each cation as correspond to its charge and they are uniformly distributed over the anions around each cation then a divalent oxygen ion receives two of them, a monovalent anion such as fluorine or 

Silicates. The most extensive group of polynuclear iso-complexes is without doubt the silicates. The structural unit is here always the oxygen tetrahedron around the silicon ion at the centre. It has been concluded from X-ray investigation (see p. 26) that the bonding in these groups is certainly for the half ionogenic. On the basis of what has been said above we can give the formulae of a number of silicates in a simple manner. 

In the application to natural minerals some complications occur, however, through multiple mixed crystal formation. Thus, in particular, a part of the Si4+ in the oxygen tetrahedra is often arbitrarily replaced by Al3+ but there is often aluminium as well which does not belong to the tetrahedra of the skeleton. Furthermore a part of the oxygen ions is sometimes present as OH, sometimes isomorphously replaced by F, which is of the same size. The OH and F never form part of the skeleton of oxygen tetrahedra in fact this would be in contradiction with the above formula for zA, which can only be one in this case. In older chemical analyses and the empirical formulae based on them the OH ions were, however, often incorrectly reckoned as water of crystallization. The earlier division into 

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