In crystals of silicates and aluminates the distribution of aluminum in the centers of tetrahedra is not entirely at random, but must obey certain restrictions not easily understood on the basis of the Pauling electrostatic valence rule. Whenever two tetrahedra are linked by one oxygen bridge, the center of only one of them can be occupied by aluminum; the other center must be occupied by silicon, or another small ion of electrovalence four or more, e.g. phosphorus. Likewise, whenever two aluminum ions are neighbours to the same oxygen anion, at least one of them must have a coordination number larger than four, that is, five or six, towards oxygen. These rules explain the maximum substitution of 50% of the silicon in three-dimensional frameworks and plane networks of tetrahedra by aluminum. For 50% substitution, rigorous alternation between silicon and aluminum tetrahedra becomes necessary; this explains the experimental fact that the unit cell of anorthite has double the size of that of albite. Agreement of several known structures with these rules is shown. One exception is explained.

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