Atomic Sizes are Suitable

When crystallization occurs when stone melts solidify, aggregates of solid matter form with a three-dimensional structure. Atoms and ions of rare metals are caught in this growing crystal structure if, and only if, their sizes are suitable. Atoms that are too big or too small remain in the melt. The aluminum/gaUium pair is a classic example. The ion Ga, with a radius of 0.61 A, is easily hidden in a structure in which 

A1 ions, radius 0.53 A, predominate. All aluminum minerals contain up to 100 g/tonne of gallium, and gallium is found with aluminum in all industrial processes. This explains why gallium is, in fact, common but is perceived as rare. 

If two ions have the same radius and the same charge, they will enter into a crystallizing mineral with equal ease. 

If two ions have similar but unequal radii and the same charge, the smaller ion forms a stronger bond and will preferentially be concentrated in the early formed parts of a crystallizing mineral. However, extensive substitution does not occur between elements whose radii differ by more than 15%. 

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