In the document Historical Individuum, Here-and-now Individuum in the Critical Series of documents in First Part of Website , we discussed the phenomenon of substitution in crystals and what this phenomenon metaphysically means.
Here we reproduce the conclusion of that discussion :
When, in crystallization, at some moment in time, substitution sets in, the space group symmetry will remain the same, because substitution is only possible if the new atoms are very similar to the original ones. The crystal lattice should not 'be aware' that from that moment on many of its lattice points are now occupied by a new species of atom. So the Space Group remains the same. Only the chemical composition changes. But because this chemical change is just a substitution of atoms by similar atoms, we can, in a way, say that the chemical composition did not change at all. This (way of expressing ourselves) can be accomplished by including into the specification of the chemical composition of a given crystal the equivalence of several different atomic species, indicating the fact that they can easily substitute for each other.
Let us be more concrete by using a definite example : The crystallization of Alum from an aquous solution. Here, it is known that chromium atoms can take the place of aluminum atoms (that is, they can occupy lattice points where normally aluminum atoms should reside). Consequently there are two alums :
Potash Alum = KAl(SO4)2 . 12H2O and
Chrome Alum = KCr(SO4)2 . 12H2O .
The chemical composition of any given Alum crystal -- whether potash alum, chrome alum, or a mixture of them -- can be indicated by the following single formula :
K(Al,Cr)(SO4)2 . 12H2O .
This formula indicates the interchangeability of Al and Cr.
When, during crystallization of alum, Al (aluminum) is substituted by Cr (chromium), the Space Group remains the same (because both Al and Cr are atoms, and will therefore both be more or less spherical. So no change in motif symmetry is to be expected), but (now) the chemical composition also remains the same, namely K(Al,Cr)(SO4)2 . 12H2O.
Indeed, when alum crystallizes, it remains the same individual, (the same) also qua intelligible content, whether or not substitution takes place.