Substitution

Replacement of one ion or ionic group for another in the same structural site in a mineral yielding a solid solution. Most substitution in minerals is of cations which are smaller and essentially sit in a lattice of oxygen anions. Anionic substitution does occur in halides. Substitutions are classified based on the exact nature of the processes, which include: simple substitution, coupled substitution, and interstitial substitution.

Four factors control whether substitution will occur: (1) ionic radii; (2) ionic charges; (3) temperature; and (4) natures of the bonds formed. Substitution is more likely if the two ions or ionic groups are similar in size. We can calculate the size difference as a percentage:

Substitution is common if sizes are within 15 %, limited if sizes are between 15–30 % and unlikely if sizes differ by >30 %. Similarly, substitution is more likely if the charge difference is 0 or 1; larger differences make substitution unlikely. At higher temperatures, the sites into which the ions must fit are larger (bonds lengthen: thermal expansion) and substitution is easier. Increasing temperature effectively makes the size constraint less rigid, providing an additional ~10% leeway. Lastly, substitution is unlikely if the types of bonds formed by the two ions are very different (essentially a function of electronegativity). The chart below applies to terrestrial rocks.