Coupled Substitution

Substitution in which the charges of substituting ions are not same and charge balance is achieved by a second substitution on a different crystallographic site. The most common example in the solid solution series of the plagioclase feldspars: anorthite, CaAl2Si2O8, to albite, NaAlSi3O8. Here, there are two substitutions taking place: Na1+ « Ca2+ (in the A site) and Al3+ « Si4+ (in the tetrahedral site). In order to maintain charge balance these combine to yield:

  • Na1+ + Si4+ ↔ Ca2+ + Al3+

Other examples of common coupled substitutions include:

  • Ca2+ + Mg2+ ↔ Na1+ + Al3+ (in clinopyroxene)
  • 2Al3+ + Mg2+ ↔ 2Fe2+ + Ti4+ (in spinel)

Coupled substitutions may also involve introducing vacancies (symbolized as □) into a crystal structure. For example the following substitution occurs in amphiboles:

  • 0 + Si4+ ↔ Na1+ + Al3+

Some or all content above used with permission from J. H. Wittke.