Coupled Substitution

SubstitutionReplacement 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 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 solutionCompositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with series of the plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite feldspars: anorthiteRare compositional variety of plagioclase and the calcium end-member of the plagioclase feldspar mineral series with the formula CaAl2Si2O8. Anorthite is found in mafic igneous rocks such as anorthosite. Anorthite is abundant on the Moon and in lunar meteorites. However, anorthite is very rare on Earth since it weathers rapidly, 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 spinelMg-Al oxide, MgAl2O4, found in CAIs.)

Coupled substitutions may also involve introducing vacancies (symbolized as □) into a crystal structureMutual arrangement of atoms, molecules or ions that are packed together in a crystal lattice to form a crystal.. For example the following substitution occurs in amphiboles:

• □⁰ + Si⁴⁺ ↔ Na¹⁺ + Al³⁺