Coupled substitution

Coupled substitution is the geological process by which two elements simultaneous substitute into a crystal in order to maintain overall electrical neutrality and keep the charge constant. In forming a solid solution series, ionic size is more important than ionic charge, as this can be compensated for elsewhere in the structure.

To make a geometrically stable structure in a mineral, atoms must fit together in terms of both their size and charge. The atoms have to fit together so that their electron shells can interact with one another and they also have to produce a neutral molecule. For these reasons the sizes and electron shell structure of atoms determine what element combinations are possible and the geometrical form that various minerals take. Because electrons are donated and received, it is the ionic radius of the element that controls the size and determines how atoms fit together in minerals.

For example, when a plagioclase feldspar solid solution series forms, albite (Na Al Si₃O₈) can change to anorthite (Ca Al₂Si₂O₈) by having Al³⁺
replace Si⁴⁺
. However, this leaves a negative charge that has to be balanced by the (coupled) substitution of Ca²⁺
for Na⁺
.