Isomorphous substitution

Isomorphous substitution is the replacement of one ion by another of similar size within a crystal structure, preserving the host structure while potentially changing its chemistry and charge.

Ionic radius: A measure of the effective size of an ion in a crystal or solution. Similar ionic radii let different ions occupy the same crystallographic site.
Clay mineral: A fine-grained hydrous aluminosilicate mineral with a layered or framework structure. Layered clays commonly host substitutions that generate permanent charge.
Olivine: A silicate mineral group with the general formula (Mg,Fe)₂SiO₄ and an orthorhombic structure. Magnesium and iron substitute extensively on the same sites across the olivine series.
Interstitial defect: A crystal defect in which an extra atom or ion occupies a space between regular lattice sites. Interstitial ions enter empty spaces rather than replacing occupants of crystal sites.
Crystal structure: The ordered three-dimensional arrangement of atoms, ions, or molecules in a crystalline solid. Substitution changes occupants while the host arrangement remains substantially intact.
Cation-exchange capacity: A measure of a material's capacity to retain and exchange positively charged ions. Substitution-generated charge helps determine how many cations a clay can exchange.
Plagioclase: A feldspar mineral series ranging from sodium-rich albite to calcium-rich anorthite. Its coupled sodium-calcium and silicon-aluminum substitutions span a major mineral series.
Vacancy defect: A crystal defect caused by an atom or ion missing from its regular lattice site. A vacancy leaves a site empty instead of filling it with a substitute ion.
Coordination number: The number of nearest-neighbor atoms or ions surrounding a particle in a structure. A substitute must fit the coordination environment of the site it occupies.
Zeolite: A crystalline porous aluminosilicate with a framework containing channels and cavities. Framework substitutions alter zeolite charge and the ions held in its pores.

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