The substitution of one type of atom in a crystal by another with a similar size, valency, and electronegativity. Isomorphic substitution is very common in minerals and solid solutions, resulting in solids with a wide range of chemical compositions. The substitutions must be in accord with the overall charge neutrality of the solid (see aluminosilicates). In practice, similar size is the most important determining factor in isomorphous substitution. For example, sodium ions are more frequently replaced with calcium ions than with potassium ions. Guidelines for isomorphous substitution have been given by Goldschmidt’s rules for minerals and the very similar Hume-Rothery rules for solid solutions.