Isomorphism is the phenomenon where minerals have the same crystal structure and specific sites can be occupied by different elements or ions. This is also known as solid solution. There are rules for ionic substitution in minerals: 1) substitution is enhanced at higher temperatures as sites stretch, 2) electrical neutrality must be maintained, and 3) substitution is more extensive if substituting ions are similar in size. Isomorphism types include substitutional, interstitial, and omission solid solutions. Mineral formulas can be calculated from elemental weight percentages using steps to determine molecular, cation, and atomic proportions. Compositions are graphically represented using binary and ternary diagrams.