Replacement 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 cations or anions replacing one another have the same charge and similar radii (within ~15%). Simple substitutions may result in complete or partial Compositional 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. The following simple substitutions are commonly complete: Fe2+ (0.78 Å) ↔ Mg2+ (0.72 Å), Fe2+ (0.78 Å) ↔ Mn2+ (0.83 Å), and Br- (1.96 Å) ↔ Cl- (1.81 Å). In contrast, the following substitutions are usually partial because they significantly differ in ionic size: Na+ (1.18 Å) ↔ K+ (1.51 Å), Mg2+ (0.72 Å) ↔ Mn2+ (0.83 Å), I- (2.20 Å) ↔ Cl- (1.81 Å).