Summary: The four principles governing the cation distribution in various octahedral metal sites in the amphibole structure are: Ionic size—the M₄ position prefers large cations like Ca²⁺, Na⁺; charge balance—when the M₄ position is occupied by a monovalent ion, like Na⁺, Li⁺, the neighbouring M₂ position is occupied by a trivalent ion like Al³⁺, Fe³⁺; role of the strongly polarizing cations—cations like Fe²⁺ prefer the M₄ site, since they tend to form covalent bonds with the O₄ oxygens; and the role of the (OH) ions—these form part of the brucite layer and repel large or strongly polarizing cations, which tend to distort the regularity of the sheet structure.

The Al-Si distribution in the two different tetrahedral sites are governed by three principles: relief of the Si-O-Si bond strain—Al³⁺ in the Si₁ site causes fewer Si⁴⁺ cations to share more than two anions with other Si⁴⁺ cations; charge balance—with the introduction of alkali ions in the ‘A’ position, Al³⁺ is introduced in the Si₁ position, since the coordinating oxygens around the A position mostly belong to Si₁; and steric considerations—occupation of the M₄ site by a large cation tends to rotate the Si₂ tetrahedron anticlockwise; Al³⁺ substitution in the Si₁ site rotates the Si₁ tetrahedron clockwise and thus the regularity and the constancy of the repeat distance of the silicate double chain is maintained.

Based on these principles, a scheme of cation distribution in the M₁, M₂, M₃, M₄, A, Si₁, and Si₂ positions in the amphibole structure has been derived for most of the common amphibole varieties, except richterite, whose position is anomalous from a crystallochemical point of view.