AbstractThe distribution of Al³⁺ and Fe³⁺ between octahedral and tetrahedral sites of dioctahedral smectites was shown to be controlled by a partition coefficient Kd_(4/6) = [(Fe³⁺)₄×(Al³⁺)₆]/[(Fe³⁺)₆×(Al³⁺)₄]. The Kd_(4/6) value is near 0.006 for natural dioctahedral smectites, formed between 2 and ~100°C, and near 0.0174 for smectites synthesized at 200°C. These Kd_(4/6) values, obtained from both chemical and spectroscopic data, were consistent with those calculated using the ionic radii of cations and Brice’s model (Brice, 1975). The partition coefficient approach explained well why for natural and synthetic dioctahedral smectites no tetrahedral Fe³⁺ is detected when the total Fe³⁺ content is below 3 atoms per unit cell (24 oxygen atoms).