Fe3+/Al3+ partitioning between tetrahedral and octahedral sites in dioctahedral smectites

The 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)x(Al3+)(6)]/[(Fe3+)(6)x(Al3+)(4)]. The Kd((4/6)) value is near 0.006 for natural dioctahedral smectites, formed between 2 and similar to 100 degrees C, and near 0.0174 for smectites synthesized at 200 degrees 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).