Site occupancies by iron in nontronites

Twelve nontronites and two ferruginous smectites have been characterized with respect to Fe 3+ occupancy of tetrahedral sites. The techniques used were near infrared, Fe-K X-ray absorption near-edge and X-ray absorption fine-structure spectroscopies, along with two X-ray diffraction techniques. The results show that calculations of the structural formulae of many nontronites should be adjusted to include Fe 3+ in tetrahedral sites. The nontronite from Spokane County, Washington, (∼44% Fe 2 O 3 ) is essentially an end-member with its non-siliceous tetrahedral sites occupied by Fe 3+ . Samples with chemical compositions similar to Garfield nontronite (∼36.5% Fe 2 O 3 ) may have small amounts ( < 5% of total Fe 3+ ) of tetrahedral Fe 3+ . Tetrahedral Fe 3+ is unlikely to be present in samples containing less than ∼34% Fe 2 O 3 .