Site occupancies by iron in nontronites

Twelve nontronites and two ferruginous smectites have been characterized with respect to Fe3+ 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 Fe3+ in tetrahedral sites. The nontronite from Spokane County, Washington, (similar to44% Fe2O3) is essentially an end-member with its non-siliceous tetrahedral sites occupied by Fe3+. Samples with chemical compositions similar to Garfield nontronite (similar to36.5% Fe2O3) may have small amounts (<5% of total Fe3+) of tetrahedral Fe3+. Tetrahedral Fe3+ is unlikely to be present in samples containing less than similar to 34% Fe2O3.