Identification of Ferrihydrite in Soils by Dissolution Kinetics, Differential X‐ray Diffraction, and Mössbauer Spectroscopy

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Abstract
Ferrihydrite is a poorly crystalline, natural Fe3+ oxide which occurs in ochreous spring precipitates and hydromorphic soils of humid temperate climates. The identification of ferrihydrite in soils is complicated by its association with goethite, quartz, and layer silicates.The following criteria were used to identify ferrihydrite in Fe‐oxide accumulations from soils: high solubility in acid oxalate, five to six broad x‐ray diffraction lines, and the existence of a typical magnetic hyperfine field distribution at 4K in Mössbauer spectra rather than a discrete field value. Identification of low concentrations of ferrihydrite (≲ 20% oxalate‐soluble Fe) by x‐ray diffraction was made possible by subtracting diffraction data obtained after oxalate treatment from data obtained before such a treatment (differential XRD). Oxalate treatment preferentially dissolves ferrihydrite over goethite. This led to an increase in the quadrupole splitting observed in Mössbauer spectra from ∼0.10 to ∼0.23 mm s−1, resulted in a significantly narrower field distribution, and intensified the goethite DTA peak.

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