Ferrihydrite (Fhy) is the initial Fe-oxyhydroxide in supergene soils, and its structural instability is affected by many environmental factors. However, little attention has been paid to how these Fe-oxyhydroxide trans-formations are induced by Fe2+ cations when they combine with clay minerals, especially organoclay minerals, in soil solutions. This study investigated Fhy transformation in the presence of Fe2+ with and without the clay minerals montmorillonite (Mnt) and organo-Mnt (OMnt) under anaerobic conditions at pH 6.5. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and Mossbauer spectroscopy were used to characterize the Fhy phase transformation. The results showed that goethite (Goe), lepidocrocite (Lep), and hematite (Hem) were the main products in the pure Fhy system. While Goe and Lep were the main transformation products, and Hem was not found in the presence of Mnt and OMnt. Fhy transformation was due to Fe2+ adsorption, followed by atomic exchange and electron transfer between Fe2+ and structural Fe(III) in Fhy. The presence of Mnt and OMnt dispersed Fhy particles, and some even combined with Fhy. Particularly, the presence of organic matter increased the dispersion of Fhy to a greater extent and occupied the interlayer sites of Mnt, which thus reduced the specific surface area and the Fe2+ adsorption ca-pacity. Moreover, the Fe2+ adsorption sites, electronic transfer pathway, and the atomic exchange level were also affected by the presence of Mnt and OMnt, which further influenced the transformation products of Fhy accordingly. The presence of Mnt was more conducive to the formation of Lep, while OMnt was more conducive to the formation of Goe. These findings provide new insights into the stability of Fhy in clay-based anaerobic environments as well as the geochemical behavior of Fe in response to Fe cycling.

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