【Objective】Fe(II)-induced transformations of amorphous ferrihydrite to more crystalline iron oxide phases is a widely occurring geochemical process in soils under reduced conditions, and play an important role in regulating the biogeochemical processes of nutrient elements and pollutants. As one of the main species of Fe(II) in soils, Fe(II)-containing clay minerals are ubiquitous in soils under reduced conditions. However, the catalytic properties of structural Fe(II) in clay minerals for ferrihydrite transformation and its influencing factors are still not fully understood.【Method】In this study, the transformation of ferrihydrite induced by the structural Fe(II) in reduced montmorillonite (rSWy-2), which was produced by a chemical method, were investigated at neutral pH under anoxic conditions. Also, the influencing factors including types of cations and anions, organic matter, and As(III) on the transformation were studied.【Result】The X-ray diffraction (XRD) and chemical extraction analyses results showed that the structural Fe(II) in rSWy-2 with low Fe content can catalyze the transformation from ferrihydrite to more crystalline lepidocrocite, and 83.3% of initial ferrihydrite converted to lepidocrocite after mixing reaction for 96 h. XRD, high-resolution transmission electron microscope (HRTEM), scanning transmission electron microscopy (STEM) and surface adsorbed Fe(II) content analyses showed that the processes of ferrihydrite transformation induced by the structural Fe(II) in rSWy-2 mainly included three stages: Firstly, positively charged ferrihydrite nanoparticles adsorption on negatively charged rSWy-2 surface through electrostatic interaction. Secondly, interfacial electron transfer from the structural Fe(II) in rSWy-2 to the adsorbed ferrihydrite and partly reducing Fe(III) to surface adsorbed Fe(II). Finally, the surface adsorbed Fe(II) catalyzed the transformation of ferrihydrite to more crystalline lepidocrocite phases. HRTEM analyses showed that the formed lepidocrocite phases presented nanoplates with a size range of 100～200 nm. XRD and chemical extraction analyses results showed that Na⁺ and Cl^- ions in the mineral suspension had a weak effect on the catalyzed transformation from ferrihydrite to lepidocrocite phases by the structural Fe(II) in rSWy-2. In contrast, Ca²⁺, SO₄^2-, organic matter, and As(III) all had obvious inhibition on the transformation due to their strong interactions with minerals.【Conclusion】The structural Fe(II) in rSWy-2 could catalyze the transformation of ferrihydrite to lepidocrocite at neutral pH under anoxic conditions, and coexisting divalent cations and anions, As(III), and organic matter could inhibit the transformation reaction. The results provide a theoretical basis for further understanding the role of iron-bearing clay minerals in regulating abiotic transformations of iron oxides in soils under anaerobic reduction conditions.