Understanding aquatic chemistry, the natural cycling and fractionation of elements, and energy flow in soils almost always involves, by necessity, attention to the governing influence of electron transfer processes. Interaction of aqueous Fe(II) with Fe(III)-(oxyhydr)oxide minerals yields the enigmatic "sorbed Fe(II)" species and a rapid underlying redox exchange of soluble and insoluble Fe atoms that directly controls iron mineral transformations, metal incorporation, and diverse biogeochemical processes. Capitalizing on team momentum that unveiled the role of electrical conduction underlying goethite (Gt) / hematite (Hm) recrystallization, we propose atomic-to-mesoscale research on the challenging topic of isolating electron and mass transfer pathways that control rates and products of Fe(II)-catalyzed transformation of ferrihydrite (Fh) to more crystalline phases. We will connect macroscopic Fe isotope exchange measurements, in situ XRD, and Mossbauer measurements to microscopic atom exchange fronts probed on individual particles/crystallites by APT and NanoSIMS; we will visualize and understand coupled dissolution, nucleation, and growth processes using quasi in situ HRTEM on individual transforming particles; and we will unveil the residence, local structure, and role of sorbed Fe(II) over time by developing AIMD-informed site-selective EXAFS for sorbed Fe(II). The research plan integrates EMSL experimental and computational resources to unravel for the first time the microscopic basis for directed nucleation of lepidocrocite (Lp) and goethite (Gt), and the orders of magnitude accelerating effect of Fe(II). This topic lies at the heart of micronutrient metal uptake or occlusion by the mineral fraction and the overall bioavailability of iron that can define biogeochemical hotspots in soils and sediments.