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Polycrystalline ice formation concentrates mineral nanoparticles into liquid boundaries between growing ice crystals. Here we show that minutes of freezing dictate iron mineral fate over subsequent months of aqueous aging. A single freeze–thaw cycle irreversibly aggregates ferrihydrite through converging physical and chemical mechanisms. Freeze concentration collapses electrostatic barriers while cryosuction strips hydration layers and compresses nanoparticles into micrometer-scale planar aggregates. Chemical evidence points to interfacial (hydr)oxo bridging, alongside hydrogen bonding, that resists disaggregation. These mechanisms lock nanoparticles into mesocrystal-like assemblages that retain their nanoscale identity but inhibit dissolution–reprecipitation to goethite, instead favoring solid-state transformation to hematite. Ice formation thus acts as a geochemical reactor, driving aggregation and interfacial bonding that redirect iron speciation, with broad implications for nutrient cycling and carbon preservation across the cryosphere.