Persistent anoxia and the lack of a skeletal silica sink through the Precambrian would have promoted a variety of reactions between iron and dissolved silica through much of Earth’s early history. However, although both iron and silica have each left clear fingerprints in the Precambrian record, evidence for their interaction, and the attendant biogeochemical consequences, is cryptic. Here, experimental evidence is presented showing that Fe2+ and SiO2(aq) in anoxic seawater–derived solutions promote rapid nucleation of a hydrous Fe(II)-silicate gel at 25 °C. By merging experimental data with crystallographic constraints, we observe that structural rearrangement and dehydration produce Fe-rich serpentine nanoparticles within the gel, which eventally aggregate to form the mineral greenalite. This nonclassical crystal growth pathway is consistent with the crystal structure of greenalite and with its syndepositional origin in iron formation. A mechanistic underpinning for greenalite precipitation also permits new constraints on the chemistry of ferruginous Precambrian waters. For example, greenalite may have nucleated from waters with a pH as high as 7.7–8.3, implicating alkalinity as a key trigger in coupling and decoupling Fe and Si during the anoxic deposition of several late Archean and Paleoproterozoic iron formations. The common, though not exclusive, association of greenalite with deeper-water iron formation facies (i.e., below the fair-weather wave base) suggests that the upwelling of silica-rich alkaline water masses played an important role in driving precipitation. More broadly, our results prompt a reconsideration of the inorganic reactions that determine the upper limits on water-column Fe2+ concentrations in nonsulfidic seawater. The primary precipitation of greenalite and/or siderite would set a ceiling for dissolved Fe2+ that is sensitive to pH, and higher than previously estimated. These results indicate that a better understanding of greenalite distributions in chemical and siliciclastic sediments will help to disentangle the coevolution of redox and acid-base chemistries through the Precambrian.