The origin of greenalite in iron-rich Precambrian sedimentary rocks, and its significance in tracking Earth’s oxygenation, is the subject of vigorous debate. While known as a common mineral of the ~1.88 Ga granular iron formations (GIFs) of the Lake Superior district, North America, greenalite was poorly documented in ferruginous cherts and banded IFs (BIFs) deposited prior to the Great Oxidation Event (GOE) at ~2.4 Ga. The advent of nanoscale electron microscopy revealed greenalite nanoparticles “hidden in plain sight,” previously overlooked in well-preserved, pre-GOE BIFs and ferruginous cherts due to their minute size. Here, we document the occurrence of primary greenalite in ancient anoxic and ferruginous sediments and its decline from the rock record as Earth’s surface and oceans became oxygenated.

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