Author name: Nicholas J. Tosca

Over the last decade, high-resolution petrographic examinations of the sedimentary record revealed that greenalite was deposited across several continental margins and throughout many Archean successions. What physical and chemical processes could be responsible for this distribution? Combined sedimentological observations and geochemical results identify and strongly constrain greenalite’s origins in Precambrian sediments, specifically for iron formation deposits. Although greenalite often formed as a pore water or bottom water precipitate, the Precambrian greenalite factory may have resided at the interface between subseafloor hydrothermal vent fluids and anoxic seawater. Once formed, however, greenalite’s stratigraphic distribution was ultimately controlled by its susceptibility to oxidation, a property first recognised by geologists over 120 years ago.

After years of relative obscurity, greenalite is stepping into the limelight. Although first identified in late Paleoproterozoic iron formations over 120 years ago, its true extent has until recently remained hidden due to its minute crystal size and inconspicuous optical properties. In the last decade, nanoparticulate greenalite has become a prime candidate in the deposition of iron formations. Together with experiments and modeling, greenalite is shedding new light on the composition of the early oceans, the role of biology in iron deposition, and H₂
production during serpentinization. While the origin of greenalite is hotly debated, greenalite’s antiquity makes it an invaluable guide into environmental conditions on primordial Earth during the emergence and early evolution of life.