Thematic Articles

Clays have long been implicated in the story of life’s origin. This idea gained support when experiments suggested that tiny crystals of acid-preactivated montmorillonite catalyze the growth of prebiotic polymers. From a geological viewpoint, there are good reasons to consider another clay—greenalite (Fe₃Si₂O₅(OH)₄). Model predictions and observations from ancient sedimentary rocks indicate that nanoparticulate greenalite was a major phase produced during hydrothermal venting in ancient oceans and lakes. Greenalite is an iron-rich, redox-active mineral whose modulated crystal structure provides surfaces with repetitive, parallel grooves of the right size and orientation to align and potentially facilitate the assembly of long, linear biopolymers, thereby addressing a significant challenge for prebiotic chemistry—the synthesis of polymers with genetic and catalytic functions essential for life.

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.

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.

The unusual structural properties of the Fe-Mg serpentine minerals permit significant chemical variability, but the mechanisms and extent of elemental substitution have only recently come to light. New results show that greenalite forms solid solutions with the Fe(III) end-member hisingerite, cronstedtite, and Mg-serpentines, with the composition depending on formation conditions. Leveraging this new mineralogical context enables quantitative estimation of H₂ production on Earth and Mars. Together, these advances indicate that greenalite solid solutions in ancient rocks produced and released H₂ and thus contributed to planetary habitability. Examination of Martian rocks and analogous Earth materials shows greenalite-hisingerite minerals were responsible for H₂ fluxes to the ancient Martian atmosphere and could be important contributors to planetary habitability throughout the Solar System.

Greenalite is a chemically simple but structurally complex sheet silicate with a general formula of

Fe²⁺_{(3−x−y−z)}
Fe³⁺_x
Mg_y
□_z
Si₂
O_(3.5+x−2z)
(OH)_(6−x+2z)
.
Originally characterized as a serpentine from X-ray powder diffraction data, detailed interrogation of its structure through electron microscopy has revealed complex yet systematic disorder within tetrahedral-octahedral layers, and disorder in the stacking patterns of those layers along the crystallographic c-axis. These features arise from the misfit in lateral dimensions between oxygens coordinating relatively large Fe²⁺ octahedra and those forming the basal plane of Si tetrahedra, and result in a composition that deviates significantly from that of an ideal serpentine-group mineral. Continued interrogation of greenalite’s structure and chemistry will be fundamental to resolving problems related to its formation and stability in natural systems.

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.