Author name: Birger Rasmussen

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.

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.