Ferric iron triggers greenalite formation in simulated Archean seawater

Sedimentary rock deposits provide the best records of (bio)geochemical cycles in the ancient ocean. Studies of these sedimentary archives show that greenalite, an Fe(II) silicate with low levels of Fe(III), was an early chemical precipitate from the Archean ocean. To better understand the formation of greenalite, we explored controls on iron silicate precipitation through experiments in simulated Archean seawater under exclusively ferrous conditions or supplemented with low Fe(III). Our results confirm a pH-driven process promoting the precipitation of iron-rich silicate phases, and they also reveal an important mechanism in which minor concentrations of Fe(III) promote the precipitation of well-ordered greenalite among other phases. This discovery of an Fe(III)-triggering iron silicate formation process suggests that Archean greenalite could represent signals of iron oxidation reactions, potentially mediated by life, in circumneutral ancient seawater.

Automated summary

Sedimentary rock deposits provide detailed records of (bio)geochemical cycles in the ancient ocean, with studies showing that greenalite was an early chemical precipitate from the Archean ocean. Experiments in simulated Archean seawater revealed that a pH-driven process promotes the precipitation of iron-rich silicate phases, while minor concentrations of Fe(III) can also promote the formation of well-ordered greenalite among other phases. This suggests that greenalite in the Archean ocean may represent signals of iron oxidation reactions potentially mediated by life in circumneutral seawater.