FERRIC IRON CATALYZES THE FORMATION OF IRON-RICH SILICATES UNDER ARCHEAN OCEAN-LIKE CONDITIONS

Microbial life and early bio(geo)chemical cycles evolved in the Archean [4.0 – 2.5 billion years ago (Ga)] ocean. Chemical precipitates from this time, such as the iron- and silica-rich Banded Iron Formations (BIFs), are our best records of marine (bio)geochemical processes. Historically, Fe(III) oxide minerals in BIFs have been interpreted as the primary BIF precipitate, but recent studies propose iron-silicates trapped in well-preserved BIF cherts were the primary Archean minerals from 3.45 to ~2.4 Ga¹. Earlier work characterized these nanoparticle inclusions as a 7Å-layered clay, greenalite [Fe²⁺₃Si₂O₅(OH)₄], with 10-20% Fe(III)². Here, we investigated the potential implications of iron redox reactions on BIF deposition by testing the role of abiotic iron oxidation in catalyzing iron-silicate precipitation. We conducted anoxic laboratory experiments under relevant Archean seawater conditions with high ferrous iron and silica concentrations at pH 6.5, 7, and 7.5 to understand the formation of iron-rich silicates. Under strictly ferruginous conditions, we observed minimal precipitation with just ~7 mg/L of precipitate formed. However, experiments starting with 5% of the iron added as Fe(III) formed substantially more precipitate, 14-31 mg/L, with rising pH leading to increasing precipitate mass and decreasing bulk Fe(III) content. Scanning electron microscopy (SEM) imaging of these low-temperature Fe(II,III) precipitates suggest iron clay formation at pH 7. After hydrothermal crystallization, high-resolution transmission electron microscopy (HRTEM) imaging and electron and X-ray diffraction (XRD) of our experiments revealed that, at pH 6.5 and 7, iron-rich 7Å-layered silicates similar to those found in the rock record formed alongside iron oxides phases such as magnetite and hematite. Our results suggest that iron oxidation, potentially triggered by iron-oxidizing photosynthesis or through interactions with oxygen, may be key for the formation of the iron-silicates found in Archean BIFs.

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