Fig. 5: Formation of the alkaline hydrothermal vents (AHV) analogs in the course of time and evolution of the pH in solution.

A Schematic representation of AHV analogs formation in a pH gradient. The dark blue area made of nanocrystalline magnetite is formed after 1 h. The blue area is formed after 5 h, according to magnetite growth. The cyan area formed after 24 h consists of alternating layers of green rust and magnetite, resulting in a laminated structure. Finally, the yellow area is dominated by amakinite and metallic iron nails (in red), surrounded by magnetite. For each area, values of Eh and pH are estimated based on a representative ternary system retrieved from iron Pourbaix diagrams^28,43. B pH monitoring of the alkaline solution (in orange) and of the acidic iron-rich solution (in blue) during the formation of the AHV analogs for 72 h. The error bars represent the standard deviation (SD) over two replicates.