Fig. 4: Scanning electron microscopy observations of the alkaline hydrothermal vents (AHV) analogs formed after 48 h, 72 h and 10 days.

A AHV analog formed after 48 h. The cyan area from (A) in (B) presents laminations formed by the alternation of two minerals, as in the 24 h analog (see Fig. 3). C The yellow area from (A) absent from shorter time samples is characterized by a dense, homogeneous texture which induces fractures in the material. D Formation of metallic iron nails starting 48 h. E AHV analog formed after 72 h, showing the same cyan and yellow areas as in (A). Red arrows indicate metallic iron, as shown at higher magnification in (F). G AHV analog formed after 10 days, harboring the cyan and yellow areas as in (A) and (E), with metallic iron nails indicated by red arrows. H Magnetite nanoparticles surrounding metallic iron.