Optical micrographs of thin sections of four different H₂ generating rocks (H₂_GR) belonging to categories H₂_GR1-4 showing different mineral assemblages in Cross-Polarized Light (CPL), Plane Polarized Light (PPL), and Reflected Light (RL). (a) a serpentinite (CPL) where olivine was altered into serpentine. (b) a BIF (RL) with a replacement of magnetite into maghemite, (c) a granite with biotite and zircon (PPL), and (d) an organic matter-rich rock (PPL). Bt: biotite, Fsp: feldspar, Mag: magnetite, Mgh, maghemite, OM: organic matter, Ms: muscovite, Ol: olivine, Qz: quartz, Srp: serpentine, Zrn: zircon.