Iron-Rich Clay Minerals Mediate Abiotic Methane Formation through Distinct Fe Coordination Environments

Methane (CH 4 ) plays a central role in the global carbon cycle, yet the contribution of abiotic processes to natural CH 4 emissions remains poorly constrained, particularly in iron-rich sedimentary environments. Here, we demonstrate that ubiquitous iron-rich clay minerals, including Fe 3+ -exchanged montmorillonite (Fe 3+ -MMT) and ferric nontronite (NAu), catalyze abiotic CH 4 formation from methylated organic substrates under environmentally relevant redox conditions. Both minerals enhance CH 4 formation relative to dissolved Fe 3+ but exhibit contrasting formation kinetics and product selectivity arising from differences in Fe coordination and mineral structure. Fe 3+ -MMT generates rapid CH 4 pulses through interlayer-confined redox cycling, whereas NAu supports slower yet sustained CH 4 formation through gradual activation of structural Fe. Spectroscopic analyses combined with density functional theory calculations reveal that mineral structure regulates Fe(IV)═O generation and stabilizes methyl radicals, thereby suppressing overoxidation. Compared with homogeneous Fe 3+ systems, clay-catalyzed reactions reduced CO/CO 2 formation by 42–62%. CH 4 yields are further modulated by pH, temperature, inorganic cations, and organic ligands. These findings identify iron-rich clays as mineralogical controls on abiotic CH 4 -forming pathways and highlight their potential role in sedimentary carbon cycling and methylated organic compound transformation.