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

Methane (CH4) plays a central role in the global carbon cycle, yet the contribution of abiotic processes to natural CH4 emissions remains poorly constrained, particularly in iron-rich sedimentary environments. Here, we demonstrate that ubiquitous iron-rich clay minerals, including Fe3+-exchanged montmorillonite (Fe3+-MMT) and ferric nontronite (NAu), catalyze abiotic CH4 formation from methylated organic substrates under environmentally relevant redox conditions. Both minerals enhance CH4 formation relative to dissolved Fe3+ but exhibit contrasting formation kinetics and product selectivity arising from differences in Fe coordination and mineral structure. Fe3+-MMT generates rapid CH4 pulses through interlayer-confined redox cycling, whereas NAu supports slower yet sustained CH4 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 Fe3+ systems, clay-catalyzed reactions reduced CO/CO2 formation by 42-62%. CH4 yields are further modulated by pH, temperature, inorganic cations, and organic ligands. These findings identify iron-rich clays as mineralogical controls on abiotic CH4-forming pathways and highlight their potential role in sedimentary carbon cycling and methylated organic compound transformation.