Redox Cycling Driven Transformation of Layered Manganese Oxides to Tunnel Structures.

Mn oxides are among the most ubiquitous minerals on Earth and play critical roles in numerous elemental cycles in biotic/abiotic loops as the key redox center. Yet, it has long puzzled geochemists why the laboratory synthesis of todorokite, a tunnel-structured Mn oxide, is extremely difficult while it is the dominant form over other tunneled phases in low-temperature natural environments. This study employs a novel electrochemical method to mimic the cyclic redox reactions occurring over long geological time scales in an accelerated manner. The results revealed that the kinetics and electron flux of the cyclic redox reaction are key to the layer-to-tunnel structure transformation of Mn oxides, provided new insights for natural biotic and abiotic redox reactions, and explained the dominance of todorokite in nature.