Carboxyl-stabilized Mn redox cycling promotes a metastable kutnahorite-to-dolomite pathway

Abstract

Fine-crystalline, fabric-preserving dolostones in deep-time successions are difficult to reconcile with high-temperature burial models, suggesting the existence of a low-temperature formation pathway capable of overcoming both the kinetic hydration barrier of Mg2+ and the thermodynamic miscibility gap separating calcite from ordered dolomite. Here, we demonstrate a kinetically favourable route to self-assembling dolomite driven by the synergy of manganese redox cycling and carboxyl functionalization. Using a bio-inspired electrochemical reactor, we show that electrochemical valence-state modulation selectively regulates Mn2+ co-precipitation with dolomite reactants. Unlike inorganic controls where manganese is rapidly sequestered into non-templating phases, the functionalized system transiently stabilizes reactive Mn(III) intermediates. This sustains redox cycling and prevents irreversible oxide immobilization, which templates the nucleation of spheroidal, metastable magnesian-kutnahorite. Nanostructural characterization reveals a core–shell architecture where this metastable, isostructural precursor serves as a lattice-distorted scaffold, enabling the rapid heteroepitaxial growth of substitutionally disordered manganoan dolomite cortices. Mechanistically, localized acidity from redox cycling triggers a “proton-driven cation pump”, actively releasing Mg2+ (and Ca2+) from the functionalized hydrogel reservoir to the mineralization front. This electrochemical route offers an extrapolable geological framework that links the massive fabric-retentive dolostones of the Precambrian to ancient redox-stratified shallow oceans, while explaining their punctuated scarcity in the Phanerozoic as a consequence of global oxygenation decoupling the manganese redox shuttle from shallow-marine environments.