Organic imprint: Natural matter dictates crystallization of manganese oxides on hematite.

Mineral-organic interfaces regulate crystallization processes central to mineral evolution and biogeochemical cycling, yet how organic matter (OM) actively directs nucleation and growth pathways remains unresolved. Here, we reveal a pivotal role of humic acid (HA), a representative OM, in orchestrating the crystallization of Mn oxides on hematite by influencing both the spatial distribution of Mn ions and the subsequent pathways of Mn oxides nucleation and growth. Our findings reveal that mineral-associated organic matter (MAOM) facilitates the formation of fine-grained Mn oxides through a distinct three-step mechanism: i) formation of Mn-rich and Mn-poor OM domains on hematite surface, ii) catalytic oxidation of Mn(II) and nucleation of Mn ions to form primary particles within the Mn-rich domain, and iii) subsequent assembly of Mn oxide particles via particle attachment, leading to a structurally integrated Mn oxide phase (hausmannite and manganite). Spectroscopic analysis and molecular dynamics simulations further reveal that HA adopts facet-specific conformations, which modulate Mn accumulation and particle assembly. Our findings demonstrate that OM not only associates with minerals but can modify mineral-OM interfacial reactivity and crystallization pathways, offering insights into mineral crystallization mechanisms, soil evolution, and mineral-OM interactions.