Defect-mediated carbonation pathways at the brucite-gas interface.

HYPOTHESIS: Mineral carbonation is governed by water-mediated interfacial transport and nucleation processes. In bulk aqueous environments, dissolution, transport, and precipitation occur in spatially separated domains, preventing direct resolution of interfacial reaction pathways. We hypothesize that under water-lean conditions local mass redistribution can be directly resolved at the reactive interface, enabling carbonation pathways to be determined through direct tracking of redistributed and newly formed interfacial volumes. EXPERIMENTS: To isolate the interfacial carbonation pathway, we performed atomic force microscopy on freshly cleaved brucite [Mg(OH)2] single crystals exposed to CO2 under hydrothermal equilibrium conditions and specific humidity ∼2%. The same surface regions were tracked before and after the treatments, enabling direct nanoscale visualization of morphological evolution due to carbonation while suppressing bulk-solution processes. FINDINGS: Carbonation initiates preferentially at surface step edges, where defect-mediated nucleation produces nanophases with possible amorphous structure whose volume scales with step height. These transient layers remain permeable to mass transport, enabling sustained subsurface growth beneath crystal terraces instead of immediate surface passivation. Upon aging, the amorphous precursor crystallizes into oriented needle-like aggregates at the brucite‑carbonate interface, revealing a non-classical growth pathway governed by defect-controlled flux localization and transport likely through nanoconfined interfacial water films. These findings establish a direct mechanistic link between surface defects, plausible water molecule mediation, and interfacial carbonate growth dynamics, providing a framework for understanding solid-gas mineralization reactions in confined aqueous environments.