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Understanding precipitation formation at lanthanum hydroxide (La(OH)3) nanoparticle–solution interfaces plays a crucial role in catalysis, adsorption, and electrochemical energy storage applications. Liquid-phase transmission electron microscopy enables powerful visualization with high resolution. However, direct atomic-scale imaging of the interfacial metal (hydro)oxide nanostructure in solutions has been a major challenge due to their beam-driven dissolution. Combining focused ion beam and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we present an atomic-scale study of precipitation formation at La(OH)3 nanoparticle interfaces after reaction with phosphate. The structure transformation is observed to occur at high- and low-crystalline La(OH)3 nanoparticle surfaces. Low-crystalline La(OH)3 mostly transformed and high-crystalline ones partly converted to LaPO4 precipitations on the outer surface. The long-term structure evolution shows the low transformation of high-crystalline La(OH)3 nanoparticles to LaPO4 precipitation. Because precipitation at solid–solution interfaces is common in nature and industry, these results could provide valuable references for their atomic-scale observation.

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The multi‐valent manganese is ubiquitous in various marine environments and is involved in many biogeochemical processes with electron transfers between different oxidation states. The involvement of Mn (IV) in the anaerobic oxidation of methane (AOM) in cold seeps has long been proposed by experimental simulation and was suggested to serve as an important sink of methane in the paleo‐oceanic environment. Nonetheless, metal‐dependent AOM processes still lack explicit geological evidence. Meanwhile, research on micronodules gained momentum in recent years, but the authigenic manganese micronodules in active cold seeps were yet not well studied. In this study, Mn‐rich sediments were discovered at the active Haima cold seeps area in the South China Sea, from which Mn‐micronodules were then extracted. These micronodules are irregular or subspheroidal with rough and porous surfaces and are mainly composed of 7 Å and 10 Å phyllomanganates. They are enriched in Mn, and deficient in Fe, Co, Ni, Cu, and REYs. Characteristic microbial‐like mineralization structures are also observed. Based on these mineralogical/geochemical features and the abnormally high growth rate (calculated by the cobalt chronometer algorithm), these authigenic manganese micronodules were probably formed rapidly via biomineralization and chemical precipitation and were supplied with sufficient Mn by the metal‐rich seepage fluids. The occurrence of these micronodules demonstrates manganese bacterial activities, and indirectly reflects metal‐dependent AOM.

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