Abstract: Fe-Si-Mn oxyhydroxides deposited from deep-sea low-temperature hydrothermal fluids are widely distributed on the global seafloor, yet the formation mechanisms of Mn-rich oxides within these deposits remain unclear. This study used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to perform high-resolution characterization of the elemental and mineralogical composition of the microstructures of Mn-rich oxides in Fe-Si-Mn oxyhydroxide deposit from the Longqi Hydrothermal Field on the Southwest Indian Ridge. The Mn-rich oxides contain abundant spheres composed of poorly crystallized Na-bearing δ-MnO₂ with surface pores, which have carbon-rich interiors and more crystalline, carbon-depleted outer layers. Fe-Si oxyhydroxide layers of varying thicknesses and Fe-Si oxyhydroxide filaments formed by neutrophilic microaerophilic Fe-oxidizing bacteria are on the surfaces of the Mn oxide spheres. Results suggest that the Mn oxide spheres formed mainly via microbial Mn oxidation, followed by autocatalytic Mn oxidation, forming more crystalline outer layers. Their surface structures influenced the thickness of the secondary Fe-Si oxyhydroxide precipitates by altering surface area and pore space in contact with hydrothermal fluids. The surface pores may have provided more stable microenvironments for the attachment of Fe-oxidizing bacteria. This study offered important scientific evidence into microbial roles in Mn oxide formation and the characteristics and post-depositional processes of biogenic Mn oxides in deep-sea low-temperature hydrothermal environments.