The mineralogical and micromorphological characteristics of Si-Fe-Mn oxyhydroxides from the dacite-hosted PACMANUS hydrothermal field were analyzed.The samples are poorly crystallized Si-Fe-Mn oxyhydroxides with minor birnessite, todorokite, nontronite, goethite, and opal-A.There are some microtextures which are rather like fossil microbes such as the filamentous silica and the hollow pipes.Flakes of nontronite crystals are found either forming a honeycomb texture or distrib- uted on the surface of the hollow pipes.Nontronite is the product precipitated from low-temperature hydrothermal fluids, and microbes may play a role in its formation.Si-Fe-Mn oxyhydroxides have two kinds of nuclei: Si-Mn nuclei and Si nuclei, both enveloped by the similar Si-Fe outer layer, existing in the rod-shaped oxyhydroxide and spheroidal oxyhydroxide, respectively.In the Si-Mn nuclei, the concentration of SiO2 is between 39.32 wt% and 86.31 wt%, and MnO concentration is between 4.97 wt% and 27.01 wt%, but Fe2O3 concentration is very low (0.54 wt%-3.43 wt%).In the Si nucleus the concentration of SiO2 is 90.17 wt%, but concentration of MnO and Fe2O3 are low, with 0.06 wt% and 3.47 wt%, respectively.The formation of the Si-Mn nucleus is closely related to microbes, whereas the Si nucleus is of inorganic origin. The mineralogical and micromorphological characteristics of Si-Fe-Mn oxyhydroxides from the dacite-hosted PACMANUS hydrothermal field were analyzed. The samples are poorly crystallized Si-Fe-Mn oxyhydroxides with minor birnessite, todorokite, nontronite, goethite, and opal-A.There are some microtextures which are rather like fossil microbes, such as the hollow pipes.Flakes of nontronite crystals are found either forming a honeycomb texture or distrib- uted on the surface of the hollow pipes.Nontronite is the product precipitated from low- Fe-Mn oxyhydroxides have two kinds of nuclei: Si-Mn nuclei and Si nuclei, both enveloped by the similar Si-Fe outer layer, existing in the rod- shaped oxyhydroxide and spheroidal oxyhydroxide, respectively. In the Si-Mn nuclei, the concentration of SiO2 is between 39.32 wt% and 86.31 wt%, and the MnO concentration is between 4.97 wt% and 27.01 wt%, but The concentration of SiO 2 is 90.17 wt%, but the concentration of MnO and Fe 2 O 3 are low, with 0.06 wt% and 3.47 wt% respectively. of the Si-Mn nucleus is closely related to microbes, while the Si nucleus is of inorganic origin.