Mineralogy and crystallochemistry of low-temperature ferro-manganese deposits of the relict hydrothermal mound from the TAG hydrothermal field, mid Atlantic ridge

Low-temperature Fe-Mn deposits from the TAG hydrothermal field are composed of bacteria-like particles of protoferrihydrite, nontronite, goethite, and thin plated aggregates of birnessite. Particles of prototoferrihydrite (mineral formed at lower Eh than other iron oxides) containing much silica were replaced in situ by nontronite or goethite during alteration. Birnessite precipitation at the final stages of the hydrothermal process was partly replaced by vernadite due to bacterial activity. Protoferrihydrite together with silica precipitated within the whole hydrothermal field. As to birnessite, it precipitated locally. Thin films composed of Fe-vernadite and Mn-ferroxygite were found on the surfaces of hydrothermal deposits. They indicate the cessation of hydrothermal activity.