MINERALOGY AND CRYSTALLOCHEMISTRY OF LOW- TEMPERATURE FERROMANGANESE 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 com posed of bacteria-like particles of protoferrihydrite, nontronite, goe thite, and thin plated aggregates of birnessite. Particles of prototof errihydrite (mineral formed at lower Eh than other iron oxides) contai ning much silica were replaced in situ by nontronite or goethite durin g alteration. Birnessite precipitation at the final stages of the hydr othermal process was partly replaced by vernadite due to bacterial act ivity. Protoferrihydrite together with silica precipitated within the whole hydrothermal field. As to birnessite, it precipitated locally. T hin films composed of Fe-vernadite and Mn-ferroxygite were found on th e surfaces of hydrothermal deposits. They indicate the cessation of hy drothermal activity.