Clay minerals in vesicles and miarolitic voids form black halos in zero age oceanic basalts. They correspond to mixtures of Fe-smectites with celadonite or glauconite. They are Fe-rich and present variable Al, Mg and K contents. The observation of these clay minerals at scales ranging from that of petrographic microscope to that of high resolution transmission electron microscope indicates that their formation mechanisms are very complex : they form not only by precipitating directly from hydrothermal fluids mixed with seawater but also by replacing basalt primary pyroxene and mesostasis