The clay–iron interactions in presence of an alkaline (Ca, Na) fluid were experimentally investigated at three temperatures (80, 150 and 300°C) for three durations of 3, 6 and 9 months. The starting product, a bentonite (MX-80), was mixed with iron powder (metallic iron and magnetite) and placed in contact with a metallic iron plate. Mineralogical changes have been characterised by XRD and SEM imaging, and the crystal chemistry of clay particles and by-products has been determined by EELS- and EDS-TEM. The rate and nature of the clay transformations depend on two critical parameters: temperature and distance to metallic iron plate. At 300°C, run products are dominated by zeolite (mordenite-type zeolite) and Fe-rich vermiculite, the iron/magnesium ratio of vermiculite increasing at proximity of the iron plate. At 80 and 150°C, di-octahedral smectite remains the predominant clay, but is progressively enriched in Fe, especially at proximity of the iron plate. Besides the clay transformation, the progressive dissolution of feldspars and of the metallic iron powder is observed. Thus, the dissolution of metallic iron results in the formation of magnetite and Fe-rich clays (Fe-rich smectite, Fe-vermiculite), which are different from those (Fe-saponite, chlorite) produced in experiments carried out with neutral pH solutions.