The Morrón de Mateo bentonite deposit has been studied as a natural analogue of the thermal effect on the bentonite barrier of a geological radioactive waste repository. This deposit was intruded by a volcanic dome that induced hydrothermal activity affecting the smectite clay minerals close to the dome. Previous studies of proximal bentonites indicated that Al-montmorillonites were transformed into Fe-rich smectites with intermediate composition between beidellite and saponite through gradual steps formed by smectites increasingly rich in Mg and Fe. In order to confirm the suggested transformation and the Fe distribution into the smectites, infrared and Mössbauer spectroscopy studies were performed. Infrared spectra of samples away from the dome show typical bands for montmorillonite type with prevailing Al in octahedral positions, while proximal samples also show bands of Fe-rich smectites. Mössbauer data confirm that Fe present in the fine fraction of bentonites is fundamentally located in the smectites structure, mainly as octahedrally coordinated Fe(III). Proximal smectites have a considerably more distorted octahedral environment for Fe(III) which probably stemmed from a significant degree of substitution of Al by Fe(III). These results confirm that an alteration process occurred related to the volcanic intrusion which produced an increase in temperature and Fe-rich solutions responsible for the transformation of Al-montmorillonites.