Abstract: The pink clays from the Tagus basin, Spain, were characterized by X-ray difraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Chemical data were obtained by plasma emission spectroscopy and analytical electron microscopy (AEM), and specific surface and cation-exchange capacity were measured also. The data indicate that these pink clays are primarily stevensite. This Mg-rich smectite is characterized by poor crystallinity, a high degree of structural disorder, trioctahedral character (pure magnesian), a very low cation-exchange capacity, a very small crystal size (which generates an abnormally high specific surface area), and a deficiency of octahedral cations. On the basis of the very small crystal size, a large number of edge dislocations, the lack of periodicity (turbostratic) in the structure, and a cellular (spherical) texture observed by TEM, we consider this occurrence to be an early stage of crystallization. Unlike other precursor clay materials described in the literature, this clay is not an alteration of volcanic ash, but it was generated by precipitation from a Si- and Mg-saturated medium.