Sepiolite from Vallecas-Vicálvaro, Spain, contains 1.3% fluorine. Laser microprobe mass spectrometry of this sepiolite suggests the presence of fragments of (SiO₂)_nOMgF and (SiO₂)_nOMgOH, which are typical of the sepiolite structure. During thermal dehydroxylation, the fluorine in this sepiolite is removed simultaneously with OH groups at about 750°C. Nuclear magnetic resonance spectroscopy (NMR) of ¹⁹F indicates that the fluorine is located in the interior of the sepiolite structure, probably substituting for OH groups, and is homogeneously distributed. In the Vallecas-Vicálvaro sepiolite, about one of every four OH groups bound to Mg²⁺ is substituted by fluorine. The kinetics of extraction of Mg²⁺ and F^- ions by acid treatment (1 N HCl) shows a more rapid extraction of Mg²⁺, with a monotonous decrease of the Mg/F ratio as the extent of extraction increases. These results support the internal location of the fluorine, as suggested by the NMR data.

The adsorption isotherm of methylene blue (MB) on sepiolite gels is of the Langmuir type, indicating a great affinity of the MB towards the sepiolite; the adsorption is quantitative up to adsorption of MB of 0·1 mmol/g sepiolite. The differences observed in the absorption spectrum in the region 500–650 nm for different MB loadings are interpreted in terms of aggregation of MB molecules on the sepiolite surface with the formation of MB dimers, trimers and higher aggregates. The progressive coverage of the sepiolite surface by MB produces a sharp decrease in the viscosity of the gels, and the suspension becomes peptized for an MB loading of 0·21 mmol/g which corresponds to the adsorption of MB dimers. These results indicate that MB avoids particle-to-particle interactions. The viscosity decrease is parallel to the perturbation of the Si-OH groups on the sepiolite surface as observed by IR spectroscopy, suggesting that these silanol groups can also play an important role in the rheological behaviour of aqueous sepiolite dispersions.