Structural Fluorine in Sepiolite

Sepiolite from Vallecas-Vicálvaro, Spain, contains 1.3% fluorine. Laser microprobe mass spectrometry of this sepiolite suggests the presence of fragments of (SiO 2 ) n OMgF and (SiO 2 ) n OMgOH, 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 19 F 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 Mg 2+ is substituted by fluorine. The kinetics of extraction of Mg 2+ and F - ions by acid treatment (1 N HCl) shows a more rapid extraction of Mg 2+ , 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.