Structural Fluorine in Sepiolite

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Abstract
Sepiolite from Vallecas-Vichlvaro, Spain, contains 1.3% fluorine. Laser microprobe mass spec- trometry of this sepiolite suggests the presence of fragments of (SiO2),OMgF and (SiO~),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~ Nuclear magnetic resonance spectroscopy (NMR) of ~gF 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-Vic~lvaro 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 HC1) 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.