Structural Fluorine in Sepiolite

Julio Santaren, Jesus Sanz and Eduardo Ruiz-Hitzky
Research and Development Department, Tolsa, S.A. P.O. Box 38017, 28080 Madrid, Spain
Instituto de Ciencia de Materiales, C.S.I.C. Serrano 115 bis, 28006 Madrid, Spain

Abstract: 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.

Key Words: Fluorine • Hydroxyl • Laser microprobe mass spectrometry • Dissolution • Nuclear magnetic resonance • Sepiolite

Clays and Clay Minerals; February 1990 v. 38; no. 1; p. 63-68; DOI: 10.1346/CCMN.1990.0380109
© 1990, The Clay Minerals Society
Clay Minerals Society (www.clays.org)