A 19F Nuclear Magnetic Resonance Study of Natural Clays

Andrea Labouriau, Yong-Wah Kim, Steve Chipera, David L. Bish and William L. Earl
Chemical Sciences and Technology and Earth and Environmental Sciences Divisions Los Alamos National Laboratory, Los Alamos, New Mexico 87545, U.S.A.

Abstract: A series of natural clays, including 1:1 layer silicates (serpentines, kaolin minerals), smectites, vermiculite, micas, talc, pyrophyllite, sepiolite, and palygorskite, were studied by 19F magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The 19F chemical shift in these layer silicates is characteristic of the structure, in particular, to the local octahedral cation occupancy. Fluoride ions bonded to three Mg octahedral cations have a chemical shift of about −177 ppm and those bonded to two Al cations and a vacancy have a chemical shift of about −134 parts per million (ppm). The shift at −182.8 ppm in hectorite is apparently associated with fluoride bonded to two Al cations and a Li cation. Surprisingly, the difference in chemical shift of the interlayer and inner fluoride in 1:1 layer silicates is insufficient to distinguish these sites. Based on trends in chemical shift, it appears that fluoride substitution for inner hydroxyls in clays with octahedral substitution is not random. Fluoride is apparently preferentially associated with Mg rather than Al in the octahedral sheet as no resonance due to a fluoride bonded to two Al cations and a vacancy is observed in clays such as SAz-1.

Key Words: 27Al • Aluminum • Clay minerals • 19F • Fluorine • MAS • NMR • Solid

Clays and Clay Minerals; December 1995 v. 43; no. 6; p. 697-704; DOI: 10.1346/CCMN.1995.0430606
© 1995, The Clay Minerals Society
Clay Minerals Society (www.clays.org)