A F-19 NUCLEAR-MAGNETIC-RESONANCE STUDY OF NATURAL CLAYS

A series of natural clays, including 1:1 layer silicates (serpentines, kaolin minerals), smectites, vermiculite, micas, talc, pyrophyllite, sepiolite, and palygorskite, were studied by F-19 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The F-19 chemica l shift in these layer silicates is characteristic of the structure, i n 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 chemic al shift of about -134 parts per million (ppm). The shift at -182.8 pp m in hectorite is apparently associated with fluoride bonded to two Al cations and a Li cation. Surprisingly, the difference in chemical shi ft of the interlayer and inner fluoride in 1:1 layer silicates is insu fficient to distinguish these sites. Based on trends in chemical shift , it appears that fluoride substitution for inner hydroxyls in clays w ith octahedral substitution is not random. Fluoride is apparently pref erentially associated with Mg rather than Al in the octahedral sheet a s no resonance due to a fluoride bonded to two Al cations and a vacanc y is observed in days such as SAz-1.