Low-temperature FTIR study of kaolin-group minerals

Low-temperature FTIR spectroscopy was used to characterize the v(OH) region of kaolin-group minerals including well ordered to poorly ordered kaolins from Georgia, Brazil, and England, along with samples of discrete dickite and nacrite. Low-temperature FTIR spectra were useful in resolving dickite- and nacrite-like features present in the spectra of kaolins when cooled to <30 K. These features were not resolved at room temperature and only partially resolved at liquid N-2 temperature (77 K). The room-temperature and low-temperature positions of the v(OH) bands of kaolinite, dickite, and nacrite were linearly correlated with the interatomic OH center dot center dot center dot O distances and this relationship served as the basis for polytype/disorder identification. Dickite or dickite-like disorder was found in high Hinckley-Index kaolinite from Keokuk, Iowa, and from Cornwall, England. Dickile- and nacrite-like features were observed in both high- and low-Hinckley-index kaolinite and the amounts of these stacking sequences generally increased with decreasing Hinckley Index.