Hydroxyl Deformation in Kaolins

-The hydroxyl deformation modes of kaolins have been studied by Fourier transform (FT) Raman spectroscopy. Kaolinites showed well-resolved bands at 959, 938 and 915 cm 1 and an additional band at 923 cm ~. For dickites, well-resolved bands were observed at 955, 936.5, 915 and 903 cm I. Halloysites showed less-resolved Raman bands at 950, 938, 923, 913 and 895 cm 1. The first 3 bands were assigned to the librational modes of the 3 inner-surface hydroxyl groups, and the 915-cm ~ band was assigned to the libration of the inner hydroxyl group. The band in the 905 to 895 cm 1 range was attributed to "free" or non-hydrogen-bonded inner-surface hydroxyl groups. The 915-cm ~ band contributed --65% of the total spectral profile and was a sharp band with a bandwidth of 11.8 cm ~ for dickite, 14.0 cm ~ for kaolinites and 17.6 cm -~ for halloysites. Such small bandwidths suggest that the rotation of the inner hydroxyl group is severely restricted. For the inner-surface hydroxyl groups, it is proposed that the hydroxyl deformation modes are not coupled and that the 3 inner-surface deformation modes are attributable to the three OH2-4 hydroxyls of the kaolinite structure. For intercalates of kaolinite and halloysite with urea, a new intense band at --903 cm t was observed with concomitant loss in intensity of the bands at 959, 938 and 923 cm 1 bands. This band was assigned to the non-hydrogen-bonded hydroxyl libration of the kaolinite-urea intercalate. Infrared reflectance (IR) spectroscopy confirms these band assignments. Key Words--Deformation Mode, Dickite, FT Raman, Halloysite, Hydroxyl, Infrared, Intercalation, Kaolinite, Libration, Reflectance. I N T R O D U C T I O N The vibrational spec t roscopy of kaolins has attracted considerable interest due to its applicat ion to the structural study o f kaolin minerals. The hydroxyl bands are very sensit ive probes for dis t inguishing between kaolin clay minerals and de termining their structure (Frost 1997). The ass ignment of the hydroxyl stretching bands remains under constant review, and differ ing v iewpoints concern ing the posi t ion and orientat ion of both the inner and inner-surface hydroxyl groups exist (Giese 1988). FT Raman spect roscopy has already proven a very powerful technique in the study of kaolin minerals (Frost et al. 1993; Frost 1995). Kaolinite, as with the other kaolin clay minerals, contains 2 types o f hydroxyl groups: 1) the outer hydroxyl groups or so-called " inner-surface hydroxy l s " , des ignated OuOH, and 2) the inner hydroxyl groups, des ignated InOH. The OuOH groups are situated in the outer upper, unshared plane, whereas the InOH groups are located in the plane shared with the apical oxygens of the tetrahedral sheet. In an ordered sample such as the Georgia kaolinite (KGa-I ) , the 4 dist inct IR bands are obse rved as fol lows: the 3 higherf requency vibrations (Vl, v2, v3) are ass igned to the 3 outer (OuOH) and the v 5 band at 3620 cm-~ is ass igned to the inner hydroxyl (InOH) (Farmer 1974; Johns ton et al. 1990). The latter ass ignment is based on selective deuterat ion studies (Ledoux and Whi te 1964; Wada 1967; Whi te et al. 1970; Rouxhet et al. 1977). The c o m m o n l y accepted view is that the vl and v2 bands are the coupled ant isymmetr ic and symmetr ic vibrations (Brindley et al. 1986; Michael ian 1986). The ass ignment of the v 3 band is open to quest ion but the suggest ion has been made that the band is due to symmet ry reduct ion of the OuOH hydroxyl (Farmer and Russell 1964). These hydroxyl groups have deformation modes in the 915 to 960 cm -1 range which have been ass igned to the inner and inner-surface hydroxyl modes , respect ively, based on the select ive deuterat ion made possible by the format ion of interlayer complexes (Wada 1967). While there have been several IR studies of the hydroxyl deformat ion modes , no detailed studies of the Raman spectra o f the hydroxyl deformat ion modes have been reported. Further, studies o f halloysite hydroxyl vibrat ions are few, as are studies o f the intercalation of hal loysi tes with urea and potass ium acetate. In this paper, Raman spec t roscopy is applied to study of the a luminium hydroxyl deformat ion vibrations of kaolins, often te rmed "hydroxy l l ibrat ions" because the rotat ion of the hydroxyl group is restr icted due to hydrogen bonding. The basis o f this research is to de termine i f the hydroxyl deformat ion vibrations are coupled or, if not, i f the hydroxyl defo rma t ion v ibra t ions r ep re sen t d i sc re te h y d r o x y l groups. E X P E R I M E N T A L A suite of kaolinites dickites and hal loysi tes as shown in Tables 1 and 2 were analyzed by FF Raman spectroscopy, using the Perk in-Elmer 2000 series FT spectrometer. The latter was fitted with a Raman accessory compr is ing a Spect ron Laser Sys tems SL301 Copyright 9 1998, The Clay Minerals Society 280 Vol. 46, No. 3, 1998 Hydroxyl deformation in kaolins Table 1. Raman spectral analysis o f the hydroxyl librational region of kaolinites and dickites. 281