TheTheClayScienceSociety Clay Science Society of Japan aay Scienee 19, 39-44 (2015) -Paper- CHEMICAL BONDING AND ELECTRONIC STRUCTURES OF THE AI,Si,O,(OH), POIIYMORPHS KAOLINITE, DICKITE, NACRITE, AND HALLOYSITE BY X-RAY PHOTOELECTRON SPECTROSCOPY J. THEo KLopRoaGEa'* and BARRy J. WooDb aSZrhool Sbiences, Tlhe Uhiversity ofeueensland 9td 4072, Australia bBarry ofEarth J.VVbodl Centrefor Ml'croscc\v; and vacroana4yyis. 77ie Uhivensity qfeueenslanct Brisbane, eld 4072, Australia (Received Apri1 20, 2015. Accepted June 18, 2015) ABSTRACT A detailed analysis was undertaken of the X-ray photoelectron spectra of the three polymorphs of Al,Si,O,(OH),; kaolinite, dickite, nacrite plus the related mineral halloysite. Comparison of the spectra was made based on the chemical bonding and structural differences in the Al- and Si-coordination within each polymorph. The spectra for Si(2p) for all four polymorphs are nearly identical. consistent with the fact that all the Si atoms are in 4-fold (tetrahedral) coordination, whereas the binding energies for Al(2p) vary slightly depending on the type of polymorph and the corresponding change in the stacking order of the layers. The overall shapes of the O 1s peaks observed in the four polyrnorphs are similar. Theoretically the ratio ofoxygen atoms versus in oxygen hydroxyl groups is 5:4 (55% vs 45%). For all four polymorphs the observed values are between 53 and 58% for the oxygen atoms and 38 to 45% for the oxygen atom in the hydroxyl groups. The lower-VB spectra fbr the kaolin polymorphs are similar to those of a-SiO, in terms of binding energies, but -2 appear eV higher than that for ct-Al,O,. Compared with O ls peaks, the lower VB peaks are considerably broader (--3-4 eV FWHM), and therefore, detailed stmctures cannot be resolved. Nevertheless, this difference implies that the bonding character for the kaolin polymorphs is more covalent than that of a-Al,Oi, but similar to that of a-Si02. Key words: Al,Si,O,(OH), polymorphs,X-Ray PhotoelectronSpectroscopy, kaolinite, dickite, nacrite, hal- loysite INTRODUCTION with Si in fourfold (tetrahedral) coordination by oxygen atems only fbrrning a single 1:1 layer. Adjacent layers are weakly The four polymorphs ofAl,Si,O,(OH),, kaolinite, dickite,bonded through van der Wdals forces. The major differences nacrite and halloysite, are geologically important minerals, between kaolinite, dickite and nacrite are associated with whose crystal structures, and physical and thermodynamic different stacking orders of the 1:1 layers. Kaolinite has a properties have been extensively investigated, Kaolin group distorted IM sequence of layers with the octahedral vacancy minerals are also one of the most important and valuable in- at the B site in every layer. Dickite has the same 1Mstacking dustrial minerals with a wide range of applications. These ap- sequence of layers as kaolinite, but the vacancy alternates reg- plieations include their use in the fabrication of paper, paints ularly between the B and C sites. In contrast to kaolinite, the and inks, rubber and plastic, ceramic raw material, fibreglass,crystal structure of dickite is well defined and less strained. cracking catalysts, cosmetics, medicines, etc. (Harvey and The stacking sequence of layers in nacrite is quite different Murray, 1997; Murray, 1999; Murray, 2000) from those in kaolinite and dickite. Nacrite has the ideal 6R Kaolinite is triclinic with spacegroup Pl, while dickite,polytype sequence in which each layer is shifted by one third nacrite and halloysite are monoclinic with spacegroup Cc ofthe 8.9A lateral repeat relative to the layen The octahedral (Table 1). All four polymorphs have the same basic building vacancy alternates regularly between the B and C sites, but blocks ofa sheet with Al in sixfbld (octahedral) coordination every octahedral sheet is rotated 180 degrees. The pattern of by oxygen atoms and hydroxyl groups and a second sheet vacancies reduces the symmetry to monoclinic. Halloysite is in general a hydrated form of kaolinite. Halloysite has mainly Corresponding author: J. Theo Kleprogge, Scheol of Earth Sciences. irregular layer stacking but with a limited tendency for 2Ml The University of Queensland, Qld 4072, Australia stacking in small domains, The presence of interlayer water NII-ElectronicNII-Electionic Library Service TheTheClayScience Clay Science Society of Japan 40 .L T KIoprogge et al. TABLE l. Crystallographic cell data for the four pelymorphs kaolinite, dickite,nacrite and halloysite (Anthony et al., 2003) Space- groupa b c tt s Tz KaoliniteDickiteNacriteHalroysitePlCcCcCc 5.155.1508.9e95.148.958.9405.1468.90 7.3914.42415.69714.991.so9oo9oo104.5-10so 9oo9oo9oo9oo2444 96e44, !!3042' 9oe 101.ge may be related to the presence ofsrnall amounts ofAl(IV) that EXPERIMENTAL are balaneed by exchangeable cations. Water is only weakly bonded and therefbre halloysite easily dehydrates upon heat- Mineralsamples ing or exposure to ultrahigh vacuum such as used in X-ray Kaolinite KGa-1 was obtained from the Clay Minerals Repository. The dickite used was the photoelectron spectroscopy. SocietySourceClays Over the years a significant arnount of research has been Saint Claire dickite frorn Pennsylvania, USA. Halloysite was devoted to the study ofthe kaolin polymorphs by vibrationai from Matauri Bay, New Zealand. The nacrite was from Tuni- spectroscopic techniques such as Mid-infrared, Near- infra- sia (orginally from the van der Marel collection) The purity of red, Raman and Infrared Emission spectroscopy (see e.g. all samples was confirmed by X-ray diffi'action, showing no Johnston et al., 1985; Johnston et al., 1990; Frost and v.d. other minerals present, prior to the XPS analyses. Gaast, 1997; Frost et al., 1993; Kloprogge, 2005; Kloprogge and Frost, 1999; Frost, 1995; Frost and Vassallo, 1996; Frost I\]PS analysis and Kloprogge, 2000). These techniques have allowed one The minerals were analyzed in freshly powdered fbrm in to gain a better understanding of especially the behaviour of order to prevent surface oxidation changes. The samples were the hydroxyl groups associated with the octahedral sheet. The finely crushed and, using a clean SS spatula a small amount current status of the assignment of the kaolinite hydroxyl- of sample was loaded onto a special double sided adhesive -5 stretching frequencies and the physics ofcollection of spectral tape covering mm diarneter. The area examined by XPS is data has been recently reviewed by Farmer (1998), Infrared --O.7 mm x O.3 mm. Typical duration fbr the powders being load-lock (IR) spectra of well-ordered kaolinites generally show four exposed in air (prior to evacuation in the sample OH-stretching frequencies, near 3697, 3670, 3652 and 3620 chamber) was less than 1O min. The samples were not Argon cm'i. Raman spectra of the same kaolinites mostly show Ion Beam eleaned because the use ofArgon Ion Beam clean- an additional band near 3686 cm-i, which, in the coarsely ing in most cases results in ion-induced damage to the sarnple, crystalline Keokuk kaolinite, largely replaces the 3695 cm'i in particular ion-beam reduction. were outgassed under band.Itisshown thatthe 3686 cm'] band can be ascribed to Priorto the analysis the samples a transverse optical crysta1 vibration involving the in-phase vacuum fbr 72 hours, The XPS analyses were performed on stretching vibration ofthe three inner-surface hydroxyl groups a Kratos AXIS UItra with a monochromatic Al X-ray source in the unit-cell. at 150 W. Each analysis startecl with a survey scan flrom O to The differences in the chemical bonding and electronic 1 200 eV with a dwell time of 1OO milliseconds, pass energy of struetures of the four Al,Si,O,(OH), polyrnorphs are funda- 160 eV at steps of 1 eV with 1 sweep. For the high resolution mental to the understanding oftheir therrnodynarnic propertiesanalysis the number of sweeps was increased, the pass energy meV and the dwell time and stability relations. X-ray photoelectron spectroscopy was loweredto20 eV at steps of 100 The Kratos Axis Ultra, (XPS) is a technique to probe the chemical bonding of spe- was changed to 250 milliseconds. cific elements, However, application of the XPS technique to XPS used for the analysis, has a built-in patented coaxial low minerals, like the Al,Si20,(OH), polymorphs, is difficult due energy electron charge compensation system which proyides to sample charging during the analysis. Detailed analyses of a high flux of electrons of uniform charge density. It uses a below the sample and, the photoelectron speetra obtained from alumino-silicate glass magnetic immersionlenssituated have been reported (Yagi et a!,, 2001; Miura et al., 2000). In low energy electrons from a filament located at the base ofthe glass forrn, however, Al and Si polyhedra are all eomer linked,photoelectron input lens, are iniected into the magnetic field. `overcompensation' therefore the present study from four different polymorphs It is operated in such a way that occurs the can give further insight into the chemistry and electronic which results in fuIl charge neutralization and photoelec- C 1s structure of bonding of eorner-linked tetrahedral and partialtron peaks move down seale a few eVL The principal peak corner-linkedoctahedra, from advantageous carbon is used as the reference at 284.8 eV A detailed study has been undertaken using X-ray photo- to calibrate the spectra. electron spectroscopy of the fbur A12Si,O,(OH), Band component analysis was undertaken using the Jandel polymorphs `Peakfit' which enabled the type of fitting to probe the differences in their chemical bonding, and com- software package, allows specific to be pared their electronic structures with those of a-SiO, (quartz)functionto be selected and parameters Band fitting was done using a and a-A120, (corundum, which was described in earlier work; fixedor varied aceordingly. cross-product funetion with the minimum (Kloprogge et al., 2006) using proper charge compensation Lorentz-Gauss techniques.