Periodic ab initio Hartree-Fock calculations of the low-symmetry mineral kaolinite

482

278

Abstract--The crystal structure of Keokuk kaolinite, including all H atoms, was refined in space group C1 using low-temperature (1.5 K) neutron powder diffraction data (~ = 1.9102 A) and Rietveld refinement/ difference-Fourier methods to R,,,p = 1.78%, reduced x 2 = 3.32. Unit-cell parameters are: a = 5,1535 ( (5) ik ~. Unit-cell parameters show that most of the thermal contraction occurred along the [001] direction, apparently due to a decrease in the interlayer distance. The non-H structure is very similar to published C1 structures, considering the low temperature of data collection, but the H atom positions are distinct. The inner OH group is essentially in the plane of the layers, and the inner-surface OH groups make angles of 600-73 ~ with the (001) plane. Difference-Fourier maps show minor anisotropy of the inner-OH group in the [001] direction, but the inner-surface OH groups appear to have their largest vibrational (or positional disorder) component parallel to the layers. Although no data indicate a split position of any of the H sites in kaolinite, there is support for limited random positional disorder of the H atoms. However, these data provided no support for a space group symmetry lower than C1.

Section: Oh Orientationssupporting

confidence: 75%

Rietveld Refinement of the Kaolinite Structure at 1.5 K

Bish

1993

482

278

“…The angle between the O-H(1) bond and the (001) plane is 12 ~ (Table 6) and, based on the difference map, the estimated standard deviation is near 5 ~ . Thus, the observed angle of 12(5) ~ is higher than the angle (0.34 ~ observed by Bish (1993) or that calculated (3.8 ~ by Hobbs et al (1997) and reported (3.8 ~ by Hess and Saunders (1992). However, large temperature differences between the experimental conditions makes comparison difficult.…”

Section: Position Of the Intralayer Hydrogenmentioning

confidence: 59%

Refinement of the Kaolinite Structure From Single-Crystal Synchrotron Data

Neder¹,

Burghammer²,

Grasl³

et al. 1999

172

Abstract--The crystal structure of single crystals of kaolinite from Keokuk, Iowa, was refined using data measured at the microfocus X-ray beamline at the ESRE Grenoble, France (X = 0.6883, T -room temperatu[e). The volume of the crystals was 8 and 0.8 txm 3, respectively. Unit-cell parameters are: a = 5.154(9) A, b = 8.942(4) A, c = 7.401(10) A, c~ ~ 91.69(9) ~ , 13 = 104.61(5) ~ "/ -89.82(4) ~ . Space group C1 is consistent with the observed data. All non-hydrogen atoms were independently refined with anisotropic displacement parameters. The positions and isotropic displacement parameters for the three interlayer H atoms were refined a/so. The position of the intralayer H was found by difference-Fourier methods, although refinement was not possible. Difference-Fourier maps suggested large anisotropic displacement vectors of this intralayer H, however, no evidence for a second maximum was found. The diffraction patterns show diffuse scattering in streaks parallel to [001]* through hkl reflections with hk 0, which is caused by stacking faults. No twinning was observed for either of the two crystals.

“…In this paper's discussion of intercalated halloysites, this convention is adopted, and the additional bands are simply defined as Vx, as they appear in the spectra. These InOH are bound to the A1 atoms and extend towards the intralayer cavity of the kaolinite (Hess and Saunders 1992). For kaolinites, the InOH plane essentially lies parallel to the 001 plane and calculations predict the OH group to be at an angle of +3.1 degrees to this plane (Hess and Saunders 1992).…”

Section: Halloysite Hydroxyl Regionmentioning

confidence: 99%

Intercalation of Halloysite: A Raman Spectroscopic Study

Frost

Kristóf

1997

Abstract--Intercalates from an ordered halloysite with urea and potassium acetate were studied using Raman microscopy. The urea intercalate showed new Raman bands at 3387, 3410, 3497 and 3598 cm -~ which were attributed to the formation of a urea-Si205 complex. New Raman bands were observed at 3585 and 3602 cm t for the potassium acetate intercalate with concomitant loss of intensity of the bands at 3635, 3655, 3675 and 3696 cm ~. These new bands were attributed to the hydrogen bonds formed between the acetate and the inner surface hydroxyl groups. Remarkable changes in intensity in the lattice region of the halloysite were observed, the foremost being the reduction of the intensity of the bands at 243, 271 and 336 cm -t. Pronounced changes in the bands at 913 and 143 cm t attributed to the A1-OH librations were also observed.It is proposed that 2 distinct types of intercalation were present, as exemplified by: 1) urea intercalate, where the intercalating molecule hydrogen bonds to the Si-O of the halloysite layers and 2) potassium acetate intercalate, where the molecule is hydrogen-bonded to the inner surface hydroxyls of the halloysite layer and interacts with the tetrahedral sheet of the next adjacent halloysite layer. The Raman spectra of the intercalated halloysite strongly resembled that of an intercalated kaolinite.