Rietveld Refinement of the Kaolinite Structure at 1.5 K

<jats:title>Abstract</jats:title><jats:p>The crystal structure of Keokuk kaolinite, including all H atoms, was refined in space group <jats:italic>C1</jats:italic> using low-temperature (1.5 K) neutron powder diffraction data (λ = 1.9102 Å) and Rietveld refinement/difference-Fourier methods to R<jats:sub>wp</jats:sub> = 1.78%, reduced χ<jats:sup>2</jats:sup> = 3.32. Unit-cell parameters are: <jats:italic>a</jats:italic> = 5.1535(3) Å, <jats:italic>b</jats:italic> = 8.9419(5) Å, <jats:italic>c</jats:italic> = 7.3906(4) Å, <jats:italic>α</jats:italic> = 91.926(2)°, <jats:italic>β</jats:italic> = 105.046(2)°, <jats:italic>γ</jats:italic> = 89.797(2)°, and <jats:italic>V</jats:italic> = 328.70(5) Å<jats:sup>3</jats:sup>. 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 <jats:italic>C1</jats:italic> 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 60°–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 <jats:italic>random</jats:italic> positional disorder of the H atoms. However, these data provided no support for a space group symmetry lower than <jats:italic>C1</jats:italic>.</jats:p>