RIETVELD REFINEMENT OF THE KAOLINITE STRUCTURE AT 1.5-K

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 (lambda = 1.9102 angstrom) and Rietveld refinement/difference-Fourier methods to R(wp) = 1.78%, reduced chi2 = 3.32. Unit-cell parameters are: a = 5.1535(3) angstrom, b = 8.9419(5) angstrom, c = 7.3906(4) angstrom, alpha = 91.926(2)-degrees, beta = 105.046(2)-degrees, gamma = 89.797(2)-degrees, and V = 328.70(5) angstrom3. 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 Cl 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-degrees-73-degrees with the (00 1) 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 Cl.