Rietveld R E F I N E M E N T of the Kaolinite S T R U C T U R E 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 (~ = 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(3) /~, b = 8.9419(5) &, c = 7.3906(4) &, a = 91.926(2) ~ = 105.046(2) ~ 7 = 89-797(2) ~ and V= 328.70(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. Key Words--Crystal structure, H positions, Kaolinite, Low temperature, Neutron powder diffraction, Rietveld refinement. I N T R O D U C T I O N Since Pauling first outlined the crystal structure of kaolinite over sixty years ago (Pauling, 1930), many studies have been conducted to clarify the details of its structure. The first comprehensive study (Brindley and Robinson, 1946) used X-ray powder diffraction data to locate the non-H atoms in kaolinite. Since then, little additional information has been obtained on the non-H structure, but there remains little agreement on the locations o f the H atoms. H-atom orientations were originally inferred from infrared spectroscopic data, and positions were later modeled by Giese and Datta (1973) and Giese (1982) using electrostatic energy calculations. More recently, Suitch and Young (1983) and Young and Hewat (1988) refined H-atom positions using neutron powder diffraction data but assuming the space group of P1. H-atom positions were determined also in the Rietveld study of the St. Austell kaolinite (Adams, 1983), but the diffraction pattern exhibited two-dimensional diffraction effects that are not modeled by the Rietveld method. In addition, the St. Austell material consists of at least two variants of kaolinite (Planqon et al., 1989). Consequently, there is uncertainty in the derived atomic positions. Because of the lack of agreement in the literature concerning the H positions, a Rietveld refinement of the kaolinite structure was undertaken in space group C1 using lowtemperature (1.5 K) neutron powder diffraction data kindly provided by Professor R. Young and A. Hewat. THE SPACE G R O U P OF KAOLINITE A crystal structure refinement requires the deterruination of the space group consistent with available Copyright 9 1993, The Clay Minerals Society observations. Although most investigators have accepted space group C1, Suitch and Young (1983) and Young and Hewat (1988) assumed space group P1. This assumption requires twice the number of independent atoms in the asymmetric unit by allowing crystallographically identical atoms in C1 to occupy nonequivalent sites in P1. Although Young and coworkers concluded that the non-H portion of the structure largely obeys C-centering, the important result of the P1 refinements was that OH groups that are equivalent in C1 exhibited two different orientations in P1. These orientations have important effects, for example, in vibrational spectroscopy (e.g., Brindley et aL, 1986; Johnston et al., 1990), the calculation of X-ray powder diffraction patterns (e.g., Bookin et aL, 1989; Plancon et aL, 1989), and electrostatic modeling of the structure (e.g., Abbott, 1989; Guthrie and Bish, 1991). Thus, it is crucial to examine the available evidence concerning the true space group. Differentiation between a P1 and C1 space group involves the determination that h + k --odd diffraction data are systematically absent for all hkl reflections, which indicates a C-centered lattice. The search for reflections violating C-centering is potentially complicated by the fact that the violating reflections in kaolinite have been postulated to arise primarily from the H atoms (Suitch and Young, 1983; Young and Hewat, 1988), which do not contribute greatly to X-ray diffraction intensities. However, it is possible that deviations in the Young and Hewat (1988) model from a C-centered lattice may also be due to small distortions of the heavier atoms. To test this possibility, an X-ray powder diffraction pattern was calculated for C u K a radiation using POWD10 (Smith et al., 1983)