RIETVELD REFINEMENT OF NON-HYDROGEN ATOMIC POSITIONS IN KAOLINITE

from Keokuk, Iowa, was refined in space group C1 using CuKa X-ray powder diffraction data and Rietveld refinement techniques (Rwp = 12.3%). A distance least-squares (DLS) model for the initial atomic oordinates avoided a false minimum and unrealistic results characteristic ofprevious Rietveld refinements of kaolinite. All Keokuk samples ex-amined contain small amounts of dickite. The profile of the mixture was calculated using a fixed set of dickite atomic parameters and by constraining the dickite profile parameters tothose refined for kaolinite. Results indicate that the individual kaolinite layer is very similar to the dickite layer; bond distances, which are similar to dickite and nacrite, are: Si-O, 1.60"1.63 A; Al-O, 1.87-1.97 ~. The tetrahedral rotation angle is 6.9 ~ compared with 7.3 ~ for dickite and 7.4 ~ for nacrite. Previous Rictveld refinements apparently suffered from poor preferred orientation models, failure to consider the presence of dickite as a second phase, and, most importantly, refinement toa false-minimum structure. The conclusions based on the earlier efinements must therefore be r+evaluated, especially regarding the kaolinite space group and hydrogen positions. Existing X-ray and electron diffraction and spectroscopic data support the presence of C-centering in kaolinite. Key Words--Crystal structure, Dickite, Kaolinite, Rietveld refinement, X-ray powder diffraction