Rietveld Refinement of Non-Hydrogen Atomic Positions in Kaolinite

Abstract The structure of kaolinite (non-hydrogen atoms only) from Keokuk, Iowa, was refined in space group C 1 using CuK α X-ray powder diffraction data and Rietveld refinement techniques (R wp = 12.3%). A distance least-squares (DLS) model for the initial atomic coordinates avoided a false minimum and unrealistic results characteristic of previous Rietveld refinements of kaolinite. All Keokuk samples examined 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 to those 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 Å; Al-O, 1.87-1.97 Å. The tetrahedral rotation angle is 6.9°, compared with 7.3° for dickite and 7.4° for nacrite. Previous Rietveld refinements apparently suffered from poor preferred orientation models, failure to consider the presence of dickite as a second phase, and, most importantly, refinement to a false-minimum structure. The conclusions based on the earlier refinements must therefore be reevaluated, 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.