X-ray powder diffraction study reveals non-hydrogen atomic positions in kaolinite, indicating that earlier models suffered from false-minimum errors and undetected dickite.
Key Points
Refine the non-hydrogen atomic structure of kaolinite and resolve false-minimum artifacts and phase-contamination issues present in earlier crystallographic refinements.
Analyzed natural kaolinite samples from Keokuk, Iowa, using CuKα X-ray powder diffraction data refined in space group C1 via Rietveld techniques.
Utilized a distance least-squares (DLS) model for initial atomic coordinates to prevent convergence toward unrealistic false minima.
Modeled dickite as a secondary phase using fixed atomic coordinates and profile parameters constrained directly to those of kaolinite.
Achieved a Rietveld refinement weighted profile residual (Rwp) of 12.3%, yielding Si-O bond lengths of 1.60–1.63 Å and Al-O bond lengths of 1.87–1.97 Å.
Identified a tetrahedral rotation angle of 6.9° (relative to 7.3° in dickite and 7.4° in nacrite), establishing strong structural homology across the individual layers.
Demonstrated that previous structural models were biased by poor preferred-orientation corrections and unaccounted dickite, confirming C-centering in kaolinite.