Ab Initio Computational Crystallography of 2:1 Clay Minerals: 1. Pyrophyllite-1Tc

Abstract

The interpretation of diffraction and spectroscopic data on clay minerals can be facilitated by first-principles quantum mechanical simulations based on density functional theory (DFT), which now has achieved sufficient accuracy to predict crystallographic properties of these minerals without recourse to empirical parametrization. To test the accuracy of DFT for simulating 2:1 clay minerals, we have performed calculations on a related layer aluminosilicate for which high-quality crystallographic data are available, pyrophyllite-1Tc. The computational scheme included a plane-wave basis set along with new pseudopotentials for Si, Al, and O generated using the Perdew−Wang 91 generalized gradient approximation for the core electron exchange-correlation functional, the same approximation that was used for the valence electrons. Our calculations allowed full structural relaxation and, therefore, were appropriate for comparison to ambient-pressure crystallographic data. The simulation results gave an excellent account of bond lengths and angles when compared to single-crystal X-ray diffraction data. Subtle features of the pyrophyllite structure, such as tetrahedral rotation and corrugation of the basal planes, also were captured accurately. The orientation angle of the structural OH groups in pyrophyllite was predicted to be 25.4° relative to the ab plane but was found to exist within a very broad minimum in respect to its effect on the total energy of the crystal. Overall we conclude from our results that high-quality DFT optimizations now can provide significant crystallographic information to aid in the interpretation of diffraction and spectroscopic data on layer type aluminosilicates.