Crystal structure of nacrite from the electron diffraction data

Abstract

The crystal structure of the nacrite mineral of the kaolin group (space group Cc; R = 2.76%; 634 unique reflections) is refined from the digital oblique-texture electron diffraction patterns obtained with the use of imaging plates. The maxima characterizing the locations and potentials of the hydrogen atoms of the hydroxyl groups are revealed from the difference Fourier-potential syntheses. The O-H interatomic distances and the angles of inclination of the O-H bond with respect to the ab plane are equal to 0.97 Å and −18.3° for the inner hydroxyl group and 0.92, 0.85, 0.93 Å and 60.8°, 67.8°, 58.4° for the outer hydroxyl groups, respectively. The O_donor-O_acceptor interatomic distances are 2.940, 2.949, and 3.121 Å. It is established that the electrostatic potential distributions of the hydrogen atoms of the inner hydroxyl group and one of the outer hydroxyl groups located in the vicinity of the symmetry pseudoplane m of the layer are characterized by anisotropy, which can indicate a statistical distribution of these hydrogen atoms.