The crystal structure of wavellite
Abstract
A needle-like crystal of wavellite from Arkansas was used for this investigation. The lattice constants of a = 9.62 Å, b = 17.36 Å, and c = 6.99 Å and the space group of Pcmn were obtained. Intensities of 765 reflections were collected with an equi-inclination, single-crystal diffractometer, using CuKα radiation.
Three-dimensional Patterson maps were prepared and the structure was determined by using the minimum-function method and subsequent Fourier syntheses. The atomic coordinates were refined, first with isotropic then with anisotropic temperature factors, using a full-matrix least-squares method. A final R of 5.5% was obtained for all reflections.
The two aluminum atoms in the structure are octahedrally coordinated: one is bonded to two O, two (OH), and two H2O; and the other to three O, two (OH), and one H2O. The phosphorus is in tetrahedral coordination with oxygen. The Al octahedra, linked through (OH) corners, form chains parallel to the c axis, and the P tetrahedra are attached to this chain by sharing O atoms of subsequent octahedra. An extra H2O molecule occupies the large cavity between the chains and, as indicated by a high temperature factor, it has a statistical distribution within this cavity.
