AFM observations and simulations of jarosite growth at the molecular scale: probing the basis for the incorporation of foreign ions into jarosite as a storage mineral

The growth of K-jarosite was evaluated using scanning electron microscopy (SEM), atomic force microscopy (AFM), and molecular simulations. SEM micrographs show crystals with almost quadratic and, to a minor extent, triangular faces. These were identified to be {012} and {001} faces, respectively, using an X-ray single-crystal diffractometer. These results are in agreement with molecular simulations that show that the dipole-free {012} face is the most stable surface, followed by the {001} surface, that can lower its dipole moment perpendicular to the surface during relaxation. On the {012} face, the most stable step directions were identified. From these, an idealized equilibrium growth island can be constructed when the corner energies are minimized by the successive removal of corner ions until a stoichiometric growth island is obtained. Such an equilibrium growth island is the basis for developing a spiral growth model that explains the mechanism of spiral formation as observed using AFM. Furthermore, reactive sites that are potential candidates for the incorporation of foreign ions can be located.