Microtopography of the barite (001) face during growth: AFM observations and PBC theory

Abstract

Under moderate supersaturation conditions, crystal growth on the barite (0 0 1) surface takes place by the development of two-dimensional nucleation simultaneously with the advancement of molecular-height cleavage steps on the surface. The most frequent growth steps have a height of a half-unit cell, as has been predicted by periodic bond chain (PBC) theory, and they are parallel to the S1 2 0T PBC directions. Along opposite directions the velocity of S1 2 0T growth steps is strongly anisotropic. Moreover, the directions of fast growth alternate for successive elementary growth layers. The anisotropy of the growth rates can be explained by taking into account the crystallographic features and orientation of the complete PBC within each (0 0 2) elementary growth slice. On the other hand, the alternation of the fast growth direction for S1 2 0T steps in successive d002 growth layers is related to the existence of a 21 screw axis perpendicular to the (0 0 1) surface. Two-dimensional nucleation on the barite (0 0 1) surface is characterized by the development of islands with a circular sector shape and half-unit cell in height. The two-dimensional islands nucleated on the initial surface show the same orientation. As growth proceeds, islands coalesce and a homogeneous layer with a thickness of 3.5 A is formed. Nucleation on this new surface produces islands oriented in the opposite sense to those in the previous layer. Goniometric measurements and X-ray di¤raction experiments conÞrm that the straight edges of the islands are parallel to the [1 2 0] and [1 2 0] crystallographically equivalent directions. The third side of each island is curved, rough and tangent to [010]. Both the morphology and development of two-dimensional nuclei on the barite (001) face clearly indicate that the growth process is structurally controlled. The asymmetry of [120], [120] and [010] PBCs and their crystallographic features can be considered as responsible for the geometry and spread of the circular sector islands formed on each elementary (002) growth layer.