Publication: Microtopography of the barite (001) face during growth:
AFM observations and PBC theory
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Publication Date
1998
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Elsevier Science B.V.
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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.