Growth of nordstrandite: effect of chloride on surface morphology

The effect of chloride on the growth of the Al(OH)3 mineral nordstrandite in ethylenediamine solutions has been investigated. Atomic force microscopy (AFM) observations revealed a difference in the morphology of monolayer steps on the basal faces of the crystals. The steps are made up of straight segments, down to a horizontal resolution of a few nanometers. These segments are shorter for the crystals precipitated in the presence of chloride. However, they show no difference in crystallographic orientation compared to those of the crystals synthesized without added chloride. Double steps are more prevalent for crystals synthesized with higher chloride concentrations. For crystals synthesized in the control solutions, two-dimensional nucleation on the surfaces only occurred at a few points and concentric steps were commonly observed. However, for crystals synthesized in the presence of chloride, the nucleation sites were more evenly spread. The observations suggest that the differences in the step morphology between the nordstrandite crystals synthesized in the presence of chloride and those synthesized in the control solutions are due to the adsorption of chloride at the steps. For all twinned crystals, two-dimensional nucleation occurred preferentially at point sources within the re-entrant angles. Spiral growth was only observed in one case.