In Situ Observation of Step Dynamics on Gypsum Crystals

In this work we studied the kinetics of gypsum crystals growing from aqueous solutions as a function of temperature and supersaturation. Laser confocal differential interference contrast microscopy (LCM-DIM) and atomic force microscopy (AFM) were used to observe in situ the step advancement and the evolution of the surface morphology of the {010} face. We found that, for the experimental conditions used in this study, 2D nucleation is the main step generation mechanism, even at low supersaturations, and only a few spiral hillocks were observed. Due to the elongated morphology of 2D islands along the c-axis and the frequent nucleation of multilayer 2D islands, {010} faces growing from a supersatured solution developed a “hill and valley” topography. This type of surface topography is observed at all temperatures. The step kinetic coefficient, βst, was determined in the temperature range 20−80 °C, and a steep increment in the kinetic coefficient is found with increasing temperature. From these data, the activation barrier for incorporation of building units in the {010} face was determined to be 70.7 ± 5.0 kJ/mol. Analysis of the kinetic data shows that at low temperatures (≤40 °C) growth of the {010} face is dominated by a mixed regime and at higher temperatures (>40 °C) growth is controlled solely by diffusion.