Quantitative 3D Visualization of the Growth of Individual Gypsum Microcrystals: Effect of Ca²⁺:SO₄²– Ratio on Kinetics and Crystal Morphology

The kinetics of crystal growth of gypsum is determined by measuring the 3D time-evolution of isolated microcrystals (∼10 μm characteristic dimension) by in situ AFM. By coupling such measurements to a well-posed diffusion model, the importance of mass transport to the overall rate can be elucidated readily. Indeed, because microscale interfaces that act as source or sink sites are characterized by intrinsically high diffusion rates, it is possible to study crystal growth free from mass transport effects in many instances. In the present study, a particular focus is to elucidate how the ratio of Ca2+ to SO42– ions at constant supersaturation influences the rate of growth at the major crystal faces of gypsum. It is found that growth at the {100} and {001} faces, in particular, is highly sensitive to solution stoichiometry, resulting in needle-like crystals forming in Ca2+-rich solutions and plate-like crystals forming in SO42–-rich solutions. The maximum growth rate occurs with a stoichiometric solution of Ca2+:SO42– The much slower growing basal {010} face also shows a stoichiometry dependence and growth is found to occur at step sites in growth hillocks. Importantly, overall growth rates derived by measuring the volumetric expansion of microcrystals by 3D in situ AFM are in reasonable agreement with previous bulk studies on suspensions. This study is a further illustration that the study of individual microcrystals is a powerful approach for resolving face-specific kinetics and in providing a link between microscopic observations and macroscopic rates in bulk systems. In this study it has further been possible to link face-specific kinetics to the resulting crystal morphology.