Information about crystal growth rates and the influence of impurities are essential for the design of industrial crystallization processes. Here, the influence of trivalent metal ions Fe3+ in solution and adsorbed in the crystal on the growth and dissolution rate is investigated. The results clearly show kinetic and thermodynamic effects caused by impurities which have to be taken into account in the equipment design. Furthermore, effects of growth rate dispersion must be excluded by investigation of crystal collectives. Additionally, the crystal growth data should be obtained from real starting solutions.