The effects of inorganic impurities on the crystallization of calcium sulfates in strong HCl (6.3 mol L-1)-CaCl2 (1.8 mol L-1) solutions were investigated. The impurities considered relate to hydrochloric acid leaching of apatite-type ores for the extraction of rare earth elements. The impurities investigated were K+, Mg2+, Sr2+, Ba2+, Al3+, Fe2+, Fe3+, La3+, Y3+, F- (fluoride), and PO43- (phosphate). The investigation was done in the context of a continuous steady-state crystallization process. Therefore, temperature-controlled, semibatch crystal growth experiments with regulated reagent addition, to ensure nearly constant supersaturation, were performed. The experiments were conducted at 40 and 80 degrees C corresponding, respectively, to crystallization of calcium sulfate dihydrate (DH) and calcium sulfate hemihydrate (HH). Among all impurities investigated, phosphate and strontium were found to have the most significant effects, with La3+ and Y3+ having some modest effects. Phosphate (added as phosphoric acid) was found to accelerate the growth kinetics of dihydrate up to a certain concentration level (0.3 mol L-1), subsequently causing a retardation effect over the concentration range from 0.3 to 1.0 mol L-1. In contrast, phosphate had no effect on the growth kinetics of hemihydrate. In the meantime, phosphate uptake increased with increasing impurity concentration in the range up to 0.2 mol L-1 and then plateaued at 0.02 mol(phosphate) mol(solid)(-1). X-ray photoelectron spectroscopy (XPS) analysis provided evidence of the presence of a surface calcium phosphate species. On the other hand, the uptake of Sr2+ by dihydrate was much more extensive than that of phosphate (approximate to 5-10X). In this case, substitution rather than adsorption was the mechanism of uptake, reflecting the similar ionic radii between calcium and strontium. At phosphate and strontium concentrations >approximate to 0.2 mol L-1, partial transformation of dihydrate to hemihydrate was induced. Finally, La3+ and Y3+ were found to be incorporated at trace level amounts into dihydrate crystals causing crystal morphology changes but not promoting phase transformation.