There is a poor understanding of the effect of impurities in phosphate rock on the crystal habitof gypsum during wet-process phosphoric acid production. The present work studies the effects of Al3+, SiF6 2-, and F- on the habit of gypsum in an MSMPR crystallizer. It is found that the gypsum crystal shape changes little in the presence of Al3+ alone. In the presence of both Al3+ and F-, the shape of the gypsum crystal changes greatly and forms sphere-shaped aggregates instead of the elongated needles formed in the presence of Al3+. In the presence of F-, Al3+, and SiF62-ions, the gypsum crystal shape is similar to that in the presence of both F- and Al3+ ions. In addition, the effects of Al3+, SiF6 2-, and F- on the crystallization kinetics of gypsum are also investigated.