Please help mother answer science fair question about salt crystals?

I have been searching for 4 weeks for info on the net about salt crystals. My sons project is due in the morning and it is now midnight. The question is...Doe Iodine in Iodized salt affect the size and or shape of the crystals compare to non iodized salt? We made 3 of each kind . The iodized made perfect cubes abou the size of a pencil eraser, the non iodized made tiny cubes and snowflakes. Considering Iodine is an impurity and the impurites should fall to the bottom of the solution and not be a factor, why were the iodized salt crystals nicer? There is 1 piece of info on the net abou this, which is not concrete. anyone have the answer and or info on the net....HELP! 8hours til school!