Abstract: The effect of metallic impurities ions，such as Mg2+，Al3+，and Fe3+ on phase transformation time（PTT） and the change of crystal morphology in phase transformation process of CaSO4·2H2O to CaSO4·0.5H2O in CaCl2-HCl-H2O electrolyte solution （3.5 mol/kg CaCl2，0.5 mol/kg HCl） at 70 ℃ was investigated. The PTT was determined by the change of crystallization water con- tent and the crystal morphology was characterized through the images of the crystals.Results indicated that the metallic impurity ions accelerated the phase transformation process.And the PTT shortened to 1.5 h and 1.0 h respectively when the mass fractions of ionic impurities Mg2+ and Al3+ exceeded 0.01%，and shortened to 1.0 h when the mass fractions of Fe3+ exceeded to 0.001%；the sequence of acceleration was followed as Al3+＞Fe3+＞Mg2+ with lower mass fraction of metallic ions; the crystal morphology gradually became needle-like from plate-like with the phase transformation process，and finally remained the needle-like shape.

Key words: CaCl2-HCl, calcium sulfate, ionic impurity, phase transformation, crystal morphology