The effects of NO3- and SO42- ions on the H+ concentration in organic phase and crystal morphology of solid product nesquehonite (MgCO3 center dot 3H(2)O) in the coupled reaction-extraction crystallization process were investigated. During the coupled process, the H+ concentration in organic phase increased with the increase in concentration of NO3- ions and decreased with the increase in concentration of SO42- ions, indicating that NO3- ions promoted the carbonization of MgCl2 while SO42- ions inhibited the same. The rodlike MgCO3 center dot 3H(2)O crystals became short and thick in the presence of both NO3- and SO42- ions. Energy-dispersive X-ray spectroscopy (EDS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and molecular dynamic simulation were used to clarify the mechanism by which impurity ions modified the structure. The results indicated that the NO3- and SO42- anions could be selectively adsorbed on the top facet (011) of MgCO3 center dot 3H(2)O, preventing the growth of crystal surfaces, which results in the decrease in length and increase in diameter of MgCO3 center dot 3H(2)O product. The morphology modification by NO3- ion was mainly due to the selective adsorption of NO3 ions. Both selective adsorption of ions and the incorporation of ions accounted for the morphology modification by SO42- ions.