The crystallization of magnesium hydroxide [Mg(OH)(2)] from the aqueous solutions of magnesium chloride (MgCl2) or magnesium dodecyl sulfate [Mg(DS)(2)] was conducted by means of the slowly diffusing of ammonia gas at 30 degrees C. When the aqueous solution of 5.0 x 10(-3) mol/L MgCl2 was used as a reaction medium, the sunflower-shaped superstructures of thin and branched Mg(OH)(2) sheets were obtained at 2 d; the prolongation of reaction time and the increase of MgCl2 concentration caused the formation and the consequent agglomeration of Mg(OH)(2) platelets. Interestingly, the aqueous solution of 5.0 x 10(-3) mol/L Mg(DS)(2) could be referred as the counterpart of 5.0 x 10(-2) mol/L MgCl2 at the surface nucleation of Mg(OH)(2). The precipitates sampled from Mg(DS)(2) solutions at 2 or 3 d presented the lamellar aggregates of Mg(OH)(2) platelets; while these sampled at 6 and 12 d showed the column-shaped structures of Mg(OH)(2) under the transmission electron microscopy (TEM) observation, suggesting the further growth of Mg(OH)(2) platelets along the c-axis perpendicular to the [0 0 1] face. Meanwhile, the nucleation, the crystal orientation, and the crystallographic defect were also discussed, indicating the different crystallization habits of Mg(OH)(2) in the aqueous solutions of different magnesium sources. (c) 2008 Elsevier B.V. All rights reserved.