The synthesis of aragonite precipitated calcium carbonate by treating a suspension of Ca(OH)(2) with CO2 gas was investigated with regard to the effects of Mg2+ ions and organic additives on polymorphism and alternative orientations. In the presence of a small amount of Mg2+, Mg-calcite formed, but as the Mg2+ ion concentration increased, the amount of Mg-calcite decreased and the amount of aragonite increased. Thus, the formation of Mg-calcite is suppressed and only aragonite is formed in the presence of 60 mol% MgCl2. As the Mg2+ ion concentration increased, the aragonite that formed was found to have decreased in terms of its longitude and aspect ratio. Furthermore, the effect of Mg2+ ions in conjunction with organic additives was also investigated with regard to polymorphs and morphology and the structure-forming properties of the organic additives. (C) 2008 Elsevier B.V. All rights reserved.