The role of pH and Mg on the stability and crystallization of amorphous calcium carbonate

The effects of pH and Mg on the crystallization of amorphous calcium carbonate (ACC) to vaterite and/or calcite were studied using a combination of in situ time resolved synchrotron-based techniques and electron microscopy. The experiments showed that Mg increased the stability of ACC and favoured the formation of calcite over vaterite. A neutral (similar to 7) starting pH during mixing promoted the transformation of ACC into calcite via a dissolution/reprecipitation mechanism. Conversely, when ACC formed in a solution that started with a high initial pH (similar to 11.5), the transformation to calcite occurred via metastable vaterite, which formed via a spherulitic growth mechanism. In a second stage this vaterite transformed to calcite via a surface-controlled dissolution and recrystallization mechanism. These crystallization pathways can be explained as a consequence of the pH-dependent composition, local structure, stability and dissolution rates of ACC. (c) 2011 Elsevier B.V. All rights reserved.