Dehydration and crystallization of amorphous calcium carbonate in solution and in air

• Published in: Nature communications Vol. 5; no. 1; p. 3169
• Main Authors: Ihli, Johannes, Wong, Wai Ching, Noel, Elizabeth H., Kim, Yi-Yeoun, Kulak, Alexander N., Christenson, Hugo K., Duer, Melinda J., Meldrum, Fiona C.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 28.01.2014; Nature Publishing Group; Nature Pub. Group
• Subjects: 140/131; 140/133; 639/301/54/989; 639/301/54/991; 639/638/263; Crystallization; Humanities and Social Sciences; multidisciplinary; NMR; Nuclear magnetic resonance; Science; Science (multidisciplinary); United Kingdom > UK
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms4169

The mechanisms by which amorphous intermediates transform into crystalline materials are poorly understood. Currently, attracting enormous interest is the crystallization of amorphous calcium carbonate, a key intermediary in synthetic, biological and environmental systems. Here we attempt to unify many contrasting and apparently contradictory studies by investigating this process in detail. We show that amorphous calcium carbonate can dehydrate before crystallizing, both in solution and in air, while thermal analyses and solid-state nuclear magnetic resonance measurements reveal that its water is present in distinct environments. Loss of the final water fraction—comprising less than 15% of the total—then triggers crystallization. The high activation energy of this step suggests that it occurs by partial dissolution/recrystallization, mediated by surface water, and the majority of the particle then crystallizes by a solid-state transformation. Such mechanisms are likely to be widespread in solid-state reactions and their characterization will facilitate greater control over these processes. The crystallization of amorphous calcium carbonate is a widely studied process. Here, the authors probe the mechanism and show that transformation to calcite is preceded by dehydration, even in solution, and that loss of the final water fraction triggers crystallization.