The crystallization behavior of CaCO₃ is of tremendous importance due to its relevance in biomineralization, various industrial processes and its potential use as a carbon dioxide sequestering agent. CaCO₃ is thought to crystallize according to the Ostwald’s rule of stages. The least stable amorphous phase (ACC) forms first and transforms stepwise through the various metastable phases into calcite, which is the thermodynamically stable form of CaCO₃ at ambient conditions.

Much research interest has also been devoted to Mg-ACC and Mg-calcite because the magnesium level seems to play a pivotal role in biogenic CaCO₃, which contains in some cases up to 40% Mg.

However, the synthesis of high-magnesian calcite represents a challenge because on the one hand Mg-calcite is more soluble and hence less stable than calcite and on the other hand the dehydration of Mg²⁺ is connected with a high energy barrier and thus hindered kinetically.

We have chosen a new approach to avoid the latter problem by precipitating and crystallizing Ca_(1-x)Mg_xCO₃ in an ionic liquid for the first time. The weakly coordinating anion allows the precipitation of high-mangnesian ACC, which was subsequently crystallized under various conditions. The two side phases CaCO₃ and MgCO₃ have also been precipitated. An astonishing stability of the respective amorphous phases was revealed, even though reaction conditions were thermodynamically controlled (T>100°C, t= 7days).

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