Abstract
The stabilizing effect of magnesium ions on amorphous calcium carbonate has been studied extensively due to its widespread occurrence in biogenic minerals. It has long been suggested that magnesium...
Highlights
Magnesium has long been examined as an additive in calcium carbonates, generally studied for its stabilizer effect on the amorphous phase[1−4] but is found as an additive in, for example, biogenic calcium carbonates.[5−7] Magnesium ions are well-known to increase the lifetime of the amorphous phase[1,8] and even control the polymorph selection during crystallization.[9]
Recent studies have shown that diffusion of water is significantly faster in amorphous magnesium carbonate (AMC) compared to amorphous calcium carbonate (ACC) and even compared to mixed phases with as much as 73% Mg/(Ca + Mg), suggesting that calcium reduces water mobility more effectively than magnesium, despite the lower dehydration energy.[2,12]
We find that for x = 1, that is, for AMC, the low coordination number of magnesium limits the number of water molecules that can bind to the cation as most of the sites are occupied by the anions, resulting in ∼35% of the water only bound by hydrogen bonds
Summary
Magnesium has long been examined as an additive in calcium carbonates, generally studied for its stabilizer effect on the amorphous phase[1−4] but is found as an additive in, for example, biogenic calcium carbonates.[5−7] Magnesium ions are well-known to increase the lifetime of the amorphous phase[1,8] and even control the polymorph selection during crystallization.[9]. In the mixed phase (x = 0.5, ACMC) the magnesium ion showed a similar local structure to the one in AMC, but here, the calcium ion binds more water molecules, and the amount of water not binding to cations is reduced to ∼10%. This explains the faster diffusion of water in the AMC phase with respect to ACC and ACMC and suggests that the stabilizing effect of magnesium is more likely connected to the presence of the hydroxide ion rather than the strong cation−water interaction. Neutron pdf was measured at the SANDALS Instrument[29] at ISIS (STFC Rutherford Appleton Laboratory, Didcot, UK).
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