​Minerals are crucial for many organisms. Amorphous calcium carbonate (ACC) is one of the most widespread biogenic minerals and it plays a central role in the formation of skeletal elements of many animals, for example molluscs. Commonly seen in ACC is the stabilising effect of magnesium ions. This was thought to be due to magnesium’s high dehydration energy causing it to bind water more strongly, but recent work suggests other factors may be at play. 

By studying the effect of increasing the magnesium content in a mixed magnesium-calcium system, this study was able to show that the stabilising effect is actually unlikely to be due to the magnesium ion alone but also to the hydroxide ions that co-precipitate with it. Moreover, using a combination of neutron and X-ray total scattering with computational fitting, the researchers were able to see that the small size of the magnesium ion restricts the number of water molecules that can bind to it. 

As a result, roughly a third of water molecules in the pure magnesium phase are bound exclusively to other water molecules and anions. When the larger calcium ion is introduced, this proportion goes down to closer to 10%. As the interaction of water with other water molecules and anions is a hydrogen bond, which is weaker than the coordinative interaction with cations, water is also able to diffuse faster through the pure magnesium phase than through those containing calcium. 

Related publication: “Small Ionic Radius Limits Magnesium Water Interaction in Amorphous Calcium/ Magnesium Carbonates.” J. Phys. Chem. C, 124, 11, 6141–6144 (2020) 

​DOI: 10.1021/acs.jpcc.9b11594