UK scientists are a step closer to understanding how and why nature makes amorphous calcium carbonate.

Calcium carbonate is abundant. It's found in minerals such as chalk, limestone and marble. It's the main component of shells and is commonly used medicinally as a calcium supplement or as an antacid.  And it's also important in industry, where it is used in coatings and fillers, and where it can precipitate out as an unwanted product.

Calcium carbonate is found in different forms, the majority of which have a regular, crystalline structure. But one form - amorphous calcium carbonate - has a highly disordered structure.

It's this disordered form of calcium carbonate that attracted the attention of Fiona Meldrum at the University of Bristol, UK, and colleagues. 

Since organisms produce amorphous calcium carbonate, despite the fact that synthetic amorphous calcium carbonate is typically unstable and crystallises rapidly, there must be some advantage to this phase which could be used synthetically, said Meldrum.

The amorphous calcium carbonate made by living organisms contains organic macromolecules plus significant quantities of magnesium and phosphate, believed to stabilise it. With this in mind Meldrum and co-workers looked at how the additives poly(aspartic acid) and magnesium change the way calcium carbonate crystallises.  

The researchers found that the effect of the additives on the initial amorphous phase formed, can go on to influence the final crystal form of calcium carbonate. 

We are now closer to understanding the role additives play in controlling calcium carbonate precipitation via an amorphous precursor phase, said Meldrum.

Sarah Corcoran