A tale of two CaCO3’s — Calcite and Aragonite

Aragonite (left) and calcite (right), both CaCO₃; image by author

Here in this photo, we clearly have two different minerals–one aragonite and the other calcite. However, they both have the same chemical formula of CaCO₃. So how can it be that these two different minerals are both calcium carbonate?

Aragonite and calcite are polymorphs of calcium carbonate. A polymorph is a solid material that can exist in more than one crystal structure. So therefore, both can be CaCO₃ without having to have the same crystal shape thereby creating some differences.

Aragonite and calcite will both react to HCl by bubbling and fizzing, but aragonite has a rather different crystal lattice than calcite does.

While calcite’s structure is rather nicely organized and compact, aragonite’s is a little bit all over the place and less stable:

This difference in structure the allows calcite and aragonite to have some different properties, such as aragonite having a slightly higher hardness than calcite (3.5-4 compared to 3).

Nonetheless, both minerals are very important for shelly invertebrates, as they construct their shells out of calcium carbonate.