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.