Calcite is a common but spectacular mineral. Its formula is CaCO3, and it is a trigonal-rhombohedral mineral. This crystal structure takes on many different forms – over 300 of them! Here’s a fun fact: aside from taste, geology students can often tell the difference between calcite and halite by their crystal structure. Halite, also known as table salt, forms cubes, while calcite forms rhombohedrons, which look like a sheared deck of cards.

We mentioned polymorphs in the last mineral spotlight, and calcite also has a polymorph. To review, a polymorph is sort of like a fraternal twin, made out of the same stuff, but looking slightly (or a lot!) different. Calcite’s polymorph is aragonite. Aragonite has an orthorhombic crystal structure and it often forms acicular crystals, which means they radiate. The museum hosts a beautiful specimen of white aragonite.

Calcite has a hardness of 3, so it is fairly soft. It is also fluorescent, which means it responds to different wavelengths of ultraviolet light to produce spectacular colors. One of calcite’s practical uses, though, is that it is one of the main components of limestone, which is used to make cement. We have an abundance of gorgeous specimens in the museum, so don’t forget to drop by!