Calcite and Dolomite: Carbonate Minerals

Calcite and dolomite are very similar minerals. Both have the same hardness (H = 3), the same rhombohedral cleavage, and are found in identical geologic settings. The best way to tell one from the other is the acid test: a drop of 1 M HCl on calcite produces an instant, obvious fizz; a drop on dolomite produces slow or no obvious bubbling. Frequently, you have to use a knife point to powder a small area of dolomite to get a visible reaction to a drop of acid.
This exceptionally clear calcite is known as Iceland Spar. It is an excellent example of the rhombohedral cleavage of calcite (three cleavage planes, none at right angles). Also, it shows how you can see double vision when you look through 2 of 3 sides of a calcite rhomb.

Photo: Dept. of Physics, University of Naples.

Like many minerals, calcite comes in a variety of colors. Although it is tough to see in the photo, each of these pieces forms a rhombohedron. No cubes allowed!
If you find a big block of whitish mineral and want to know if it is calcite, look where it has broken. Here you can see several rhombohedra corners poking out. Look for the non-90° angles between the cleavage planes!
The dark mineral is biotite and the white mineral is calcite. The calcite is readily identified by its rhombohedral cleavages displayed near the bottom of this photograph. It is highly unusual to find biotite and calcite together.

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