Is it Calcite or Dolomite?

These Carbonates have Chemistry.

- Bob Jones

Calcite, a calcium carbonate, is the most common of the carbonates. We admire its lovely crystal forms—all 600 of them. We also use it in huge quantities in its massive form, limestone.

Dolomite, a calcium, magnesium carbonate, is not as common as calcite. It also forms in lovely crystals, but not in as wide a variety as calcite. We also use dolomite in its massive form, dolostone. Which brings up two questions: “How do we distinguish calcite from dolomite?” and “How do we distinguish between limestone and dolostone?”

The first question is relatively easy to answer. Dolomite never forms in scalenohedral, or “dogtooth”, crystals, which is one of the common forms of calcite. Also, calcite forms flat, rhomboid “poker chip” crystals, but dolomite does not. Calcite responds vigorously to acid, while dolomite responds less vigorously. Calcite is 3 on the Mohs Scale of Hardness, but dolomite is Mohs 4-4.5. One other difference you may see is the smoothness of the crystal faces on your specimen. Dolomite crystals, because of an unusual internal structure, may have slightly curved crystal faces, while calcite does not. These are some simple tests you can use to determine the identity of a carbonate you own.