One of the most challenging, yet rewarding aspects of being a rockhound, is successfully identifying similar minerals. Rock & Gem's prolific consulting editor Bob Jones provides an insightful and interesting review of the differences between calcite and dolomite.
Is It Calcite or Dolomite?
These Carbonates Have Chemistry
By Admin -
August 27, 2019
The white dolomite provides a nice contrast to the fine siderite rhomb in this North Carolina specimen. (IROCKS.com)
Story by 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?”
Distinguishing Traits
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. Calcite also 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 only 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.
A rich amber-colored, 8.5-cm.-long calcite scalenohedron from the North-End Denton mine (Harris Creek district), Illinois, stands majestically on a thin layer of limestone, along with druzy “ruby jack” sphalerite.(irocks.com)
The second question is considerably more difficult to answer. Limestone and dolostone are both carbonate rocks, but externally there is little difference. The distinction really lies within the atomic structure of these carbonates. Scientists have a name for what’s happening: isomorphic substitution. Certain elements have the ability to substitute for each other in the atomic structure of a developing mineral’s molecules. This is possible because the atoms of different elements ... "
https://www.rockngem.com/is-it-calcite- ... cite082919
