The Guide to Rocks and Minerals

In focus: Dolomite

Chemical Composition: CaMg(CO3)2
Group: Carbonate
Colour: White, Gray, Reddish white, Brownish white, Gray
Streak: white
Hardness: 3.5-4
Density: 2.8-2.9
Luster: Vitreous
Cleavage: Perfect
Acid sensitive: no

Dolomite is a calcium-magnesium carbonate. The mineral was fist identified by the French mineralogist Deodat de Dolomieu in 1791 and named after the discoverer. Dolomite is a common sedimentary rock that can be found in massive beds several hundred feet thick . These beds are found all over the world and are called (appropriately enough) Dolomite or dolomitic limestone. Dolomite is the main ingredient of dolomitic marble and can also occur in hydrothermal veins, with manganite, and in some serpentinites.

Except for the pink variety, Dolomite can closely resemble calcite. However, there are a number of distinguishing features. Firstly, Dolomite contains magnesium ions. The magnesium ions are not the same size as calcium ions and the two seem incompatible in the same layer. As a result magnesium and calcium create separate layers (see the illustration below). Secondly, Dolomite reacts slowly with acid. It dissolves slowly in cold hydrochloric acid, but the reaction speeds up in hot acid. Finally, Dolomite forms rhombohedrons as its typical crystal habit, but many of these crystals curve into saddle-shaped crystals - a feature unique to Dolomite.

As mentioned above, the magnesium ions are bigger than calcium ions and as a result these do not fit well together in one crystal layer. Dolomite crystal structure is therefore composed of alternate layers of magnesium, carbonate and calcium. This is not unique to Dolomite but it is a defining feature of the Dolomite group of minerals in which the Dolomite is the main member. The Dolomite group of minerals is also know as AB carbonate group [AB(CO₃)₂], where A mineral can be either calcium, barium and/or strontium and the B can be either iron, magnesium, zinc and/or manganese Other minerals in Dolomite group are: Ankerite, Benstonite, Huntite, Kutnohorite, Minrecordite and Norsethite

Dolomite crystal structure

Theoretically, Dolomite should contain about 54.35% of CaCO₃ and 45.65% of MgCO₃ but the the percentage of magnesium and lime (CaC0₃) can vary considerably in different Dolomite deposits. Deposits which contain about 10% more of CaCO₃ over the theoretical composition are termed calcitic Dolomite, high-calcium Dolomite or lime-Dolomite. Deposits characterised by a decreased percentage of MgCO₃ are called dolomitic limestone. When the Dolomites contain only between 5% and 10% of MgCO₃ they are known as magnesian limestones.

Mineral uses

A major source of magnesium, particularly for agricultural and pharmaceutical applications.
An ornamental and structural stone
In metallurgy for extracting certain metals from their ores
To a lesser extent, in the glass industry, especially in sheet-glass manufacture

Formation of Dolomite

Although Dolomite is a common sedimentary rock, the formation of Dolomite is a much more complex process than with other sedimentary rocks. For example, limestones are continuously formed, but at present there is no formation of Dolomite taking place on the surface of the earth. So why not? The answer to that lies in the way that Dolomite forms. It appears that Dolomite is one one of the relatively few sedimentary rocks that undergoes a significant mineralogical change after it is deposited. The pre-Dolomites originally deposit as calcite/aragonite-rich limestones which then undergo a secondary process known as diagenesis. This process is not metamorphism, but something similar. This results in the addition of magnesium to the calcite and/or aragonite and probably requires magnesium rich ground water that has a significant amount of salinity. Warm, tropical environments near the ocean are probably the best places for Dolomite formation.

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