What Is Dolomite?

Dolomite is a mineral composed of calcium, magnesium, carbon and oxygen, with the chemical formula CaMg(CO₃)₂. Its chemical name is calcium magnesium carbonate. The mineral is named after the French mineralogist Deodat Dolomieu, who provided the first description of it in 1791. It can occur as a sedimentary rock, which is found in vast deposits in many parts of the world, and also as crystals. In its pure form it is colorless or white, but the rock and crystals often contain impurities, especially iron, that impart color; it is commonly pinkish in hue, but can be yellow, gray, brown or black.

The mineral is similar in many respects to calcite or calcium carbonate (CaCO₃), which also forms rock deposits — limestone and chalk — or crystals. Like limestone, dolomite has some solubility in water, especially if the water is slightly acidic, and can form caves with stalactites, stalagmites and other typical limestone features. The rock form is sometimes known as dolostone. Crystals of dolomite are curved into a saddle shape, due to the strain placed on the crystal structure by the differing sizes of the calcium and magnesium atoms. This allows the crystalline forms of the two minerals to be easily distinguished from one another.

The rock forms of calcite and dolomite can look very similar; however, they can be distinguished by their reactions with acids. Calcite reacts vigorously with strong acids, and noticeably with weak acids, releasing carbon dioxide (CO₂) and producing effervescence. Dolomite will react only with strong acids and then only slowly and when powdered. The two can thus be distinguished by placing samples in a weak acid, such as vinegar.

The formation of dolomite presents a puzzle for geologists. Limestone is formed mainly by the secretion of calcium carbonate by marine organisms. Dolomite also seems to form mostly in marine environments, but despite being found in huge deposits, it appears that — in contrast to limestone — it is not forming in significant amounts today. It is thought that limestone may form first, and then, if conditions are right, dolomite forms gradually through the replacement of some of the calcium in the limestone by magnesium.

One of the main uses of dolomite is in crushed rock for the construction industry. It is also used in the manufacture of refractory brick. The material is heated to drive off carbon dioxide, leaving a mixture of calcium and magnesium oxides, which — due to their high melting points — are excellent raw material for the brick. It provides a cheap source of magnesium for various applications, including the manufacture of the metal itself.