Dolomite


 * For the 18th century French naturalist Deodat de Dolomieu. For the 1975 blaxploitation film, see Dolemite.



Dolomite is the name of both a carbonate rock and a mineral consisting of calcium magnesium carbonate (formula: CaMg(CO3)2) found in crystals. Dolomite rock (also dolostone) is composed predominantly of the mineral dolomite. Limestone which is partially replaced by dolomite is referred to as dolomitic limestone, or in old U.S. geologic literature as magnesian limestone.

Dolomite mineral crystallizes in the trigonal - rhombohedral system. It forms white, gray to pink, commonly curved, crystals although it is usually massive. It has physical properties similar to those of the mineral calcite, but does not rapidly dissolve or effervesce (fizz) in dilute hydrochloric acid. The Mohs hardness is 3.5 to 4 and the specific gravity is 2.85.

Dolomite was first described in 1791 as the rock by the French naturalist and geologist, Deodat Guy Tancrede de Gratet de Dolomieu (1750-1801) for exposures in the Dolomite Alps of northern Italy.

The dolomite problem
There is significant uncertainty regarding the cause of dolomite formation. Vast deposits are present in the geological record, but the mineral is relatively rare in modern environments. This is referred to as the "Dolomite Problem". Dolomite accounts for about 10% of all sedimentary rock, including much that would have been produced near the surface of the Earth. However, laboratory synthesis of undisputed dolomite has been carried out only at temperatures of greater than 100 degrees Celsius, conditions typical of burial in sedimentary basins - even though much dolomite in the rock record appears to have formed in low-temperature conditions.

Modern dolomite does occur as a precipitating mineral in specialized environments on the surface of the earth today. In the 1950's and 60's, dolomite was found to be forming in highly saline lakes in the Coorong region of South Australia. Dolomite crystals also occur in deep-sea sediments, where organic matter content is high. This dolomite is termed "organogenic" dolomite.

Recent research has found modern dolomite formation under anaerobic conditions in supersaturated saline lagoons along the Rio de Janeiro coast of Brazil, namely, Lagoa Vermelha and Brejo do Espinho. One interesting reported case was the formation of dolomite in the kidneys of a dalmatian dog. This was believed to be due to chemical processes triggered by bacteria. Dolomite has been speculated to develop under these conditions with the help of sulfate-reducing bacteria. This joins other research in pointing out many new interesting links between large-scale geology and small-scale microbiology (see geomicrobiology).

The actual role of bacteria in the low-temperature formation of dolomite remains to be demonstrated. The specific mechanism of dolomitizaqtion, involving sulfate-reducing bacteria, has not yet been demonstrated.

Dolomite appears to form in many different types of environment and can have varying structural, textural and chemical characteristics. Some researchers have stated "there are dolomites and dolomites," meaning that there may not be one single mechanism by which dolomite can form. Much modern dolomite differs significantly from the bulk of the dolomite found in the rock record, leading researchers to speculate that environments where dolomite formed in the geologic past differ significantly from those where it forms today.

Reproducible laboratory syntheses of dolomite (and magnesite) leads first to the initial precipitation of a metastable "precursor" (such as magnesium calcite), to be changed gradually into more and more of the stable phase (such as dolomite or magnesite) during periodical intervals of dissolution and reprecipitation. The general principle governing the course of this irreversible geochemical reaction has been coined "Ostwald's Step Rule".

Uses
Dolomite is used as an ornamental stone and as a raw material for the manufacture of cement. It is also a source of magnesium oxide. It is an important petroleum reservoir rock and a host rock for large strata-bound MVT base metal (lead, zinc, copper, etc.) deposits. It is sometimes used in place of calcite in the production of iron and steel where it serves as a flux to remove impurities and assists in reducing iron ore. It is sometimes sold as a dietary supplement as a source of calcium and magnesium.