A mineral formed from calcium magnesium carbonate (Ca,Mg)CO3. The term is also applied to sedimentary carbonate rocks with more than 50% dolomite. It is usually formed by the alteration of calcite (CaCO3).
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.
The mineral dolomite crystallizes in the trigonal - rhombohedral system.
Dolomite was first described in 1791 as the rock by the French naturalist and geologist, Déodat 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. 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. This suggests that the lack of dolomite that is being formed today is likely due to kinematic factors.
Modern dolomite does occur as a precipitating mineral in specialized environments on the surface of the earth today. In the 1950s and 60s, dolomite was found to be forming in highly saline lakes in the Coorong region of South Australia. 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.
The actual role of bacteria in the low-temperature formation of dolomite remains to be 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.
Uses
Dolomite is used as an ornamental stone, as a raw material for the manufacture of cement, and as a source of magnesium oxide. Where calcite limestone is uncommon or too costly, dolomite is sometime used in its place as a flux (impurity remover) for the smelting of iron and steel.
In horticulture, dolomite and dolomitic limestone are added to soils and soilless potting mixes to lower their acidity ("sweeten" them). In nutrition, dolomite is sold sometimes as a dietary supplement on the assumption that it should make a good simultaneous source of the two important elemental nutrients calcium and magnesium. However, since dolomites from Mississippi Valley-Type ore regions such as the Old Lead Belt and New Lead Belt in southeastern Missouri often include significant levels of lead and other toxic elements, users should always verify that such dolomite supplements are from non-ore regions of the world before ingesting them. The chemical processes used to create such individual supplements effectively eliminate the risk of ingesting the toxic metals often associated with raw dolomite. 489 - 493 ISBN 0-582-44210-9 Webmineral Mindat data Mineral galleries Role of Sulfate Reducing Bacteria During Microbial Dolomite Precipitation as Deduced from Culture Experiments Low temperature formation of dolomite and magnesite
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