Dolomite

Dolomite (pronounced /ˈdɒləmaɪt/) is the name of a sedimentary carbonate rock and a mineral, both composed of calcium magnesium carbonate CaMg(CO₃)₂ found in crystals.

Dolomite rock (also dolostone) is composed predominantly of the mineral dolomite. Limestone that is partially replaced by dolomite is referred to as dolomitic limestone, or in old U.S. geologic literature as magnesian limestone. 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.

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Properties

The mineral dolomite 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. Refractive index values are nω = 1.679 - 1.681 and nε = 1.500. Crystal twinning is common. A solid solution series exists between dolomite and iron rich ankerite. Small amounts of iron in the structure give the crystals a yellow to brown tint. Manganese substitutes in the structure also up to about three percent MnO. A high manganese content gives the crystals a rosy pink color noted in the image above. A series with the manganese rich kutnohorite may exist. Lead and zinc also substitute in the structure for magnesium.    

Uses

  Dolomite is used as an ornamental stone, as a raw material for the manufacture of cement, and as a source of magnesium oxide. It is an important petroleum reservoir rock, and serves as the host rock for large strata-bound Mississippi Valley-Type (MVT) ore deposits of base metals (that is, readily oxidized metals) such as lead, zinc, and copper. 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). Home and container gardening are common examples of this use.

As nutritional supplement

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 United States often include significant levels of lead and other toxic elements, users should always verify that such dolomite supplements are from non-ore regions before ingesting them. Further, laboratory experiments conducted at the University of Alberta demonstrate that dolomite is practically insoluable in stomach acid and is eliminated from the body before significant magnesium or calcium can be absorbed. A far safer strategy is to avoid using dolomite as a supplement altogether, and instead taking equivalent amounts of milk of magnesia and calcium supplements. The chemical processes used to create such individual supplements effectively eliminate the risk of ingesting the toxic metals often associated with raw dolomite.

See also

• List of minerals
• Evaporite

References

• Deer, W. A., R. A. Howie and J. Zussman (1966) An Introduction to the Rock Forming Minerals, Longman, pp. 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