Dolomite
Dolomite 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.
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.
Formation
Vast deposits are present in the geological record, but the mineral is relatively rare in modern environments. 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. The high temperature is likely to speed up the movement of calcium and magnesium ions so that they can find their places in the ordered structure within a reasonable amount of time. 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. 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 dolomitization, 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, a raw material for the manufacture of cement, and a source of magnesium oxide. It is an important petroleum reservoir rock, and it 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. In locations where calcite limestone is uncommon or too costly, dolomite may be 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
Dolomite is sometimes sold as a dietary supplement on the assumption that it should make a good simultaneous source of calcium and magnesium, two important elemental nutrients. However, dolomites from MVT ore region—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 therefore verify that such dolomite supplements are from non-ore regions of the world before ingesting them.
Furthermore, laboratory experiments conducted at the University of Alberta demonstrate that dolomite is practically insoluble in stomach acid and is eliminated from the body before significant amounts of magnesium or calcium can be absorbed. A far safer strategy is to avoid using dolomite as a supplement and instead take 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 that are often associated with raw dolomite.
See also
- Calcium
- Carbonate
- Crystal
- Magnesium
- Mineral
- Rock (geology)
ReferencesISBN links support NWE through referral fees
- Deer, W.A., R.A. Howie, and J. Zussman. 1996. An Introduction to the Rock-Forming Minerals. 2nd ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0582300940.
- Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.
- Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 978-0471721574.
- Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.
- Schumann, Walter. 2006. Gemstones of the World 3rd ed. New York: Sterling. ISBN 1402740166.
- Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.
- Mindat.org. 2007. Dolomite. Mindat.org. Retrieved April 10, 2007.
- Mineral Gallery. 2006. Dolomite. Amethyst Galleries. Retrieved April 10, 2007.
External links
- Dolomite Mineral Data - Webmineral.com. Retrieved April 10, 2007.
Credits
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
The history of this article since it was imported to New World Encyclopedia:
Note: Some restrictions may apply to use of individual images which are separately licensed.
