Dolomite

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Crystals of dolomite. The pink color arises from the presence of manganese.

Dolomite is the name for a mineral composed of calcium magnesium carbonate (CaMg(CO3)2) and for a sedimentary rock that has this mineral as its chief constituent. The rock was given this name first, but to avoid confusion it is sometimes known as dolostone. The pure mineral is white, but traces of impurities can give it a range of colors, including pink, yellow, brown, and gray.

Contents

Dolomite is used as an ornamental stone and for many practical applications. For instance, it is a raw material for the manufacture of cement, and a source of magnesium oxide. It is an important reservoir rock for petroleum, and a host rock for ores of metals such as lead, zinc, and copper. In horticulture, dolomite may be added to soils and potting mixes to lower their acidity.

Etymology

The rock dolomite was first described in 1791, by the French naturalist and geologist Déodat Gratet de Dolomieu (1750-1801), when he observed exposures in the Dolomite Alps of northern Italy.

Limestone that is partially replaced by dolomite is referred to as dolomitic limestone. In old American geologic literature, it is called magnesian limestone.

Formation

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 be more than one mechanism by which dolomite is formed.

Vast deposits of dolomite are present in the geological record, but the mineral is relatively rare in modern environments. In addition, much modern dolomite differs significantly from the bulk of the mineral in the rock record, leading researchers to speculate that the environments and mechanisms involved in the formation of dolomite in the geologic past differ significantly from those involved in its formation today.

Dolomite accounts for about ten percent of all sedimentary rock, including much that would have been produced near the surface of the Earth. Although much of the dolomite in the rock record appears to have formed under low-temperature conditions, laboratory synthesis of undisputed dolomite has been found to require temperatures above 100 degrees Celsius, conditions typical of burial in sedimentary basins. 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.

Modern dolomite has been found as a precipitating mineral in specialized environments on the surface of the Earth. In the 1950s and 1960s, 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 the content of organic matter is high. In such cases, the mineral is called "organogenic" dolomite.

Characteristics

The mineral dolomite forms curved crystals that are commonly twinned, but it is usually found in the massive state. It crystallizes in the trigonal-rhombohedral system. Its physical properties are similar to those of calcite, but it does not rapidly dissolve or effervesce (fizz) in dilute hydrochloric acid. Its Mohs hardness is in the range 3.5 to 4, and its specific gravity is 2.85. Refractive index values are nω = 1.679 - 1.681 and nε = 1.500.

Small amounts of iron in the structure give the crystals a yellow-to-brown tint. There is a solid solution series between dolomite and iron-rich ankerite.[1] In addition, manganese may substitute in the structure up to about three percent manganese oxide. A high manganese content gives the crystals a rosy pink color. A series with the manganese-rich kutnohorite[2] may exist. Lead and zinc may also substitute for magnesium in the structure.

Uses

Pale pink crystals of dolomite with a slight pearly luster

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 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 a 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 Mississippi Valley-Type 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

Notes

  1. Ankerite is a calcium, iron, magnesium, manganese carbonate mineral of the group of rhombohedral carbonates, with the formula Ca(Fe,Mg,Mn)(CO3)2. In composition, it is closely related to dolomite, but it differs in having magnesium replaced by varying amounts of iron(II) and manganese.
  2. Kutnohorite is a rare calcium manganese magnesium iron carbonate mineral, with the formula Ca(Mn,Mg,Fe)(CO3)2. It is a manganese-rich analog of the dolomite-ankerite series.

References

  • 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

All links retrieved August 22, 2013.

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