Dictionary:

dolomite

  (dō'lə-mīt', dŏl'ə-)

1. A white or light-colored mineral, essentially CaMg(CO₃)₂, used in fertilizer, as a furnace refractory, and as a construction and ceramic material.
2. A magnesia-rich sedimentary rock resembling limestone.

[French, after Déodat de Dolomieu (1750–1801), French geologist.]

The carbonate mineral CaMg(CO₃)₂. Often small amounts of iron, manganese, or excess calcium replace some of the magnesium; cobalt, zinc, lead, and barium are more rarely found. Dolomite is normally white or colorless with a specific gravity of 2.9 and a hardness of 3.5–4 on Mohs scale. It can be distinguished from calcite by its extremely slow reaction with cold dilute acid. Dolomite is a very common mineral, occurring in a variety of geologic settings. It is often found in ultrabasic igneous rocks, notably in carbonatites and serpentinites, in metamorphosed carbonate sediments, where it may recrystallize to form dolomite marbles, and in hydrothermal veins. The primary occurrence of dolomite is in sedimentary deposits, where it constitutes the major component of dolomite rock and is often present in limestones. See also Dolomite rock; Limestone; Sedimentary rocks.

Calcium magnesium carbonate.

Description

Physical Characteristics

Dolomite is a common mineral. It is also known as CaMg(CO₃)₂ and is a type of compact limestone consisting of a calcium magnesium carbonate. In combination with calcite and aragonite, dolomite makes up approximately 2% of the earth's crust. The mineral was first described by and then named after the French mineralogist and geologist Deodat de Dolomieu (1750–1801).

Dolomite is a fairly soft mineral that occurs as crystals as well as in large sedimentary rock beds several hundred feet thick. The crystals—usually rhombohedral in shape—are transparent to translucent and are colorless, white, reddish-white, brownish-white, gray, or sometimes pink. In powdered form, dolomite dissolves readily with effervescence in warm acids.

Although rock beds containing dolomite are found throughout the world, the most notable quarries are located in the Midwestern United States; Ontario, Canada; Switzerland; Pamplona, Spain; and Mexico.

Formation

Although dolomite does not form on the surface of the earth at the present time, massive layers of dolomite can be found in ancient rocks. Dolomite is one of the few sedimentary rocks that undergoes a significant mineralogical change after it is deposited. Dolomite rocks are originally deposited as calcite/aragonite-rich limestone, but during a process called diagenesis, the calcite and/or aragonite is transformed into dolomite. Magnesium-rich ground water containing a significant amount of salt is thought to be essential to dolomite formation. Thus, warm, tropical marine environments are considered the best sources of dolomite formation.

Chemical Components

Dolomite is composed of 52.06% oxygen, 13.03% carbon, 13.18% magnesium, and 21.73% calcium. Iron and manganese carbonates, barium, and lead are sometimes present as impurities.

General Use

Dolomite is commonly used in a variety of products. A few of these are listed below:

• antacids (neutralizes stomach acid)
• base for face creams, baby powders, or toothpaste
• calcium/magnesium nutritional supplements for animals and humans
• ceramic glazes on china and other dinnerware (dolomite is used as source of magnesia and calcia)
• fertilizers (dolomite added as soil nutrient)
• glass (used for high refractive optical glass)
• gypsum impressions from which dental plates are made (magnesium carbonate)
• mortar and cement
• plastics, rubbers, and adhesives

Although calcium carbonate (the kind found in dolomite) has the highest concentration of calcium by weight (40%) and is the most common preparation available, this form of calcium is relatively insoluble and can be difficult to break down in the body. In contrast, calcium citrate, although containing about half as much calcium by weight (21%), is a more soluble form. Since calcium citrate does not require gastric acid for absorption, it is considered a better source of supplemental calcium, particularly for the elderly, whose stomach acid secretions are decreased.

Calcium supplements offer many benefits and recent research even reports that calcium supplements can help prevent formation of kidney stones when combined with a fairly low animal protein, low salt diet. Doctors once advised a low-calcium diet to prevent kidney stones.

Preparations

Dolomite is generally ground into coarse or finelygrained powder and made into calcium/magnesium capsules or antacids for human consumption. The powdered form is also used in animal feed, fertilizers, and a variety of other applications.

Precautions

Nutritional Supplements

Not all commercially prepared calcium supplements are tested for heavy metal contamination. In 1981 the Federal Drug Administration (FDA) cautioned the public to limit the intake of calcium supplements made from dolomite or bone meal (ground up cow's bones) because of potentially hazardous lead levels. Additional studies show that other calcium supplements, such as carbonates and various chelates, may also contain hazardous amounts of lead.

When purchasing calcium supplements, products marked as purified (especially those made from dolomite, bone meal, or oyster shells) or those containing the USP (United States Pharmacopoeia) symbol are considered the safest. The symbol means that the vitamin and mineral manufacturer's product has voluntarily met the USP's criteria for quality, strength, and purity.

New research also encourages consumers to tell their doctors when they take antacids and calcium supplements so that physicians can watch for possible side effects or interactions with medications. Some antacids can cause side effects that eventually put patients at risk for serious problems. If a patient has a complicating problem like renal dysfunction, he or she can suffer from aluminum toxicity from certain antacids.

Ceramic Glazes

Another potential health risk associated with dolomite arises from storing food in or eating or drinking from dinnerware or cups made with glazes containing dolomite. Although it is not possible to detect a lead glaze on china with the naked eye, corroded glaze, or a dusty or chalky, gray residue on the glaze after the piece has been washed is a good indication of lead content. Although high lead toxicity is rare, trace amounts may be present. If possible, it is best to purchase dinnerware that is labeled lead-free. Also, stoneware, unless painted with decorations on the surface, are normally coated with a material that contains no lead. Glass dishes, with the exception of leaded glass and glass painted with decorations or decals, are also considered safe.

The problem is intensified if the food or beverage consumed is acidic, since acid increases lead leaching. Although other additives in glazes may contribute to the lead content (such as lead oxide or cadmium) leaching out, dolomite is a potential cause for lead toxicity.

Glazes on bathtubs also may contain harmful amounts of lead, which may leach out into the bathwater, especially if the glaze is worn. Information regarding lead content can be obtained from the manufacturer. Lead testing kits are also available by mail order or at most home and garden centers.

Fertilizers and Animal Feed

Dolomite and bone meal in fertilizers and animal feed may contaminate the soil, animals, and humans with lead and other toxic metals.

Side Effects

Indirect side effects may occur if more than the recommended dosage of any calcium supplement is taken over an extended period of time. If more than 2,000 mg/day of calcium is consumed, gastrointestinal problems can occur.

Some of the short-term symptoms of low-level lead exposure (which is particularly harmful to the young and elderly) include:

• decreased appetite
• stomachache
• sleeplessness
• constipation
• vomiting
• diarrhea
• fatigue
• irritability
• headaches

Some of the long-term effects of low-level lead exposure include:

• learning disabilities
• brain damage
• loss of IQ points
• attention deficit disorder
• hyperactive behavior
• criminal or antisocial behavior
• neurological problems

Interactions

Research on the interactions of dolomite with other drugs, vitamins, minerals, or foods is limited.

Resources

Books

Deer, W. A., R. A. Howie, and J. Zussman. "Dolomite." In An Introduction to Dolomite. Essex, England: Longman Group, 1966.

Haas, Elson M. "Calcium." In Staying Healthy With Nutrition. Berkeley, CA: Celestial Arts, 1992.

Periodicals

"Unrestricted Calcium Intake Protects Against Recurrent Kidney Stones Better than a Restricted Calcium Diet." Environmental Nutrition (March 2002): 3.

Wooten, James W. "Know Your Antacids—and Who's Taking Them." RN (March 2002): 92.

Organizations

National Lead Information Center. 801 Roeder Road, Suite 600, Silver Spring, MD 20910. (800) 424-LEAD. .

National Osteoporosis Foundation. 1232 22nd Street NW, Washington, DC 20037-1292. (202) 223-2226. .

[Article by: Genevieve Slomski; Teresa G. Odle]

For more information on dolomite, visit Britannica.com.

1. A mineral form of calcium-magnesium carbonate; a constituent of some building limestones.
2. Limestone consisting principally of the mineral dolomite; dolostone.

Environment

Sedimentary rocks, ore veins in sediments, and--rarely--in metamorphic rocks in higher-temperature surroundings.

Crystals rarer and as a rule smaller than calcite, but may range from prismatic to the basic rhombohedron. In limestone or dolomite formations in pockets (often pocket-lining crusts) of pearly, pinkish, subparallel intergrowths of curving saddle-shaped, rhombohedrons. As a bedded rock formation (often called dolostone by today's students), microcrystalline to a coarse marble.

Colorless, white, pinkish, or light tints. Luster glassy to pearly; hardness 3Ɖ-4; specific gravity 2.8; fracture conchoidal; cleavage rhombohedral. Brittle; transparent to translucent.

Calcium magnesium carbonate (30.4% CaO, 21.7% MgO, 47.9% CO ₂ , if the calcium-magnesium ratio is 1:1). It may vary slightly in either direction from this and still be called dolomite, even when some iron has also intruded.

Like calcite, except that it dissolves very slowly in cold acid, unless powdered first and dropped in test tube. Warming the acid speeds up bubbling.

The "pearl spar" white to pinkish crystal intergrowths are readily recognizable. Slow effervescence in cold acid distinguishes it from calcite (rapid) and magnesite (only in hot acid). The intermediate specific gravity will help when a pure piece can be obtained. Seems seldom to be fluorescent.

Far less common than calcite; dolomite rock beds probably form by subsequent alteration of a limestone after its deposition. The pearly clusters are especially common in the U.S., in association with galena, sphalerite, and calcite in low-temperature veins (Missouri-Kansas-Oklahoma lead district) and in pockets in limestone or dolomite quarries (Michigan, Ohio, and Rochester, New York).

Large (several inches) and fine crystals have been found in Switzerland, in pegmatitic seams in North Carolina, and in veins in Colorado. Fine clear Iceland spar-type crystals, collected in a magnesite quarry at Ergui, near Pamplona, Spain, have provided excellent specimens for contemporary collectors. Common as white rhombohedrons in Mexican lead ores with fluorite and anhydrite at Naica and Achiles Serdan (Santa Eulalia).

Dolomite differs slightly in its crystal form from the other rhombohedral carbonates and does not occur in scalenohedrons. The pink, pearly type is usually early in a mineral series, and directly coats the wall rock, underlying calcite, sphalerite, galena, fluorite, celestite, or gypsum--all of which are likely to be later in the depositional sequence. In Mexico it is more often very late.

A limestone with a high concentration of magnesium. Used as a mineral supplement for animals. Called also dolomitic limestone.

For other uses, see Dolomite (disambiguation).

Dolomite crystals

Dolomite (IPA: /ˈdɒləmʌɪt/) 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.

Dolomite from Morocco

Dolomite

Formation

Vast deposits are present in the geological record, but the mineral is relatively rare in modern environments. However, laboratory synthesis of stoichiometric 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

Pale pink crystals of dolomite with a slight pearly luster

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 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. 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

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