Dolomite

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. <http://www.epa.gov/lead/nlic.htm>.

National Osteoporosis Foundation. 1232 22nd Street NW, Washington, DC 20037-1292. (202) 223-2226. <http://www.nof.org>.

Genevieve Slomski

Teresa G. Odle

dolomite The rock dolomite, or dolostone as it is sometimes called, is closely related to limestone, but unlike limestone it is composed of more than 50 per cent of the rhombohedral carbonate mineral dolomite CaMg (CO₃)₂, rather than calcite or aragonite, which are calcium carbonate, CaCO₃.

The origin of dolomite has intrigued geologists since its discovery in 1791 by Deodat de Dolomieu in the dolomites of Italy, both the mineral and the mountains being named after the man. The understanding of dolomite was hampered because modern deposits of dolomite were unknown until the 1950s and were not extensively studied until the 1960s, when it was found forming in intertidal and supratidal deposits of the Arabian Gulf and in the Bahamas and Florida.

Primary dolomite is sometimes precipitated directly from lake or shallow lagoon marine water or from interstitial water in deep marine sediments; in the geological record it is sometimes found as a primary precipitate associated with marine sediments and as cement. Most dolomite appears, however, to have originated by reaction of earlier-formed calcium carbonate with magnesium in solution: either in brines with an Mg/Ca ratio of 10 or more, or in brackish waters with a Ca/Mg ratio of about 5. In modern environments this is achieved in brines by the removal of calcium by organisms and the precipitation of aragonite and gypsum during evaporation. In brackish waters, the mixing of marine and fresh waters produces the same result.

Dolomitization (a diagenetic process) can take place at various times and in various settings during the history of a carbonate sediment. In an arid environment, flooding of supratidal flats introduces marine waters that are subsequently concentrated by evaporation to form brines. After the precipitation of gypsum, these brines have an Mg/Ca ratio greater than 10, and dolomitize aragonitic muds: this process is evaporative dolomitization. These dolomites are usually associated with stromatolitic bedding, gypsum or anhydrite, mudcracks, and cavities produced in drying cyanobacterial mats: ‘bird's-eye’ or ‘fenestral’ structure.

Brines concentrated in coastal lagoons sink and move seawards in the subsurface to react with the carbonate sediments of barriers and nearshore areas: this is reflux dolomitization. Elsewhere, mixing of fresh and marine ground waters produces a brackish water with an Mg/Ca ratio of about 5, which on contact with carbonate sediments can cause dolomitization: this is mixing dolomitization or so-called Dorag dolomitization after the geologist who first suggested this mechanism. After burial, carbonate sediments can be dolomitized by Mg-rich waters, expelled by compaction from adjacent siliciclastic deposits: this is burial dolomitization. Even after lithification and deformation, limestones sometimes experience dolomitization. Fluids exploit unconformities, faults, joints, and other permeable pathways to produce dolomites which cut across the original stratification of the rock: tectonic dolomitization.

Dolomites are more common in Precambrian than in Phanerozoic rocks. This has been attributed to the higher partial pressure of CO₂ in the Precambrian atmosphere and the higher Mg/Ca ratio and lower SO₄ content of Precambrian sea water, as compared with the values of today.

Mole-by-mole replacement of calcium carbonate by dolomite produces a decrease in volume and hence an increase in porosity (approximately 13%). When this replacement occurs in soft sediment much of the increased porosity is lost by subsequent compaction, but when indurated sediment is replaced, the increased porosity will be preserved. Dolomite is important as a raw material for the manufacture of refractories. Also because of their increased porosity, dolomites often act as hosts for hydrocarbons and for lead–zinc and other ores.

G. Evans

Bibliography

Purser, B.,, Tucker, M. E.,, and and Zenger, D. (1994) Dolomites. International Association of Sedimentologists, Special Publication No. 21. Blackwell Scientific Publications, Oxford.
Tucker, M. E. and and Wright, V. P. (1990) Carbonate sedimentology. Blackwell Scientific Publications, Oxford.

Dolomite

The term dolomite is used both for the mineral dolomite (calcium magnesium carbonate [CaMg(CO₃)₂]) and for the rock dolomite, which consists mostly of the mineral dolomite. Dolomite rock is sometimes termed dolostone to distinguish it from the mineral dolomite, but the more confusing terminology is the more prevalent. Dolomite rock is formed from limestone (which is mostly calcite, i.e., calcium carbonate [CaCO₃]) by the replacement of about half of the limestone's calcium ions by magnesium ions. Because of its close relationship to limestone, dolomite is sometimes categorized as a type of limestone.

Limestone forms primarily in shallow seas and coastal waters where shelled marine organisms—crustaceans, mollusks, bivalves, and the like—proliferate. The shells of such creatures consist essentially of calcite. They accumulate on the sea floor in thick beds and are transformed into limestone over time. Some limestone is further transformed to dolomite by processes only partly understood. These various processes are lumped under the term dolomitization. The essential feature of all dolomitization processes is the importation of magnesium ions by water . These take up residence in the crystal structure of the limestone and convert it to dolomite.

Dolomites often occur in association with limestone, gypsum , and other rocks formed by shallow seas. Dolomite beds one or more meters thick are often sandwiched between similarly thick limestone beds. Dolomite and limestone are difficult to tell apart visually; a common field technique for distinguishing them is to drip hydrochloric acid (a hydrous solution of HCl) onto a hand sample. In response, limestone froths vigorously and dolomite weakly.

Metamorphosed limestone becomes calcite marble ; metamorphosed dolomite becomes dolomitic marble. Dolomitic marble can be converted to calcite marble by dedolomitization, that is, the leaching out of magnesium.

Dolomites are used as magnesium ores, as a source of pharmaceutical magnesia (MgO), and as a flux—aid to the removal of impurities—in metal refining.

See also Fossils and fossilization; Field methods in geology; Industrial minerals

dolomite
1. (pearlspar) Widely distributed rock-forming mineral, CaMg(CO₃)₂; sp. gr. 2.8–2.9; hardness 3.5–4.0; trigonal; usually white or colourless, but can be yellowish and brown; white streak; vitreous lustre; crystals are usually rhombohedral with curved, composite faces, also occurs massive and granular; cleavage perfect rhombohedral {1011}; usually secondary, having formed by the action of magnesium-bearing solutions on limestones (dolomitization), also occurs as a gangue mineral in hydrothermal veins particularly associated with galena and sphalerite. It dissolves very slowly in cold, dilute acid, but effervesces very readily when warmed. It is used as a building stone and in the manufacture of bricks for furnaces.

2. (dolostone) A sedimentary rock type, usually formed by the dolomitization of limestones, and commonly occurring interbedded with them. Most limestones contain some magnesium carbonate and strictly the term ‘dolomite’ refers to rocks containing 90% or more of the mineral dolomite (see 1). Dolomite that has formed soon after deposition tends to be fine-grained and to have preserved the original sedimentary structures, whereas recrystallization in late-diagenetic dolomites produces a coarser-grained rock, a loss of sedimentary structures, and an increases in porosity. See also ANKERITE.

dolomite . 1 Mineral, calcium magnesium carbonate, CaMg (CO ₃ ) ₂ . It is commonly crystalline and is white, gray, brown, or reddish in color with a vitreous to pearly luster. The magnesium is sometimes replaced in part by iron or manganese. 2 Carbonate rock composed chiefly of the mineral dolomite, similar to limestone but somewhat harder and heavier. The rock may be metamorphosed into dolomitic marble. Most dolomites probably originated from the partial replacement of the calcium in limestone by magnesium. Its chief uses are as a building stone, for the manufacture of refractory furnace linings, and as basic magnesium carbonate for pipe coverings. Formations of dolomite are very widespread (occurring in Europe, the United States, Africa, Brazil, and Mexico) and notably in the region of the Alps now called the Dolomites, where the rock was first studied by Dolomieu.

Dolomites or Dolomite Alps, Alpine group, N Italy, between the Isarco and Piave rivers, named for the dolomitic limestone of which it is composed. Famous for their strikingly bold outline (a stairstep effect created by erosion of alternate layers of soft and hard rock) and for their vivid colors at sunrise and sunset, the Dolomites are ideal for mountain climbing and skiing. Hydroelectricity is produced in the Dolomites. The Marmolada (10,964 ft/3,342 m), the highest peak, has glaciers. Cortina d'Ampezzo and other resorts are among the major tourist centers of Italy.

dolomite Carbonate mineral, calcium-magnesium carbonate, CaMg(CO₃)₂, found in altered limestones. It is usually colourless or white. A rhombohedral class prismatic crystal, it is often found as a gangue mineral in hydrothermal veins. It is also a sedimentary rock, probably formed by the alteration of limestone by seawater, where calcite has been replaced by calcium magnesium carbonate. Hardness 3.5–4; r.d. 2.8.

do·lo·mite / ˈdäləˌmīt; ˈdō-/ • n. a translucent mineral consisting of a carbonate of calcium and magnesium. ∎  a sedimentary rock formed chiefly of this mineral. DERIVATIVES: dol·o·mit·ic / ˌdäləˈmitik/ adj.

Dolomites (Dolomiti or Dolomiten) Alpine range in ne Italy. The Dolomites are composed of dolomitic limestone, eroded to form a striking landscape popular with mountaineers and tourists. There are several hydroelectric power stations. The highest peak is Marmolada, 3342m (10,964ft) high.

Dolomites (Alpi Dolomitiche), Italy A mountain range composed of dolomitic limestone which thus gave it its name. Dolomite was discovered here by, and named after, a French geologist, Dieudonné Dolomieu (1750–1801).

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