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.

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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