dolomite(redirected from Calcium magnesium carbonate)
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dolomite(dō`ləmīt', dŏl`ə–). 1 Mineral, calcium magnesium carbonate, CaMg (CO3)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.
(named after the French geologist D. Dolomieu [1750-1801]).
(1) A mineral from the carbonate group. In terms of chemical composition, dolomite is a double carbonate of calcium and magnesium: CaMg [CO3]2. It crystallizes in the trigonal system, forming well-defined rhombohedral crystals. In nature, dolomite is encountered in macrocrystalline, finely crystalline, and cryptocrystalline aggregates and sometimes as a rock-forming mineral in oolitic, reniform, alveolar, and other habits. The large crystalline aggregates are usually encountered in hydrothermal formations as well as in carbonate layers that have undergone substantial recrystallization and metamorphism.
Dolomite has a grayish-white color, sometimes with a yellowish, brownish, or greenish cast. Its hardness on the mineralogical scale is 3.5-4, and its density is 2,800-2,900 kg/m3. In contrast to calcite, it does not effervesce in cold hydrochloric acid but does dissolve upon heating.
(2) A sedimentary rock, composed principally of the mineral dolomite (at least 90 percent). If the rock contains 50-90 percent dolomite, it is called limestone dolomite; if it contains even less dolomite, then the rock is known as dolomitized limestone. The most common admixture is calcite, often anhydrite or gypsum, and sometimes authigenic silica. In structure and porosity, dolomite may be compact with a predominance of the basic mineral mass or cavernous and porous with a predominance of cementing material. In terms of origin, dolomite is divided into two genetic groups: exogenous and endogenous. The principal mass of dolomite was formed exogenously in the seas, lagoons, and salty lakes (not connected with seas) with the diagenetic transformation of lime mud under the conditions of increased water salinity. These dolomites usually occur within limestone sequences in the form of beds—sometimes as very extensive, bedlike lenses and crystal accumulations—and also in clays and in detrital and sulfate rocks (anhydrites). The exogenous dolomites also arise as a result of the epigenic dolomitization of limestones. The endogenous dolomites are formed as a result of hydrothermal and hydrothermal-metasomatic processes. This results in the formation of veins, irregularly shaped bodies, and stockworks.
Dolomites have been discovered in the sedimentary sequences of all geological periods, but they are particularly widespread in Precambrian and Paleozoic deposits. Dolomite deposits are extremely numerous both in the USSR and abroad.
Dolomite has extensive practical use. It is used in burnt form as a refractory for lining metallurgical furnaces, is part of the charge for glass of increased hardness and strength, and is used in the production of refractory glaze and magnesia alba and as ashlar, rubblestone, and gravel for concrete. Dolomite and particularly dolomitized limestones are used in blast furnace smelting as fluxes and in agriculture as additives neutralizing acid soils. Abroad (in the United States), dolomite is raw material for obtaining magnesium.
REFERENCESStrakhov, N. M. “O tipakh i genezise dolomitovykh porod.” Trudy Geologicheskogo in-ta AN SSSR, issue 4. Moscow, 1956.
Kurs mestorozhdenii ne metallic he skikh poleznykh iskopaemykh. Moscow, 1969.
G. I. TEODOROVICH