Dolomite /ˈdÉ'lÉ™maɪt/ is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally CaMg(CO3)2. The word dolomite is also used to describe the sedimentary carbonate rock, which is composed predominantly of the mineral dolomite (also known as dolostone).
History
Most probably the mineral dolomite was first described by Carl Linnaeus in 1768. In 1778, it was described by the Austrian naturalist Belsazar Hacquet as the "stinking stone" (German: Stinkstein, Latin: lapis suillus). In 1791, it was described as a rock by the French naturalist and geologist, Déodat Gratet de Dolomieu (1750â€"1801) first from buildings in the old city of Rome and later as samples collected in what is now known as the Dolomite Alps of northern Italy. The mineral was given its name in March 1792 by Nicolas de Saussure, naming it after De Dolomieu. Hacquet and Dolomieu met in Laibach (Ljubljana) in 1784, which may have contributed to De Dolomieu's work.
Properties
The mineral dolomite crystallizes in the trigonal-rhombohedral system. It forms white, tan, gray, or pink crystals. Dolomite is a double carbonate, having an alternating structural arrangement of calcium and magnesium ions. It does not rapidly dissolve or effervesce (fizz) in dilute hydrochloric acid as calcite does. Crystal twinning is common.
Solid solution exists between dolomite, the iron-dominant ankerite and the manganese-dominant kutnohorite. 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. Lead, zinc, and cobalt also substitute in the structure for magnesium. The mineral dolomite is closely related to huntite Mg3Ca(CO3)4.
Because dolomite can be dissolved by slightly acidic water, areas of dolomite are important as aquifers and contribute to karst terrain formation.
Formation
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. It is often thought that dolomite will develop only with the help of sulfate-reducing bacteria (e.g. Desulfovibrio brasiliensis). However, promising new research on low-temperature dolomite formation indicates that low-temperature dolomite may occur in natural environments rich in organic matter and microbial cell surfaces. This occurs as a result of magnesium complexation by carboxyl groups associated with organic matter.
Vast deposits of dolomite are present in the geological record, but the mineral is relatively rare in modern environments. Reproducible, inorganic low-temperature syntheses of dolomite and magnesite were published for the first time in 1999. Those laboratory experiments showed how the initial precipitation of a metastable "precursor" (such as magnesium calcite) will change gradually into more and more of the stable phase (such as dolomite or magnesite) during periodical intervals of dissolution and re-precipitation. The general principle governing the course of this irreversible geochemical reaction has been coined "breaking Ostwald's step rule".
There is some evidence for a biogenic occurrence of dolomite. One example is that of the formation of dolomite in the urinary bladder of a Dalmatian dog, possibly as the result of an illness or infection.
Formation of dolomite from solution and its link with biogenic dolomite
Recently, it has been experimentally discovered that the direct crystallization of dolomite can occur from solution at temperatures between 60â€"220 °C. Dolomite forms through a three-stage process: 1) A nanoparticulate magnesium-deficient, amorphous calcium carbonate (Mg-ACC) forms; 2) After a temperature-dependent induction time, this Mg-ACC partially dehydrates and orders prior to its rapid (<5 min) crystallization to non-stoichiometric proto-dolomite (with a lower Mg/Ca ratio compared to dolomite). This occurs via the dissolution of Mg-ACC, followed by the secondary nucleation of proto-dolomite from solution. 3) Finally, the proto-dolomite crystallization proceeds via spherulitic growth that follows a growth front nucleation mechanism with a de-nuovo and continuous formation of nanocrystalline proto-dolomite subunits that form spherical aggregates. In stage three of the reaction, the proto-dolomite transforms to highly crystalline and stoichiometric dolomite on a much longer timescale (hours to days), via an Ostwald-ripening mechanism. Such a three-stage crystallization can explain microbially induced proto-dolomites observed in modern hypersaline settings and may also be the route by which the Cryogenian cap dolomite deposits of the Neoproterozoic formed.
Uses
Dolomite is used as an ornamental stone, a concrete aggregate, and a source of magnesium oxide, as well as in the Pidgeon process for the production of magnesium. 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 such as lead, zinc, and copper. Where calcite limestone is uncommon or too costly, dolomite is sometimes used in its place as a flux for the smelting of iron and steel. Large quantities of processed dolomite are used in the production of float glass.
In horticulture, dolomite and dolomitic limestone are added to soils and soilless potting mixes as a pH buffer and as a magnesium source. Home and container gardening are common examples of this use.
Dolomite is also used as the substrate in marine (saltwater) aquariums to help buffer changes in pH of the water.
Calcined dolomite is also used as a catalyst for destruction of tar in the gasification of biomass at high temperature.
Particle physics researchers like to build particle detectors under layers of dolomite to enable the detectors to detect the highest possible number of exotic particles. Because dolomite contains relatively minor quantities of radioactive materials, it can insulate against interference from cosmic rays without adding to background radiation levels.
Dolomite is a popular choice for motorcycle speedway tracks throughout Australia and New Zealand.
Dolomite is used in the ceramic industry and in studio pottery as a glaze ingredient.
See also
- Dolomitization
- Evaporite
- List of minerals
- Magnesian Limestone
- Main Dolomite
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