Dolomite (rock)
Dolomite (also known as dolomite rock, dolostone or dolomitic rock) is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite, CaMg(CO3)2. It occurs widely, often in association with limestone and evaporites, though it is less abundant than limestone and rare in Cenozoic rock beds (beds less than about 66 million years in age). The first geologist to distinguish dolomite from limestone was Déodat Gratet de Dolomieu; a French mineralogist and geologist whom it is named after. He recognized and described the distinct characteristics of dolomite in the late 18th century, differentiating it from limestone.
Most dolomite was formed as a magnesium replacement of limestone or of
Dolomite is resistant to
Name
Dolomite takes its name from the 18th-century French mineralogist Déodat Gratet de Dolomieu (1750–1801), who was one of the first to describe the mineral.[8][9]
The term dolomite refers to both the calcium-magnesium carbonate mineral and to sedimentary rock formed predominantly of this mineral. The term dolostone was introduced in 1948 to avoid confusion between the two. However, the usage of the term dolostone is controversial, because the name dolomite was first applied to the rock during the late 18th century and thus has technical precedence. The use of the term dolostone was not recommended by the Glossary of Geology published by the
In old
Description
Dolomite rock is defined as
Dolomite outcrops are recognized in the
Under the microscope, thin sections of dolomite usually show individual grains that are well-shaped rhombs, with considerable pore space. As a result, subsurface dolomite is generally more porous than subsurface limestone and makes up 80% of carbonate rock petroleum reservoirs.[16] This texture contrasts with limestone, which is usually a mixture of grains, micrite (very fine-grained carbonate mud) and sparry cement. The optical properties of calcite and mineral dolomite are difficult to distinguish, but calcite almost never crystallizes as regular rhombs, and calcite is stained by Alizarin Red S while dolomite grains are not.[17] Dolomite rock consisting of well-formed grains with planar surfaces is described as planar or idiotopic dolomite, while dolomite consisting of poorly-formed grains with irregular surfaces is described as nonplanar or xenotopic dolomite.[15] The latter likely forms by recrystallization of existing dolomite at elevated temperature (over 50 to 100 °C (122 to 212 °F)).[17]
The texture of dolomite often shows that it is secondary, formed by replacement of calcium by magnesium in limestone. The preservation of the original limestone texture can range from almost perfectly preserved to completely destroyed.[18] Under a microscope, dolomite rhombs are sometimes seen to replace oolites or skeletal particles of the original limestone.[19] There is sometimes selective replacement of fossils, with the fossil remaining mostly calcite and the surrounding matrix composed of dolomite grains. Sometimes dolomite rhombs are seen cut across the fossil outline. However, some dolomite shows no textural indications that it was formed by replacement of limestone.[17]
Occurrence and origin
Dolomite is widespread in its occurrences, though not as common as limestone.
Many dolomites show clear textural indications that they are secondary dolomites, formed by replacement of limestone. However, although much research has gone into understanding this process of dolomitization, the process remains poorly understood. There are also fine-grained dolomites showing no textural indications that they formed by replacement, and it is uncertain whether they formed by replacement of limestone that left no textural traces or are true primary dolomites. This dolomite problem was first recognized over two centuries ago but is still not fully resolved.[21]
The dolomitization reaction
- 2CaCO3 + Mg2+ → CaMg(CO3)2 + Ca2+
is thermodynamically favorable, with a Gibbs free energy of about -2.2 kcal/mol. In theory, ordinary seawater contains sufficient dissolved magnesium to cause dolomitization. However, because of the very slow rate of diffusion of ions in solid mineral grains at ordinary temperatures, the process can occur only by simultaneous dissolution of calcite and crystallization of dolomite. This in turn requires that large volumes of magnesium-bearing fluids are flushed through the pore space in the dolomitizing limestone.[24] Several processes have been proposed for dolomitization.
The hypersaline model (also known as the evaporative reflux model[25]) is based on the observation that dolomite is very commonly found in association with limestone and evaporites, with the limestone often interbedded with the dolomite. According to this model, dolomitization takes place in a closed basin where seawater is subject to high rates of evaporation. This results in precipitation of gypsum and aragonite, raising the magnesium to calcium ratio of the remaining brine. The brine is also dense, so it sinks into the pore space of any underlying limestone (seepage refluxion), flushing out the existing pore fluid and causing dolomitization. The Permian Basin of North America has been put forward as an example of an environment in which this process took place.[25] A variant of this model has been proposed for sabkha environments in which brine is sucked up into the dolomitizing limestone by evaporation of capillary fluids, a process called evaporative pumping.[25]
Another model is the mixing-zone or Dorag model, in which meteoric water mixes with seawater already present in the pore space, increasing the chemical activity of magnesium relative to calcium and causing dolomitization. The formation of Pleistocene dolomite reefs in Jamaica has been attributed to this process. However, this model has been heavily criticized,[26] with one 2004 review paper describing it bluntly as "a myth".[27] A 2021 paper argued that the mixing zone serves as domain of intense microbial activity which promotes dolomitization.[28]
A third model postulates that normal seawater is the dolomitizing fluid, and the necessary large volumes are flushed through the dolomitizing limestone through tidal pumping. Dolomite formation at Sugarloaf Key may be an example of this process. A similar process might occur during rises in sea level, as large volumes of water move through limestone platform rock.[29]
Regardless of the mechanism of dolomitization, the tendency of carbonate rock to be either almost all calcite or almost all dolomite suggests that, once the process is started, it completes rapidly.[30] The process likely occurs at shallow depths of burial, under 100 meters (330 ft), where there is an inexhaustible supply of magnesium-rich seawater and the original limestone is more likely to be porous. On the other hand, dolomitization can proceed rapidly at the greater temperatures characterizing deeper burial, if a mechanism exists to flush magnesium-bearing fluids through the beds.[31]
Mineral dolomite has a 12% to 13% smaller volume than calcite per alkali cation. Thus dolomitization likely increases porosity and contributes to the sugary texture of dolomite.[16]
The dolomite problem and primary dolomite
Dolomite is supersaturated in normal seawater by a factor of greater than ten, but dolomite is not seen to precipitate in the oceans. Likewise, geologists have not been successful at precipitating dolomite from seawater at normal temperatures and pressures in laboratory experiments. This is likely due to a very high
The magnesium ion is a relatively small ion, and it acquires a tightly bound
It is possible that microorganisms are capable of precipitating primary dolomite.
Dedolomitization
Dolomitization can sometimes be reversed, and a dolomite bed converted back to limestone. This is indicated by a texture of pseudomorphs of mineral dolomite that have been replaced with calcite. Dedolomitized limestone is typically associated with gypsum or oxidized pyrite, and dedolomitization is thought to occur at very shallow depths through infiltration of surface water with a very high ratio of calcium to magnesium. [42]
Uses
Dolomite is used for many of the same purposes as limestone, including as
Caves in dolomite rock
As with limestone
Dolomite speleothems
Both calcium and magnesium go into solution when dolomite rock is dissolved. The
See also
References
- ISBN 0-87933-416-9.
- ISBN 9780444533449.
- ^ "Dolomite. A sedimentary rock known as dolostone or dolomite rock". Geology.com. Retrieved 20 June 2014.
- ^ Fowles, Julian (25 October 1991). "Dolomite: the mineral that shouldn't exist - Scientists have never been able to make dolomite in the way the mineral forms naturally. Theories have come and gone, but the mystery of its origins remains". New Scientist. Retrieved 2021-05-31.
- S2CID 49341088.
- ^ S2CID 4371495.
- ^ .
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- ^ Saussure le fils, M. de (1792): "Analyse de la dolomite". Journal de Physique, vol. 40, pp. 161–173.
- ISBN 978-0922152896.
- ISBN 0131547283.
- ISBN 0136427103.
- ^ Blatt & Tracy 1996, p. 318.
- ^ Blatt & Tracy 1996, p. 295.
- ^ a b Boggs 2006, pp. 167–168.
- ^ a b Blatt, Middleton & Murray 1980, pp. 529–530.
- ^ a b c Blatt & Tracy 1996, p. 319.
- ^ Boggs 2006, p. 168.
- ^ Blatt, Middleton & Murray 1980, pp. 512–513.
- ^ Boggs 2006, p. 169.
- ^ a b Boggs 2006, p. 182.
- ^ Blatt & Tracy 1996, pp. 317–318.
- ^ Boggs 2006, pp. 187–188.
- ^ Blatt, Middleton & Murray 1980, pp. 518–519.
- ^ a b c Blatt & Tracy 1996, p. 321.
- ^ Boggs 2006, pp. 185–186.
- S2CID 131159219.
- S2CID 234012426.
- ^ Boggs 2006, pp. 186–187.
- ^ Blatt, Middleton & Murray 1980, pp. 517–518.
- ^ Blatt & Tracy 1996, pp. 322–323.
- ^ a b Blatt & Tracy 1996, p. 323.
- ^ Boggs 2006, pp. 182–183.
- ^ Blatt, Middleton & Murray 1980, pp. 510–511.
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- .
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- ^ Blatt, Middleton & Murray 1980, pp. 531–532.
- ^ Lamar, J.E. (1961). "Uses of limestone and dolomite" (PDF). Illinois State Geological Survey Circular. 321. Retrieved 15 September 2021.
- ISBN 9780470320488. Retrieved 14 September 2021.
- ^ ISBN 1-879961-07-5
- ISBN 0-12-406061-7
- ^ Polyak, Victor J.; Provencio, Paula (2000). "By-product materials related to H2S-H2SO4-influenced speleogenesis of Carlsbad, Lechuguilla, and other caves of the Guadalupe Mountains, New Mexico". Journal of Cave and Karst Studies. 63 (1): 23–32. Retrieved 4 April 2020.
- ^ ISBN 0-12-406061-7
Further reading
- Blatt, Harvey; Tracy, Robert J. (1996). Petrology; Igneous, Sedimentary, and Metamorphic (2nd ed.). W. H. Freeman. ISBN 0-7167-2438-3.
- ISBN 0-632-01472-5.