Dolomite (rock)
Dolomite is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite, CaMg2. It occurs widely, often in association with limestone and evaporites, though it is less abundant than limestone and rare in Cenozoic rock beds. One of the first geologists to distinguish dolomite from limestone was Déodat Gratet de Dolomieu, a French mineralogist and geologist after whom it is named. 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 lime mud before lithification. The geological process of conversion of calcite to dolomite is known as dolomitization and any intermediate product is known as dolomitic limestone. The "dolomite problem" refers to the vast worldwide depositions of dolomite in the past geologic record in contrast to the limited amounts of dolomite formed in modern times. Sulfate-reducing bacteria living in anoxic conditions can precipitate dolomite suggesting that some past dolomite deposits might be due to microbial activity. Recent laboratory research focused on the crystal growth of dolomite at the microscopic scale has revealed that multiple cycles of precipitation/dissolution can promote the growth of dolomite crystals.
Dolomite is resistant to erosion and can either contain bedded layers or be unbedded. It is less soluble than limestone in weakly acidic groundwater, but it can still develop solution features over time. Dolomite rock with a sufficient porosity can act as an oil and natural gas reservoir.
Name
Dolomite takes its name from the 18th-century French mineralogist Déodat Gratet de Dolomieu, who was one of the first to describe the mineral. Dolomite was analysed by the Swiss chemist Nicolas Théodore de Saussure who named the mineral after Dolomieu in March 1792.The term dolomite refers to both the calcium-magnesium carbonate mineral and to the 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 is not recommended by the Glossary of Geology published by the American Geological Institute.
In old USGS publications, dolomite was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolomites or magnesium-rich limestones.
Description
Dolomite rock is defined as sedimentary carbonate rock composed of more than 50% mineral dolomite. Dolomite is characterized by its nearly ideal 1:1 stoichiometric ratio of magnesium to calcium. It is distinct from high-magnesium limestone in that the magnesium and calcium form ordered layers within the individual dolomite mineral grains, rather than being arranged at random, as they are in high-magnesium calcite grains. In natural dolomite, magnesium is typically between 44 and 50 percent of total magnesium plus calcium, indicating some substitution of calcium into the magnesium layers. A small amount of ferrous iron typically substitutes for magnesium, particularly in more ancient dolomites. Carbonate rock tends to be either almost all calcite or almost all dolomite, with intermediate compositions being quite uncommon.Dolomite outcrops are recognized in the field by their softness and because dolomite bubbles feebly when a drop of dilute hydrochloric acid is dropped on it. This distinguishes dolomite from limestone, which is also soft but reacts vigorously with dilute hydrochloric acid. Dolomite usually weathers to a characteristic dull yellow-brown color due to the presence of ferrous iron. It produces scratches at a level 6 and deeper grooves at a level 7. This is released and oxidized as the dolomite weathers. Dolomite is usually granular in appearance, with a texture resembling grains of sugar.
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. This texture contrasts with limestone, which is usually a mixture of grains, micrite 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. 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. The latter likely forms by recrystallization of existing dolomite at elevated temperature.
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. Under a microscope, dolomite rhombs are sometimes seen to replace oolites or skeletal particles of the original limestone. 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.
Occurrence and origin
Dolomite is widespread in its occurrences, though not as common as limestone. It is typically found in association with limestone or evaporite beds and is often interbedded with limestone. There is no consistent trend in its abundance with age, but most dolomite appears to have formed at high stands of sea level. Little dolomite is found in Cenozoic beds, which has been a time of generally low sea levels. Times of high sea level also tend to be times of a greenhouse Earth, and it is possible that greenhouse conditions are the trigger for dolomite formation.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.
The dolomitization reaction
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. Several processes have been proposed for dolomitization.
The hypersaline model 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, 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. 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.
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, with one 2004 review paper describing it bluntly as "a myth". A 2021 paper argued that the mixing zone serves as domain of intense microbial activity which promotes dolomitization.
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, Florida, 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.
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. The process likely occurs at shallow depths of burial, under, 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.
Mineral dolomite has a 12% to 13% smaller volume than calcite per alkaline earth cation. Thus dolomitization likely increases porosity and contributes to the sugary texture of dolomite.