Доломит: состав, образование и свойства осадочной породы.

Sedimentary carbonate rock that contains a high percentage of the mineral dolomite

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 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. Recent research has revealed sulfate reducing bacteria living in anoxic conditions precipitate dolomite which indicates that some past dolomite deposits may be due to microbial activity. 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 (karst) over time. Dolomite rock can act as an oil and natural gas reservoir.

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. 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 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.

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 (mineral dolomite has a Mohs hardness of 4 or less, well below common silicate minerals) 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. 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 (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. 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 (over ). 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.

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 activation energy for nucleating crystals of dolomite. The magnesium ion is a relatively small ion, and it acquires a tightly bound hydration shell when dissolved in water. In other words, the magnesium ion is surrounded by a clump of water molecules that are strongly attracted to its positive charge. Calcium is a larger ion and this reduces the strength of binding of its hydration shell, so it is much easier for a calcium ion than a magnesium ion to shed its hydration shell and bind to a growing crystal. It is also more difficult to nucleate a seed crystal of ordered dolomite than disordered high magnesium calcite. As a result, attempts to precipitate dolomite from seawater precipitate high magnesium calcite instead. This substance, which has an excess of calcium over magnesium and lacks calcium magnesium ordering, is sometimes called protodolomite. Raising the temperature makes it easier for magnesium to shed its hydration shell, and dolomite can be precipitated from seawater at temperatures in excess of Protodolomite also rapidly converts to dolomite at temperatures of or higher. The high temperatures necessary for the formation of dolomite helps explain the rarity of Cenozoic dolomites, since Cenozoic seawater temperatures seldom exceeded 40 °C. It is possible that microorganisms are capable of precipitating primary dolomite. With time other pathways of interaction between microbial activity and dolomite formation have been added to the discord regarding their role in modulation and generation of polysaccharides, manganese and zinc within the porewater. Meanwhile, a contrary view held by other researchers is that microorganisms precipitate only high magnesium calcite but leave open the question of whether this can lead to precipitation of dolomite.

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.

Dolomite is used for many of the same purposes as limestone, including as construction aggregate; in agriculture to neutralize soil acidity and supply calcium and magnesium; as a source of carbon dioxide; as dimension stone; as a filler in fertilizers and other products; as a flux in metallurgy; and in glass manufacturing. It cannot substitute for limestone in chemical processes that require a high calcium limestone, such as manufacture of sodium carbonate. Dolomite is used for production of magnesium chemicals, such as Epsom salt, and is used as a magnesium supplement. It is also used in the manufacture of refractory materials.

As with limestone caves, natural caves and solution tubes typically form in dolomite rock as a result of the dissolution by weak carbonic acid. Caves can also, less commonly, form through dissolution of rock by sulfuric acid. Calcium carbonate speleothems (secondary deposits) in the forms of stalactites, stalagmites, flowstone etc., can also form in caves within dolomite rock. “Dolomite is a common rock type, but a relatively uncommon mineral in speleothems”.

Both calcium and magnesium go into solution when dolomite rock is dissolved. The speleothem precipitation sequence is: calcite, Mg calcite, aragonite, huntite and hydromagnesite. Hence, the most common speleothem (secondary deposit) in caves within dolomite rock karst, is calcium carbonate in the most stable polymorph form of calcite. Speleothem types known to have a dolomite constituent include: coatings, crusts, moonmilk, flowstone, coralloids, powder, spar and rafts. Although there are reports of dolomite speleothems known to exist in a number of caves around the world, they are usually in relatively small quantities and form in very fine grained deposits.