Dolomite (rock)
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.
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.
[15] Under the microscope, thin sections of dolomite usually show individual grains that are well-shaped rhombs, with considerable pore space.
[16] This texture contrasts with limestone, which is usually a mixture of grains, micrite (very fine-grained carbonate mud) and sparry cement.
[21] There is no consistent trend in its abundance with age, but most dolomite appears to have formed at high stands of sea level.
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.
According to this model, dolomitization takes place in a closed basin where seawater is subject to high rates of evaporation.
[27] A 2021 paper argued that the mixing zone serves as domain of intense microbial activity which promotes dolomitization.
A similar process might occur during rises in sea level, as large volumes of water move through limestone platform rock.
[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.
Likewise, geologists have not been successful at precipitating dolomite from seawater at normal temperatures and pressures in laboratory experiments.
In other words, the magnesium ion is surrounded by a clump of water molecules that are strongly attracted to its positive charge.
[7] This was first demonstrated in samples collected at Lagoa Vermelha, Brazil[6] in association with sulfate-reducing bacteria (Desulfovibrio), leading to the hypothesis that sulfate ion inhibits dolomite nucleation.
[36] 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,[37] manganese[38][39] and zinc[40] within the porewater.
[42] 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.
[45] Both the 'Union Internationale de Spéléologie' (UIS) and the American 'National Speleological Society' (NSS), extensively use in their publications, the terms "dolomite" or "dolomite rock" when referring to the natural bedrock containing a high percentage of CaMg(CO3)2 in which natural caves or solution tubes have formed.
[45][48] 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.
[45] 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.
