Dolomite Rock Type, Composition, Formation, Occurrence & Uses

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Dolomite is a sedimentary rock with a high percentage of dolomite mineral, CaMg(CO3)2. It is a carbonate rock that is often associated with limestone and evaporites. However, the rock is less abundant than limestone. The rock was considered to be limestone before 1778. It was BelsazarHacquet who distinguished dolomite from limestone in 1778.

Dolomite is also known as dolostone or dolomite rock. Dolomite is found in sedimentary basins worldwide. Duotone and limestone are similar rocks in various aspects. The rocks have the same color ranges (white to gray and white to light brown), have approximately the same hardness, and dissolve in dilute hydrochloric acid. Their uses are almost identical; both are crushed and cut to be used as construction material and as acid neutralizers.

The reason behind naming this rock “dolostone” or “dolomite” rock is that a mineral is also called dolomite. Some geologists are uncomfortable using the same name for a rock and a mineral. These terms make it easy to identify and explain what the writer means, but dolomite is commonly referred to as dolomite rock and dolomite mineral. You can read more about the dolomite mineral here.

dolomitic rocks from Slovakia
dolomitic rocks from Slovakia

Rock Composition

Dolomite rock is a sedimentary carbonate rock made of more than 50% mineral dolomite. Equal percentages of calcium and magnesium characterize it. Natural dolomite consists of around 44-50% magnesium and calcium, with some calcium substitution in magnesium layers. Some ancient dolomites also have small amounts of ferrous iron.

Carbonate rocks usually are 100% limestone or 100% dolomite. Intermediate compositions are quite rare. When examined with a microscope, the thinner sections in the rock usually show individual grains as three-sided figures with large pore spaces. This concludes that subsurface dolomite is more porous than subsurface limestone.

The rock’s texture is similar to limestone. It is usually a mixture of grains, cement, and fine-grained carbonate mud.

Dolomite Formation (Dolomitization)

The rock-forming process is known as dolomitization. Dolomite is a common rock, but the mineral dolomite is rarely found in sedimentary settings. Hence,  most dolomites form when lime muds or mudstones are altered by post-depositional chemical change.

These rocks also originate in the same sedimentary settings from where limestone comes. The formation sites are usually marine environments where calcium carbonate mud accumulates in the shape of shell debris, fecal matter, carbonate precipitates, and coral fragments.

It is believed that dolomite forms when calcite in carbonate mud or limestone is altered by groundwater with a high magnesium concentration. The abundant magnesium facilitates and quickens the conversion of calcite into dolomite, CaMg(CO3)2.

This process, also known as dolomitization, can completely alter a limestone into a dolomite or partially change the rock to form a dolomitic limestone.

Dolomite vs. Limestone

Dolomite is considered a type of limestone. However, some differences can help you identify both rocks.

Dolomite is slightly harder than limestone. Limestone has a hardness of 3 on the Mohs Hardness, and dolomite has a 3.5 to 4 hardness on the Mohs scale.  Even though both rocks are soluble in dilute HCL, limestone dissolves quickly and has a vigorous reaction. Dolomite takes some time to dissolve and produces weak effervescence.

Overall, these differences are not enough to successfully name a rock. It requires a chemical examination to name the rock. The rocks must be chemically analyzed before they can be named. A chemical examination that determines the calcium and magnesium content in the rocks is required to name the rocks.

Dolomite Metamorphism

The rock starts to recrystallize when exposed to metamorphic heat and pressure. The size of the crystals increases, and the rock develops a crystalline appearance. A dolomite transformed into a metamorphic rock by heat and pressure is called a dolomitic marble.

Dolomite Uses

Dolomite and limestone have similar uses. These rocks are crushed and used as an aggregate and filler in construction projects. The rocks are used in cement manufacturing, cut into slabs and blocks as dimension stones. Dolomite is preferred over limestone in these uses because it is harder and lasts longer. Moreover, it is slightly more resistant to acids. Hence, it can withstand acid rain better than limestone.

When limestone is converted into dolomite, it results in a slight volume reduction and produces a porous zone where dolomitization has occurred. These pore spaces can store oil and natural gas. Hence, dolomite is often sought during oil and gas exploration. Dolomite is a host rock for lead, copper, and zinc deposits.

Dolomite is a magnesia source in the chemical industry and a sintering agent in the steel industry. It is used as a soil conditioner in agriculture. It has limited application in the glass and ceramic industry.

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