Monday, 16 March 2015

Dolomite



What is dolomite?

Dolomite is a sedimentary rock composed primarily of calcium magnesium carbonate. The word dolomite is also referred to dolomite mineral so sometimes being confused between rock and mineral. Limestone is composed of calcium carbonate and dolomite is composed of calcium magnesium carbonate so it is thought to be originated by the post depositional alteration of limestone via magnesium rich ground water. Dolomite consist all the properties of limestone, have same hardness, reacts with hydrochloric acid and bears the same colour (white to grey or white to light brown).

Dolomitization

As limestone and dolomite shares the same depositional environment as shallow marine, warm water where organism can accumulate which in turn when deposits form carbonate. So in formation of dolomite it is thought to be the alteration of carbonate by magnesium rich water, Magnesium in the water converts calcite into dolomite, This alteration is the chemical change in the limestone which is called dolomitization. This can turn limestone into complete dolomite or can be partial alteration of the rock and is dolomitic limestone.

History

Most probably the mineral dolomite was first described by Carl Linnaeus in 1768. In 1791, it was described as a rock by the French naturalist and geologist Déodat Gratet de Dolomieu (1750–1801), first in buildings of the old city of Rome, and later as samples collected in the mountains now known as the Dolomite Alps of northern Italy. Nicolas-Théodore de Saussure first named the mineral (after Dolomieu) in March 1792.

Formation of dolomite

Modern dolomite formation has been found to occur under anaerobic conditions in supersaturated saline lagoons along the Rio de Janeiro coast of Brazil, namely, Lagoa Vermelha and Brejo do Espinho. It is often thought that dolomite will develop only with the help of sulphate-reducing bacteria (e.g. Desulfovibrio brasiliensis). However, low-temperature dolomite may occur in natural environments rich in organic matter and microbial cell surfaces. This occurs as a result of magnesium complexation by carboxyl groups associated with organic matter.
Vast deposits of dolomite are present in the geological record, but the mineral is relatively rare in modern environments. Reproducible, inorganic low-temperature syntheses of dolomite and magnesite were published for the first time in 1999. Those laboratory experiments showed how the initial precipitation of a metastable "precursor" (such as magnesium calcite) will change gradually into more and more of the stable phase (such as dolomite or magnesite) during periodical intervals of dissolution and re-precipitation. The general principle governing the course of this irreversible geochemical reaction has been coined "breaking Ostwald's step rule".
There is some evidence for a biogenic occurrence of dolomite. One example is that of the formation of dolomite in the urinary bladder of a Dalmatian dog, possibly as the result of an illness or infection.

Physical Properties of Dolomite

The physical properties of dolomite that are useful for identification are presented in the table on this page. Dolomite has three directions of perfect cleavage. This may not be evident when the dolomite is fine-grained. However, when it is coarsely crystalline the cleavage angles can easily be observed with a hand lens. Dolomite has a Mohs hardness of 3 1/2 to 4 and is sometimes found in rhombohedral crystals with curved faces. Dolomite produces a very weak reaction to cold, dilute hydrochloric acid; however, if the acid is warm or if the dolomite is powdered, a much stronger acid reaction will be observed. (Powdered dolomite can easily be produced by scratching it on a streak plate.)
Dolomite is very similar to the mineral calcite. Calcite is composed of calcium carbonate (CaCO3), while dolomite is a calcium magnesium carbonate (CaMg(CO3)2). These two minerals are one of the most common pairs to present a mineral identification challenge in the field or classroom.
The best way to tell these minerals apart is to consider their hardness and acid reaction. Calcite has a hardness of 3, while dolomite is slightly harder at 3 1/2 to 4. Calcite is also strongly reactive with cold hydrochloric acid, while dolomite will effervesce weakly with cold hydrochloric acid.

Metamorphism of Dolomite

When dolomite is subjected to heat and pressure it behaves the same way as limestone. The heat recrystallises the dolomite crystals which grows into larger crystal form. 

Uses of dolomite


Dolomite and limestone have similar uses as used in the construction purposes after being crushed into pebbles and cobbles size. These can also be used into dimension stone after cutting into regular size. Dolomite is the preference in the construction industry than that of the limestone because of its greater hardness and less chemical reactivity to acids which makes it perfect for construction uses.
Dolomitization is a process where limestone is converted, it provides opportunity for a reservoir in oil and gas industry because of the reduction in size of limestone which leaves pore spaces that are often filled by oil and gas. These are also host rock for lead, zinc and copper deposits.
Other uses of dolomite are in the chemical industry used to extract magnesia where it is served as the source rock. Steel industry use it in processing iron ore and is also used in the agriculture industry as a feed additive for live stock aiding in the egg shells which are made of calcium. It is also used in the production of glass and ceramics.

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