Dolomite
What
is dolomite?
.
Limestone is composed of calcium carbonate and dolomite is composed of calcium magnesium carbonate. 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). Dolomite and limestone have similar uses as used in the construction purposes after being crushed into pebbles and cobbles size. Dolomite is the preference in the construction industry than that of the limestone. Other uses of dolomite are in the chemical industry used to extract magnesia. Steel industry use it in processing iron ore and is also used in the agriculture industry as a feed additive for livestock aiding in the egg shells which are made of calcium. It is also used in the production of glass and ceramics.
Learning
Geology
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 livestock 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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