Limestone
is a
Sedimentary rock
, made up mostly of the minerals
calcite
and aragonite, two forms of
calcium carbonate
, CaCO
3
.
[1]
The calcium carbonate is originally produced by living organisms. Later, some of it goes into
solution
in sea water. Limestone rocks include mainly organic remains and rock.
Limestone is about 10% of the total volume of all sedimentary rocks.
[2]
[3]
Limestones are mostly made of
calcium carbonate
. It is originally made by small
photosynthetic
protists
like certain
bacteria
,
coccolithophores
, and
algae
. Then the carbonate gets used by various animals, like
bivalves
and the
polyps
which make
coral reefs
. They build their shells or framework out of CaCO
3
crystals
.
[4]
- "[Coccolithophores] were the first organisms to bring about a steady rain of calcium carbonate from the upper, illuminated zone of the ocean towards the deep sea".
[5]
p144
Carbonates are rocks formed from these sources, plus any CaCO
3
which is
precipitated
out of
solution
. Some carbonate dissolves in the water. It gets precipitated when the physical condition of the water changes. For example, if the concentration of CaCO
3
is high, then a temperature drop, or a
pH
(acidity) change, can cause precipitation.
Florida Bay
, the shallow
lagoon
behind the
Florida Keys
, is a model for the conditions which produced limestones on the
continental shelf
. Of course, all the
species
were somewhat different then but, as an
ecosystem
, it is similar.
Florida Bay has much of a type of
flowering plant
called
sea grass
. White, milky water full of calcium carbonate is also common. This is mainly formed by
Melobesia
, a small
red alga
which lives on sea grass.
[5]
p151
Finally, the organisms produce huge amounts of
slime
, which is a
polysaccharide
. There is good reason to think the slime is produced by
microorganisms
to control the way their carbonate skeletons are formed.
[5]
p139
As a side-effect the slime limits the amount of carbonate which goes into solution.
[5]
p155
Florida Bay is a flat platform which has laid down a huge amount of carbonate rock.
- "You have to drill for more than 3000 feet before you reach a different rock type, and that is of
Cretaceous
age—some 70 million years ago".
[5]
p158
Many limestones contain fossils, and some are made up almost entirely of skeletal material.
Chalk
is ~99%
coccoliths
;
Solnhofen limestone
is famous for its animal fossils; and there are many shelly limestones from different periods. Limestone is not found much before the
Cambrian
period
, because shelled animals had not yet evolved.
[6]
When limestone is
heated
to a high temperature, and squeezed at the base of a
mountain
, it recrystallizes into the
metamorphic rock
marble
. Marbles are fairly
dense
, relatively easy to cut, or be shaped into blocks, and some forms are white in colour.
[7]
Limestone is hard enough to resist
weathering
but usually soft enough to be worked (cut and carved) by stonemasons. It is used for building material. Limestone, particularly
Portland limestone
, became popular in the 19th and 20th centuries for building things such as
churches
,
banks
and
houses
. However,
acid rain
does slowly erode limestone.
Limestone is used to make
glass
and
cement
. It is also used to make
iron
in a
blast furnace
, to remove impurities from the iron
ore
.
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Limestone
.