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Limestone is a chemical sedimentary rock composed largely of the mineral
calcite (calcium
carbonate: CaCO3).
Limestone often
contain variable
amounts of silica in
the form of chert or
flint, as well as
amounts of clay,
silt and sand as
disseminations,
nodules, or layers
within the rock
unit. The primary
source of this
calcite is most
commonly marine
organisms. These
organisms secrete
shells that settle
out of the water
column and are
deposited on ocean
floors as pelagic
ooze (see lysocline
for information on
calcite
dissolution).
Secondary calcite
may also be
deposited by
supersaturated
meteoric waters
(groundwater that
precipitates the
material in caves).
This produces
speleothems such as
stalagmites and
stalactites. Another
form is composed of
oolites (oolitic
limestone) and can
be recognised by its
granular appearance.
Limestone makes up
about 10 percent of
the total volume of
all sedimentary
rocks.
Pure limestone are
white or almost
white. Because of
impurities, such as
clay, sand, organic
remains, iron oxide
and other materials,
many limestone
exhibit different
colors, especially
on weathered
surfaces. Limestone
may be crystalline,
classic, granular,
or massive,
depending on the
method of formation.
Crystals of calcite,
quartz, dolomite or
barite may line
small cavities in
the rock.
Travertine is a
banded, compact
variety of limestone
formed along
streams,
particularly where
there are waterfalls
and around hot or
cold springs.
Calcium carbonate is
deposited where
evaporation of the
water leaves a
solution that is
supersaturated with
chemical
constituents of
calcite. Tufa, a
porous or cellular
variety of
travertine, is found
near waterfalls.
Coquina is a poorly
consolidated
limestone composed
of pieces of coral
or shells.
During regional
metamorphism
limestone
recrystallizes into
marble.
Limestone is a
parent material of
Mollisol soil group.
Limestone is
sedimentary rock
consisting mostly of
organic material
such as skeletons
and shells of marine
creatures and
sediments. It is
formed by material
which settles to the
bottom of bodies of
water, and over
millions of years,
solidifies into
solid rock. Earth
movements over
extremely long
periods of earth's
history can lift
limestone miles into
the air. The summit
of Mount Everest is
limestone that
started out on an
ocean floor.
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