Limestone is a sedimentary rock, composed
mainly of skeletal fragments of marine organisms
such as coral, forams and molluscs.
Its major materials are the minerals calcite
and aragonite, which are different crystal
forms of calcium carbonate (CaCO3).
About 10% of sedimentary rocks are limestones.
The solubility of limestone in water and weak
acid solutions leads to karst landscapes,
in which water erodes the limestone over thousands
to millions of years.
Most cave systems are through limestone bedrock.
Limestone has numerous uses: as a building
material, an essential component of concrete
(Portland cement), as aggregate for the base
of roads, as white pigment or filler in products
such as toothpaste or paints, as a chemical
feedstock for the production of lime, as a
soil conditioner, or as a popular decorative
addition to rock gardens.
== Description ==
Like most other sedimentary rocks, most limestone
is composed of grains.
Most grains in limestone are skeletal fragments
of marine organisms such as coral or foraminifera.
These organisms secrete shells made of aragonite
or calcite, and leave these shells behind
when they die.
Other carbonate grains comprising limestones
are ooids, peloids, intraclasts, and extraclasts.
Limestone often contains variable amounts
of silica in the form of chert (chalcedony,
flint, jasper, etc.) or siliceous skeletal
fragment (sponge spicules, diatoms, radiolarians),
and varying amounts of clay, silt and sand
(terrestrial detritus) carried in by rivers.
Some limestones do not consist of grains,
and are formed by the chemical precipitation
of calcite or aragonite, i.e. travertine.
Secondary calcite may be deposited by supersaturated
meteoric waters (groundwater that precipitates
the material in caves).
This produces speleothems, such as stalagmites
and stalactites.
Another form taken by calcite is oolitic limestone,
which can be recognized by its granular (oolite)
appearance.
The primary source of the calcite in limestone
is most commonly marine organisms.
Some of these organisms can construct mounds
of rock known as reefs, building upon past
generations.
Below about 3,000 meters, water pressure and
temperature conditions cause the dissolution
of calcite to increase nonlinearly, so limestone
typically does not form in deeper waters (see
lysocline).
Limestones may also form in lacustrine and
evaporite depositional environments.Calcite
can be dissolved or precipitated by groundwater,
depending on several factors, including the
water temperature, pH, and dissolved ion concentrations.
Calcite exhibits an unusual characteristic
called retrograde solubility, in which it
becomes less soluble in water as the temperature
increases.
Impurities (such as clay, sand, organic remains,
iron oxide, and other materials) will cause
limestones to exhibit different colors, especially
with weathered surfaces.
Limestone may be crystalline, clastic, granular,
or massive, depending on the method of formation.
Crystals of calcite, quartz, dolomite or barite
may line small cavities in the rock.
When conditions are right for precipitation,
calcite forms mineral coatings that cement
the existing rock grains together, or it can
fill fractures.
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 supersaturated
with the 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 that occurs during
the mountain building process (orogeny), limestone
recrystallizes into marble.
Limestone is a parent material of Mollisol
soil group.
== Classification ==
Two major classification schemes, the Folk
and the Dunham, are used for identifying limestone
and carbonate rocks.
=== Folk classification ===
Robert L. Folk developed a classification
system that places primary emphasis on the
detailed composition of grains and interstitial
material in carbonate rocks.
Based on composition, there are three main
components: allochems (grains), matrix (mostly
micrite), and cement (sparite).
The Folk system uses two-part names; the first
refers to the grains and the second is the
root.
It is helpful to have a petrographic microscope
when using the Folk scheme, because it is
easier to determine the components present
in each sample.
=== Dunham classification ===
The Dunham scheme focuses on depositional
textures.
Each name is based upon the texture of the
grains that make up the limestone.
Robert J. Dunham published his system for
limestone in 1962; it focuses on the depositional
fabric of carbonate rocks.
Dunham divides the rocks into four main groups
based on relative proportions of coarser clastic
particles.
Dunham names are essentially for rock families.
His efforts deal with the question of whether
or not the grains were originally in mutual
contact, and therefore self-supporting, or
whether the rock is characterized by the presence
of frame builders and algal mats.
Unlike the Folk scheme, Dunham deals with
the original porosity of the rock.
The Dunham scheme is more useful for hand
samples because it is based on texture, not
the grains in the sample.
== Limestone landscape ==
About 10% of all sedimentary rocks are limestones.Limestone
is partially soluble, especially in acid,
and therefore forms many erosional landforms.
These include limestone pavements, pot holes,
cenotes, caves and gorges.
Such erosion landscapes are known as karsts.
Limestone is less resistant than most igneous
rocks, but more resistant than most other
sedimentary rocks.
It is therefore usually associated with hills
and downland, and occurs in regions with other
sedimentary rocks, typically clays.
Karst topography and caves develop in limestone
rocks due to their solubility in dilute acidic
groundwater.
The solubility of limestone in water and weak
acid solutions leads to karst landscapes.
Regions overlying limestone bedrock tend to
have fewer visible above-ground sources (ponds
and streams), as surface water easily drains
downward through joints in the limestone.
While draining, water and organic acid from
the soil slowly (over thousands or millions
of years) enlarges these cracks, dissolving
the calcium carbonate and carrying it away
in solution.
Most cave systems are through limestone bedrock.
Cooling groundwater or mixing of different
groundwaters will also create conditions suitable
for cave formation.
Coastal limestones are often eroded by organisms
which bore into the rock by various means.
This process is known as bioerosion.
It is most common in the tropics, and it is
known throughout the fossil record (see Taylor
and Wilson, 2003).
Bands of limestone emerge from the Earth's
surface in often spectacular rocky outcrops
and islands.
Examples include the Burren in Co. Clare,
Ireland; the Verdon Gorge in France; Malham
Cove in North Yorkshire and the Isle of Wight,
England; the Great Orme in Wales; on Fårö
near the Swedish island of Gotland, the Niagara
Escarpment in Canada/United States, Notch
Peak in Utah, the Ha Long Bay National Park
in Vietnam and the hills around the Lijiang
River and Guilin city in China.
The Florida Keys, islands off the south coast
of Florida, are composed mainly of oolitic
limestone (the Lower Keys) and the carbonate
skeletons of coral reefs (the Upper Keys),
which thrived in the area during interglacial
periods when sea level was higher than at
present.
Unique habitats are found on alvars, extremely
level expanses of limestone with thin soil
mantles.
The largest such expanse in Europe is the
Stora Alvaret on the island of Öland, Sweden.
Another area with large quantities of limestone
is the island of Gotland, Sweden.
Huge quarries in northwestern Europe, such
as those of Mount Saint Peter (Belgium/Netherlands),
extend for more than a hundred kilometers.
The world's largest limestone quarry is at
Michigan Limestone and Chemical Company in
Rogers City, Michigan.
== Uses ==
Limestone is very common in architecture,
especially in Europe and North America.
Many landmarks across the world, including
the Great Pyramid and its associated complex
in Giza, Egypt, were made of limestone.
So many buildings in Kingston, Ontario, Canada
were, and continue to be, constructed from
it that it is nicknamed the 'Limestone City'.
On the island of Malta, a variety of limestone
called Globigerina limestone was, for a long
time, the only building material available,
and is still very frequently used on all types
of buildings and sculptures.
Limestone is readily available and relatively
easy to cut into blocks or more elaborate
carving.
Ancient American sculptors valued limestone
because it was easy to work and good for fine
detail.
Going back to the Late Preclassic period (by
200–100 BCE), the Maya civilization (Ancient
Mexico) created refined sculpture using limestone
because of these excellent carving properties.
The Maya would decorate the ceilings of their
sacred buildings (known as lintels) and cover
the walls with carved limestone panels.
Carved on these sculptures were political
and social stories, and this helped communicate
messages of the king to his people.
Limestone is long-lasting and stands up well
to exposure, which explains why many limestone
ruins survive.
However, it is very heavy, making it impractical
for tall buildings, and relatively expensive
as a building material.
Limestone was most popular in the late 19th
and early 20th centuries.
Train stations, banks and other structures
from that era are normally made of limestone.
It is used as a facade on some skyscrapers,
but only in thin plates for covering, rather
than solid blocks.
In the United States, Indiana, most notably
the Bloomington area, has long been a source
of high quality quarried limestone, called
Indiana limestone.
Many famous buildings in London are built
from Portland limestone.
Limestone was also a very popular building
block in the Middle Ages in the areas where
it occurred, since it is hard, durable, and
commonly occurs in easily accessible surface
exposures.
Many medieval churches and castles in Europe
are made of limestone.
Beer stone was a popular kind of limestone
for medieval buildings in southern England.
Limestone and (to a lesser extent) marble
are reactive to acid solutions, making acid
rain a significant problem to the preservation
of artifacts made from this stone.
Many limestone statues and building surfaces
have suffered severe damage due to acid rain.
Likewise limestone gravel has been used to
protect lakes vulnerable to acid rain, acting
as a pH buffering agent.
Acid-based cleaning chemicals can also etch
limestone, which should only be cleaned with
a neutral or mild alkaline-based cleaner.
Other uses include:
It is the raw material for the manufacture
of quicklime (calcium oxide), slaked lime
(calcium hydroxide), cement and mortar.
Pulverized limestone is used as a soil conditioner
to neutralize acidic soils (agricultural lime).
Is crushed for use as aggregate—the solid
base for many roads as well as in asphalt
concrete.
Geological formations of limestone are among
the best petroleum reservoirs;
As a reagent in flue-gas desulfurization,
it reacts with sulfur dioxide for air pollution
control.
Glass making, in some circumstances, uses
limestone.
It is added to toothpaste, paper, plastics,
paint, tiles, and other materials as both
white pigment and a cheap filler.
It can suppress methane explosions in underground
coal mines.
Purified, it is added to bread and cereals
as a source of calcium.
Calcium levels in livestock feed are supplemented
with it, such as for poultry (when ground
up).
It can be used for remineralizing and increasing
the alkalinity of purified water to prevent
pipe corrosion and to restore essential nutrient
levels.
Used in blast furnaces, limestone binds with
silica and other impurities to remove them
from the iron.
It is used in sculptures because of its suitability
for carving.
=== Occupational safety and health ===
People can be exposed to limestone in the
workplace by inhalation of and eye contact
with the dust.
==== United States ====
The Occupational Safety and Health Administration
(OSHA) has set the legal limit (permissible
exposure limit) for limestone exposure in
the workplace as 15 mg/m3 total exposure and
5 mg/m3 respiratory exposure over an 8-hour
workday.
The National Institute for Occupational Safety
and Health (NIOSH) has set a recommended exposure
limit (REL) of 10 mg/m3 total exposure and
5 mg/m3 respiratory exposure over an 8-hour
workday.
== Degradation by organisms ==
The cyanobacterium Hyella balani can bore
through limestone; as can the green alga Eugamantia
sacculata and the fungus Ostracolaba implexa.
== Gallery ==
== 
See also ==
Chalk – A soft, white, porous sedimentary
rock made of calcium carbonate
Coral sand
Kurkar – Regional name for an aeolian quartz
calcrete on the Levantine coast
In Praise of Limestone
Sandstone – A clastic sedimentary rock composed
mostly of sand-sized particles
