Limestone
What is Limestone?
Limestone is a sedimentary rock
composed largely of the mineral calcite (calcium carbonate: CaCO3).
Limestone often contains variable amounts of silica in the form of
chert and/or flint, as well as varying amounts of clay, silt and sand as
disseminations, nodules, or layers within the rock. The primary source
of the calcite in limestone 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 or alternatively are
conglomerated in a coral reef. 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 taken by calcite is that of oolites (oolitic
limestone) which can be recognized by its granular appearance.
Limestone makes up about 10% of the total volume of all sedimentary
rocks.[1][2] Limestones may also form in both lacustrine and evaporite
depositional environments.
Calcite can be either dissolved by groundwater 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.
When conditions are right for precipitation, calcite forms mineral
coatings that cement the existing rock grains together or it can fill
fractures.
Karst topography and caves develop in carbonate rocks due to their
solubility in dilute acidic groundwater. Cooling groundwater or mixing
of different ground waters 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).
Because of impurities, such as clay, sand, organic remains, iron
oxide and other materials, many limestones exhibit different colors,
especially on 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.
Folk and Dunham classifications are used to describe limestones more
precisely.
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 that occurs during the mountain
building process (orogeny) limestone recrystallizes into marble.
Limestone is a parent material of Mollisol soil group.
Limestone landscape
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. Limestone is therefore
usually associated with hills and downland and occurs in regions with
other sedimentary rocks, typically clays.
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; on Fårö near the Swedish
island of Gotland, the Niagara Escarpment in Canada/United States, Notch
Peak in Utah, and the Ha Long Bay National Park in Vietnam.
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 Oland, 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.[6]
Uses of limestone
Limestone is very common in architecture, especially in North
America and Europe. Many landmarks across the world, including the
pyramids in Egypt, are made of limestone. So many buildings in Kingston,
Ontario, Canada were constructed from it that it is nicknamed the
'Limestone City'. [7] 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. It is also
long-lasting and stands up well to exposure. However, it is a very heavy
material, making it impractical for tall buildings, and relatively
expensive as a building material.
Limestone was also a very popular building block in the Middle Ages
in the areas where it occurred since it is hard, is 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 marble are very reactive to acid solutions, making
acid rain a significant problem. Many limestone statues and building
surfaces have suffered severe damage due to acid rain. Acid-based
cleaning chemicals can also etch limestone, which should only be cleaned
with a neutral or mild alkaline-based cleaner.
Other uses include:
-The manufacture of quicklime (calcium oxide) and slaked lime (calcium
hydroxide);
-Cement and mortar;
-Pulverized limestone is used as a soil conditioner to neutralize acidic
soil conditions;
-Crushed for use as aggregate—the solid base for many roads;
-Geological formations of limestone are among the best petroleum
reservoirs;
-As a reagent in desulfurizations;
-Glass making, in some circumstances;
-Added to paper, plastics, paint, tiles, and other materials as both
white pigment and a cheap filler.
-Toothpaste
-Suppression of methane explosions in underground coal mines
-Added to bread and cereals as a source of calcium
-
Courtesy of Wikipedia