Limestone (calcium carbonate CaCO 3) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO 3. Limestone forms when these minerals precipitate out of water containing dissolved calcium.
It forms through the accumulation of skeletal fragments of marine organisms, chemical precipitation from seawater, or the evaporation of calcium-rich lakes.
Limestone is found worldwide, with major deposits in North America, Europe, and Asia. It is estimated to comprise about 15% of the Earth's sedimentary crust.
Most limestones form in calm, clear, warm, shallow marine waters. That type of environment is where organisms capable of forming calcium carbonate shells and skeletons can thrive and easily extract the needed ingredients from ocean water.
When these animals die, their shell and skeletal debris accumulate as a sediment that might be lithified into limestone. Their waste products also contribute to the sediment mass.
Limestones formed from this type of sediment are biological sedimentary rocks. Their biological origin is often, but not always, revealed in the rock by the presence of fossils.
Sometimes evidence of a biological origin is destroyed by the action of currents, organisms, dissolution, or recrystallization.
Some limestones form by direct precipitation of calcium carbonate from marine or fresh water. Limestones formed this way are chemical sedimentary rocks. They are thought to be less abundant than biological limestones.
Most biological limestones contain significant amounts of directly precipitated calcium carbonate. After the biological grains have accumulated and are buried, water that is saturated with dissolved materials moves slowly through the sediment mass.
Calcium carbonate, precipitated directly from solution, forms as a "cement" that binds the biological grains together.
"Cementation" is an important step in the transformation of a sediment into a rock. If the biological grains are not cemented together, a rock will not be formed. The amount of precipitated calcium carbonate in a biological limestone can be as low as a few percent of the rock by volume, or it can be higher than 50% of the rock by volume.
Many limestone-forming environments are active on Earth today. Most of them are found in shallow parts of the ocean between 30 degrees north latitude and 30 degrees south latitude.
Limestone is forming in the Caribbean Sea, Indian Ocean, Persian Gulf, Gulf of Mexico, around Pacific Ocean islands, and within the Indonesian archipelago.
One of these areas is the Bahamas Platform, located in the Atlantic Ocean about 100 miles southeast of southern Florida (see satellite image). There, abundant corals, shellfish, algae, and other organisms produce vast amounts of calcium carbonate skeletal debris and fecal matter that completely blanket the platform. This is producing an extensive deposit of calcium carbonate sediment that has already converted to limestone at depth.
Limestone can also form through evaporation. Stalactites, stalagmites, and other cave formations (often called "speleothems") are examples of limestone that formed through evaporation.
In a cave, droplets of water seeping down from above enter the cave through fractures or other pore spaces in the cave ceiling. There they might evaporate before falling to the cave floor.
When the water evaporates, any calcium carbonate that was dissolved in the water will be deposited. Over time, this evaporative process can result in an accumulation of icicle-shaped calcium carbonate on the cave ceiling. These features are known as stalactites.
If droplets fall to the floor and evaporate there, stalagmites could eventually grow upwards from the cave floor.
The limestone that makes up these cave formations is known as "travertine," a chemical sedimentary rock. A rock known as "tufa" is a limestone formed by evaporation at a hot spring or on the shoreline of a lake in an arid area.
Limestone has numerous applications, including:
Construction: As a building material, crushed limestone is used for roads, concrete, and foundations. Agriculture: As a soil amendment to neutralize acidity and provide calcium and magnesium. Industry: As a raw material for the production of cement, lime, and glass. Environmental remediation: As a filter for water treatment and as a scrubber for air pollution control.