Carbonate rocks abound on the surface of the earth.
A carbonate is a mineral. Minerals have a uniform composition consisting of a single substance: either an element with only one kind of atom or a compound with one kind of molecule. Accordingly, apart from possible impurities, every molecule in the mineral calcium carbonate has the chemical formula CaCO3. In contrast, carbonate rocks are a mixture of minerals in which the major constituent is a carbonate. Several varieties of carbonate rock occur in abundance in the earth's crust.
Limestone
Limestone is a widely distributed sedimentary rock, meaning that it forms from sediments that settle out of water. Some limestone formations are crystalline calcite precipitates. Others originate in living organisms. For example, foraminiferans are protozoa that have a calcareous skeleton. When they die, their shells settle to the bottom, and, if enough accumulate, they can form limestone rock. The University of Wisconsin's BioWeb website states that foraminiferan skeletons are the main ingredient of the White Cliffs of Dover. Coral reefs are also limestone formations of biological origin. Snails and oysters also contribute to limestone rocks.
Karst formations consist of limestone rocks partially dissolved as a result of the action of carbonic acid, a weak acid that occurs in rainwater, especially after it comes in contact with carbon dioxide in the soil. This dissolving process results in such formations as the cenotes, or sinkholes, in the Yucatan Peninsula and extensive caverns, such as Mammoth Caves and Carlsbad Caverns in the United States. Limestone rocks often have a high percentage of a calcium carbonate mineral called calcite. Such materials as clay, pyrite or quartz may occur as minor elements in limestone rocks.
Dolostone
The term "dolomite" can refer to either a calcium magnesium carbonate mineral or a rock that contains a high percentage of this mineral substance. Semantic confusion can be avoided by calling the rock "dolostone" and reserving the term "dolomite" for the mineral. Dolostone differs from limestone in two major ways. It reacts less vigorously with hydrochloric acid than limestone, and, because of admixtures of iron, it often is brown in color instead of white like limestone, according to Professor Stephen A. Nelson of Tulane University. Dolostone is not as common as limestone, but dolomite frequently occurs in limestone, in which case the rock is called dolomitic limestone. Dolomitic limestone is more resistant to the normal action of rain and wind than ordinary limestone.
Marble
Marble is another fairly common carbonate rock. Relatively pure limestone and dolostone change into marble when they are subjected to such factors as high temperature or intense pressure. For this reason, marble is not a sedimentary rock like limestone or dolostone but a metamorphic rock, or a rock that has undergone an alteration in texture. Metamorphosed dolostone is called dolomitic marble to distinguish it from the ordinary kind. Marble has "a very tight crystalline structure and small but definite porosity," according to the U.S. General Services Administration. As a result, it withstands weathering better than the rocks from which it was derived. Since ancient times, people have quarried marble for buildings and sculpture. The marble of Mount Pentelikon near Athens adorned many ancient structures, such as the Elgin Marbles from the Parthenon and other buildings of the Athenian Acropolis.
Tags: carbonate mineral, calcium carbonate, called dolomitic, carbonate rock, high percentage
