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What is Limestone?

Introduction

Limestone, a common sedimentary rock composed primarily of the mineral calcite (CaCO3). Limestone constitutes approximately 10 percent of the sedimentary rocks exposed on the earth's surface. It forms either by direct crystallization from water (usually seawater) or by accumulation of shell and shell fragments. In the first case, it carries a record of the chemical composition of seawater and it provides evidence of how that composition has changed with time. In the second case, limestone provides a record of many important fossils. Limestone usually forms in shallow water less than 20 m (70 ft.) deep and thus also provides important geological information on the variation in sea level in the past. Limestone rocks are frequently riddled with caves. Limestone is an important building stone and is used to make cement and concrete.

When a drop of dilute hydrochloric acid is placed on a piece of limestone, the acid reacts with the calcite and forms bubbles of carbon dioxide. This "fizz" reaction is so characteristic of limestone than many geologists carry a small bottle of dilute hydrochloric acid into the field for a rapid and easy identification of limestone.

 

Composition and Origin

The principal component of limestone is the mineral calcite, but limestone frequently also contains the minerals dolomite (CaMg(CO3)2) and aragonite (CaCO3). Pure calcite, dolomite, and aragonite are clear or white. However, with impurities, they can take on a variety of colors. Consequently, limestone is commonly light colored; usually it is tan or gray. However, limestone has been found in almost every color. The color of limestone is due to impurities such as sand, clay, iron oxides and hydroxides, and organic materials.

All limestone forms from the precipitation of calcium carbonate from water. Calcium carbonate leaves solutions in many ways and each way produces a different kind of limestone. All the different ways can be classified into two major groups: either with or without the aid of a living organism.

Most limestone is formed with the help of living organisms. Many marine organisms extract calcium carbonate from seawater to make shells or bones. Mussels, clams, oysters, and corals do this. So too do microscopic organisms such as foraminifera. When the organisms die their shells and bones settle to the seafloor and accumulate there. Wave action may break the shells and bones into smaller fragments, forming a carbonate sand or mud. Over millions of years, these sediments of shells, sand, and mud may harden into limestone. Coquina is a type of limestone containing large fragments of shell and coral. Chalk is a type of limestone formed of shells of microscopic animals.

Limestone can also be formed without the aid of living organisms. If water containing calcium carbonate is evaporated, the calcium carbonate is left behind and will crystallize out of solution. For example, at Mammoth Hot Springs in Yellowstone National Park, hot water containing calcium carbonate emerges from deep underground. As the hot water evaporates and cools, it can no longer hold all of the calcium carbonate dissolved in it and some of it crystallizes out, forming limestone terraces. Limestone formed from springs is called travertine. Calcium carbonate also precipitates in shallow tropical seas and lagoons where high temperatures cause seawater to evaporate. Such limestone is called oolite. Calcium carbonate that precipitates from water dripping through caves is responsible for the formation of beautiful cave features such as stalactites and stalagmites.

 

Diagenesis of Limestone

Diagenesis is the name for those processes that affect sediment after it is deposited and prior to any metamorphism. Two processes of diagenesis are important in the formation of limestone. One is cementation, in which calcium carbonate precipitates in the pore space between the loose grains of sediment and binds them together into a hard compact rock.

The other process involves the alteration of the minerals in the limestone. When calcium carbonate precipitates, it can form two different minerals—calcite and aragonite. Calcite and aragonite are polymorphs, meaning that they have the same chemical composition, but the atoms are stacked differently in the crystal. Fresh calcium carbonate sediments sometimes contain calcite, sometimes they contain aragonite, and often they contain a mixture of the two. This is because some animals make shells of calcite while others make shells of aragonite. Similarly, the direct precipitation of calcium carbonate without the aid of organisms sometimes produces calcite, sometimes produces aragonite, and often produces a mixture of the two, depending on factors such as temperature and pressure. However, calcite is more stable than aragonite, and so, through diagenesis, aragonite slowly changes to calcite. In addition, calcite slowly absorbs magnesium from surrounding water, slowly changing to dolomite.

 

Weathering and Metamorphism

Limestone is easily weathered and eroded. Water trickling through large limestone deposits often erodes extensive underground drainage systems of sinkholes and caves. The land surface above large limestone deposits is often irregular, marked with potholes (formed when the roofs of shallow caves collapse), sinkholes, and few streams, most of which are short in length and end in a sinkhole. The irregular surface is known as karst topography, named after a limestone plateau in the Dinaric Alps of northwest Slovenia and northeast Italy. Within the caverns, secondary precipitation of calcite by percolating waters produces stalactites and stalagmites, and spectacular underground 'scenery' can result.

When limestone undergoes metamorphism, it turns into marble. If the limestone contains other materials such as sand and clay, the calcite will react with them to produce minerals such as tremolite, epidote, diopside, and grossular garnet.