Calcium carbonate limestone caves

Limestone caverns are among nature s most spectacular displays. These caves occur in many parts of the world. Examples are Carlsbad Caverns In New Mexico, Jeita Caves in Lebanon, the Blue Grotto in Italy, and the Jenolan Caves In Australia. Wherever they occur, the chemistry of their formation involves the aqueous equilibria of limestone, which Is calcium carbonate. Three such equilibria, linked to one another by Le Chatelier s principle, play essential roles In cave dynamics. 

Figure 7.1 Cave formations are caused when rainwater and carbon dioxide mix and form a weak carbonic acid, which then dissolves the calcium carbonate of limestone beneath the earth, allowing for cave formation. The photo shows stalactites and stalagmites and other formations at Luray Caverns in Virginia.

Calcium carbonate is the primary component of seashells, antacids, marble and limestone (e.g. stalactites and stalagmites in caves), blackboard chalk, scale in water pipes, and calcium supplements for people and animals. It is also used to capture S02 gas in fossil fuel burning boilers, thereby helping to prevent acid rain, and as a soil additive to provide pH adjustment and calcium to farmers soil. 

A These downward-growing, icicle-shaped structures called stalactites and the upward-growing columns called stalagmites are formed in limestone caves by the slow precipitation of calcium carbonate from dripping water. 

Limestone is mostly made up of the mineral calcite, or calcium carbonate (CaC03). As the calcium carbonate rock dissolves in the slightly acidic water, spaces and even caves develop underground. If carbonic acid dissolves all the way through the rock and into a cave below the Earth s surface, the resulting solution contains calcium hydrogen carbonate (calcium bicarbonate). 

Stalactites hang from the ceiling of limestone caves. They look like icicles and occur at cracks where water flows through the limestone and into the cave. Sometimes water containing some dissolved calcium carbonate will drip off the tip of a stalactite and onto the floor of the cave. In these places, stalagmites will form on the cave floor. Sometimes a stalactite and a stalagmite will connect and make a column. 

Calcium carbonate forms limestone, one of the most common rocks on Earth. It also forms stalactites, stalagmites, and many other types of formations found in caves. When calcium carbonate is heated, it decomposes to form solid calcium oxide and carbon dioxide gas. 

Most analytical techniques require the state of chemical equilibrium. At equilibrium, the rate of a forward process or reaction and that of the reverse process are equal. The photo at left shows the beautiful natural formation called "Frozen Niagra in Mammoth Cave National Park in Kentucky. As water seeps over the limestone surface of the cave, calcium carbonate dissolves in the water according to the chemical equilibrium... 

The flowing water becomes saturated with calcium carbonate as carbon dioxide is swept away, the reverse reaction becomes favored, and limestone is deposited in formations whose shapes are governed by the path of the flowing water. Stalactites and stalagmites are examples of similar formations found where water saturated with calcium carbonate drips from the ceiling to the floor of caves over eons. 

Acid in the Environment Carbonic acid plays a key role in the formation of caves and of stalactites and stalagmites. Carbonic acid is formed when carbon dioxide in soil is dissolved in water. When this acidic solution comes in contact with calcium carbonate—or limestone rock—it can dissolve it, eventually carving out a cave in the rock. A similar process occurs when acid rain falls on statues and eats away at the stone, as shown in Figure 16. When this acidic solution drips from the ceiling of the cave, water evaporates and carbon dioxide becomes less soluble, forcing it out of solution. The solution becomes less acidic and the limestone becomes less soluble, causing it to come out of solution. These solids form stalactites and stalagmites. 

How caves are formed Caves form in limestone regions throughout the world. Limestone is calcium carbonate, which is only slightly soluble in water. The caves that form within these rocks are called solution caves. 

Over the millennia, normal rain has created limestone caves because calcium carbonate is slightly soluble in solutions of H2CO3 ... 

Calcium carbonate (KAL-see-um CAR-bun-ate) is one of the most common compounds on Earth, making up about 7 percent of Earth s crust. It occurs in a number of minerals and other natural materials, including aragonite, calcite, chalk, limestone, marble, marl, oyster shells, pearls, and travertine. Stalactites and stalagmites found in caves are made primarily of calcium carbonate. As indicated by the melting points of aragonite and calcite, the compound s physical properties may differ somewhat depending on its crystal form. It typically occurs as an odorless, tasteless white powder or colorless crystals. 

Calcium carbonate, which contains the Ca and ions, is very common in nature, occurring in eggshells, limestone, marble, seashells, and coral. The spectacular formations seen in limestone caves are also composed of calcium carbonate. 

The gateway looked like the entrance to a cavern, but this was no ordinary cavern. No animal or man who wandered into its misty interior ever returned. Now we think we know why. Subterranean hot streams penetrate the ground surrounding the cavern. As the streams flow over deposits of limestone — or calcium carbonate — the water picks up carbon dioxide gas. Sort of a natural carbonation process. Then, as the carbonated water reaches the cavern, the pressure is released, and the gas escapes. It s kind of like opening a bottle of pop. Since carbon dioxide is heavier than air, it pushes the air out of the cave, so any person or creature who enters is quickly overcome by oxygen deprivation. 

Caves are created when limestone, which is calcium carbonate, dissolves in acidic water. The acid is produced when the carbon dioxide in water dissolves to form carbonic acid. We know that carbon dioxide is present in the air — after all, we exhale it with every breath. It dissolves in rainwater, making it slightly acidic. 

Luray Caverns, Virginia. Stalactites form like icicles when dissolved calcium carbonate is deposited by groundwater dripping from fhe ceiling of fhe limestone cave. 

Such caves are formed by the action of acidic groundwater on limestone deposits.The icicle-like formations (stalagmites and stalactites) in the caves are caused by the reprecipitation of calcium carbonate as carbon dioxide in the solution equilibrates with the surrounding air. 

---