Formation of Limestone

The chemical components of calcium carbonate — dissolved calcium ions and carbon dioxide — are widely distributed. Calcium is the fifth most common element in the earth s crust (after oxygen, silicon, aluminium and iron). It was extracted from early igneous rocks by the combined effects of erosion by the weather and corrosion by acidic gases (oxides of sulfur, oxides of nitrogen and carbon dioxide dissolved in rain water). Carbon dioxide makes up about 0.03 % by volume of the earth s atmosphere and is dissolved in both fresh and sea water. Combination of dissolved calcium ions and carbon dioxide resulted in the sedimentary deposition of calcium carbonate, which was subsequently converted into limestone rock. Early limestones (Precambrian — Table 2.1) are believed to have been deposited as precipitates of CaCOa, and/or as a result of the biochemical activity of very simple organisms, such as bacteria. 

The sedimentation of calcium carbonate occurs by two mechanisms - organic and inorganic. The organic route involves a wide variety of organisms, which build 

The above process, coupled with the fact that most carbonate-secreting organisms only thrive in clear waters — remote from rivers carrying significant amounts of solids washed from the land — accounts for the remarkably high purities of many carbonate deposits, which often exceed 98% of calcium plus magnesium carbonates. 

Carbonate sediments are also produced in a similar way by organisms in inland waters, but the resulting deposits are generally not as extensive, nor as commercially important as those produced in the marine environment. 

Inorganic precipitation of calcium carbonate occurs from both sea and inland waters (as used by geologists, precipitation refers to the relatively slow process of crystal growth on surfaces). This route has resulted in some commercially significant deposits, the most common of which are oolitic limestone and travertine (see section 2.2.1). Some minor dolomite sediments have been formed by direct precipitation from sea and lake waters. 

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The types of aqueous equilibria described in this section have been given special names, and it is essential that you be able to recognize them. Keep in mind, however, that the principles described in the previous sections apply to all chemical equilibria. Chemists categorize equilibria for convenience, but they treat all equilibria the same way. Our Chemistry and the Environment Box explores the roles of these equilibria in a spectacular natural process, the formation of limestone caverns. 

The effect of pH on the solubility of CaC03 has important environmental consequences. For instance, the formation of limestone caves, such as Mammoth Cave in Kentucky, is due to the slow dissolution of limestone (CaC03) in the slightly acidic natural water of underground streams. Marble, another form of CaC03, also dissolves in acid, which accounts for the deterioration of marble monuments on exposure to acid rain (Interlude, pages 650-651). 

Dissolution, precipitation, and deposition processes (e.g., those present during the formation of limestone caverns or in the formation of brines or high-salinity waters). 

This process and its reverse account for the formation of limestone caves and the stalactites and stalagmites found there. The acidic water (containing carbon dioxide) dissolves the underground limestone deposits, thereby forming a cavern. As the water drips from the ceiling of the cave, the carbon dioxide if ->st... 

Puri, H.S. and Collier, A., 1967. Role of microorganisms in formation of limestones. Trans. Gulf. Coast. Assoc. Geol. Soc., 17 355—367. 

Predict the effect of increasing acidity of rain on the rate of formation of limestone caves. 

As time evolved and the energy provided by the sun increased, the gradual removal of carbon dioxide from the atmosphere became critical to avoid a runaway greenhouse effect (with extremely high surface temperatures) as observed on Venus. This removal of CO2 was accomplished by weathering of calcium silicate (CaSiOs) minerals by acidic C02-rich rainwater, leading to the formation of limestone (CaC03). 

For example, consider the following heterogeneous equilibrium that is important in the formation of limestone caverns ... 

The dissolution and precipitation of limestone (CaCOs) underlie a variety of natural phenomena, such as the formation of limestone caverns. Whether a solution containing Ca and COs ions undergoes precipitation depends on the concentrations of these ions. In turn, the CC>3 ion concentration depends on the pH of the solution. To develop a better understanding of the conditions imder which CaCOs dissolves or precipitates, we must consider equilibrium relationships between Ca and COs , and between COs , HaO, and HCOs . This requirement suggests a need to combine ideas about acid-base equilibria from Chapters 16 and 17 with ideas about the new types of equilibria to be introduced in this chapter. 

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